IDENTIFICATION OF AUDIO AND ROOM PARAMETERS FOR
OPTIMUM SPEECH INTELLIGIBILITY IN ROOM
NG TSING CHUN
UNIVERSITI TEKNOLOGI MALAYSIA
PSZ 19:16 (Pind. 1/97)
UNIVERSITI TEKNOLOGI MALAYSIA
BORANG PENGESAHAN STATUS TESIS‫ט‬
JUDUL:
IDENTIFICATION OF AUDIO AND ROOM PARAMETERS
OPTIMUM SPEECH INTELLIGIBILITY IN ROOM
FOR
SESI PENGAJIAN: 2006/2007
Saya
NG TSING CHUN
(HURUF BESAR)
mengaku membenarkan tesis (PSM / Sarjana / Doktor Falsafah)* ini disimpan di Perpustakaan
Universiti Teknologi Malaysia dengan syarat-syarat kegunaan seperti berikut:
1.
2.
Tesis adalah hakmilik Universiti Teknologi Malaysia.
Perputakaan Universiti Teknologi Malaysia dibenarkan membuat salinan untuk tujuan
pengajian sahaja.
Perpustakaan dibenarkan membuat salinan tesis ini sebagai bahan pertukaran antara institusi
pengajian tinggi.
**Sila tandakan (√ )
3.
4.
√
SULIT
(Mengandungi maklumat yang berdarjah keselamatan atau
kepentingan Malaysia seperti yang termaktub di dalam AKTA
RAHSIA RASMI 1972)
TERHAD
(Mengandungi maklumat TERHAD yang telah ditentukan oleh
organisaasi / badan di mana penyelidikan dijalankan)
TIDAK TERHAD
Disahkan oleh
(TANDATANGAN PENULIS)
(TANDATANGAN PENYELIA)
Alamat tetap:
14, LORONG SATU, SITE C,
45400 SEKINCHAN, SELANGOR.
Tarikh: 11 MAY 2007
DR MOKHTAR BIN HARUN
Nama Penyelia
Tarikh: 11 MAY 2007
CATATAN: * Potong yang tidak berkenaan.
** Jika tesis ini SULIT atau TERHAD, sila lampirkan surat daripada pihak berkuasa /
organisasi berkenaan dengan menyatakan sekali sebab dan tempoh tesis ini perlu
dikelaskan sebagai SULIT atau TERHAD.
♦ Tesis dimaksudkan sebagai tesis bagi ijazah Doktor Falsafah dan Sarjana secara
penyelidikan atau disertasi bagi pengajian secara kerja kursus dan penyelidikan, atau
Laporan Projek Sarjana Muda (PSM).
“I hereby declare that I have read this thesis and in my opinion this thesis is
sufficient in terms of scope and quality for the award of the degree of Master of
Engineering (Electrical - Electronic & Telecommunication)”
Signature
: ....................................................
Name of Supervisor
:
Dr. Mokhtar bin Harun
Date
:
11 May 2007
IDENTIFICATION OF AUDIO AND ROOM PARAMETERS FOR OPTIMUM
SPEECH INTELLIGIBILITY IN ROOM
NG TSING CHUN
A project report submitted in partial fulfillment of the
requirements for the award of the degree of Master of Engineering
(Electrical – Electronic & Telecommunication)
Faculty of Electrical Engineering
Universiti Teknologi Malaysia
MAY 2007
ii
I declare that this thesis entitled “Identification of Audio and Room Parameters for
Optimum Speech Intelligibility in Room“ is the result of my own research
except as cited in the references. The thesis has not been accepted for any degree and
is not concurrently submitted in candidature of any other degree.
Signature
:
....................................................
Name
:
NG TSING CHUN
Date
:
11 May 2007
iii
ACKNOWLEDGEMENT
I wish to express my sincere appreciation to my thesis supervisor, Dr.
Mokhtar bin Harun, for encouragement, guidance, critics, advices, motivation and
mostly his patience without any haste. With his continued support and interest, this
thesis would have been successfully as presented here.
My sincere appreciation also extends to all my colleagues and others who
have provided assistance at various occasions. Their views and tips are useful indeed.
Thank you.
iv
ABSTRACT
The installation of electronic amplification system in the meeting or
conference room is intended to having a louder, clear and an even sound propagation.
Furthermore the conversation exchanged will be at ease, since speakers do not have
to raise their voice to be heard. However, the interaction between amplified sound
waves in the room and the characteristics of the room may not produce desirable
results, which is clarity of the speech. The aim of this project is to identify room and
audio parameters in meeting room, which influence conversation so that optimum
speech intelligibility can be achieved in that room. The room and audio parameters
such as room shape and size, room furnishes, reverberation time and background
noise, these characteristics will be studied so as to evaluate their effects on speech
intelligibility. CARA program is used to simulate room samples to determine which
acoustic design can achieve the optimum speech intelligibility. From the simulation
results it is found that 17 out of 18 of the room design model within the range of
acceptable speech intelligibility. The proper selection of acoustical materials for the
surfaces of ceiling, wall and floor in these meeting room models provide optimum
acoustical properties and meet the design requirements.
v
ABSTRAK
Pemasangan sistem pembesaran elektronik di dalam bilik mesyuarat atau
bilik perjumpaan adalah bertujuan untuk menghasilkan perbualan yang lebih kuat,
jelas and sama nyata di semua sudut bilik. Lagipun perbualan itu akan menjadi lebih
mudah, kerana orang yang cakap tidak perlu meninggikan suaranya supaya orang
lain boleh dengar. Tetapi, interaksi antara gelombang bunyi di dalam bilik dan juga
kelakuan bilik itu mungkin tidak dapat menghasilkan keputusan yang diingini, iaitu
kejelasan ucapan. Tujuan projek ini adalah untuk mengenalpasti ciri-ciri bilik dan
bunyi di dalam bilik mesyuarat, di mana mereka menghasilkan kesan kepada ucapan,
supaya kepandaian ucapan yang optimum dapat dicapai di dalam bilik tersebut. Ciriciri bilik dan bunyi seperti rupabentuk dan saiz bilik, perhiasan bilik, masa gemaan
dan kebisingan persekitaran, kesemua kelakuan ini akan dipelajari supaya mengenali
kesan-kesan mereka terhadap kepandaian ucapan. Program CARA digunakan untuk
simulasi terhadap model bilik tersebut dan mengenali rekabentuk akustik yang mana
satu dapat mencapai kepandaian ucapan yang optimum. Daripada keputusan simulasi
yang telah dilakukan di projek ini, 17 daripada 18 rekabentuk bilik terletak di dalam
lingkungan kepandaian ucapan yang boleh diterima. Dengan pemilihan yang
menyempurnakan bagi bahan akustik permukaaan untuk siling, dinding and lantai di
dalam model bilik mesyuarat tersebut, kandungan akustik yang optimum dapat
dicapai dan memenuhi keperluan rekabentuk itu.
vi
TABLE OF CONTENTS
TITLE
i
DECLARATION
ii
ACKNOWLEDGEMENT
iii
ABSTRACT
iv
ABSTRAK
v
TABLE OF CONTENTS
vi
LIST OF TABLES
ix
LIST OF FIGURES
xi
LIST OF SYMBOLS
xiv
CHAPTER
1
2
TITLE
PAGE
INTRODUCTION
1
1.1
Problem Statement
1
1.2
Background Study
2
1.3
Objectives of Project
3
1.4
Scope of Project
3
1.5
Layout of Thesis
4
LITERATURE REVIEW
6
2.1
Speech Intelligibility in Room
6
2.1.1 Room Acoustical Design
7
2.1.2 Speech Signal
8
Speech Intelligibility Evaluation
10
2.2.1 STI (Speech Transmission Index)
10
2.2
vii
2.2.2 Percentage Articulation
Loss of Consonants (%ALCons)
2.3
11
Reverberation
13
2.3.1 Reverberation Time
14
2.3.2 Optimum Reverberation Time
15
2.3.3 Sabine Equation
17
2.3.4 Critical Distance
18
2.3.5 Directivity
18
2.3.6 Relationship between Reverberation
Time (RT) and %ALCons
2.4
2.5
19
Room’s Acoustical Treatment
20
2.4.1 Acoustical Comfort
20
2.4.2 Ergonomics and Room Layouts
21
2.4.3 Reverberation Time and Room Acoustics
21
Surface Applied Acoustic Treatments
22
2.5.1 Sound Absorption and Absorbers
23
2.5.2 Sound Diffusion and Diffusers
24
2.5.3 Reverberation Time and Sound Absorption 25
2.5.4 Meeting Room Acoustical Treatment
2.6
26
2.5.4.1 Offices
26
2.5.4.2 Conference Rooms
27
Background Noise
27
2.6.1 Noise Reduction
28
2.6.2 Noise Reduction by Sound Absorption
29
2.6.3 Ambient Noise Level and Reverberation
Time Design Goals
3
30
METHODOLOGY
31
3.1
31
Introduction to CARA Program
3.1.1 Reverberation Time in DIN 18041 Standard 34
3.1.2 Room Usage, Shape and Size of Room
35
3.1.3 Ceiling, Floor and Walls
36
viii
3.2
3.1.4 Room Furnishes and Absorber Type
37
3.1.5 Graph of CARA Simulation
38
Room Design by CARA Program
41
3.2.1 Absorbers Type
41
3.2.2 Floor Coverings
43
3.2.3 Room Size and Furnishes
44
3.2.4 Reverberation Time, Noise Reduction
and Average Absorption Coefficient
45
3.2.5 Simulation Models of Room Design
45
4
RESULTS
47
5
ANALYSIS OF RESULTS
66
5.1
Summary 1
67
5.2
Summary 2
70
6
CONCLUSION AND RECOMMENDATION
73
6.1
74
REFERENCES
Recommendations for Future Work
75
ix
LIST OF TABLES
TABLE NO.
2.1
TITLE
Maximum ambient noise levels and optimum
reverberation time (RT) for good speech intelligibility
2.2
PAGE
8
Maximum allowable background noise levels in
accordance with DIN 18041
28
3.1
Example of room properties
36
3.2
Three types of absorbers use in simulation
42
3.3
Room dimension and number of places occupied of
simulation model
44
3.4
Simulation models of room design
46
4.1
Mineral wool ceilings without absorber, carpet floor
48
4.2
Mineral wool ceilings with 50% absorber
covered, carpet floor
4.3
49
Mineral wool ceilings with 100% absorber
covered, carpet floor
50
4.4
Mineral wool ceilings without absorber, tiled floor
51
4.5
Mineral wool ceilings with 50% absorber
covered, tiled floor
4.6
52
Mineral wool ceilings with 100% absorber
covered, tiled floor
53
4.7
Gypsum board ceilings without absorber, carpet floor
54
4.8
Gypsum board ceilings with 50% absorber
covered, carpet floor
4.9
4.10
55
Gypsum board ceilings with 100% absorber
covered, carpet floor
56
Gypsum board ceilings without absorber, tiled floor
57
x
4.11
Gypsum board ceilings with 50% absorber
covered, tiled floor
4.12
58
Gypsum board ceilings with 100% absorber
covered, tiled floor
59
4.13
Gypsum tile ceilings without absorber, carpet floor
60
4.14
Gypsum tile ceilings with 50% absorber
covered, carpet floor
4.15
61
Gypsum tile ceilings with 100% absorber
covered, carpet floor
62
4.16
Gypsum tile ceilings without absorber, tiled floor
63
4.17
Gypsum tile ceilings with 50% absorber
covered, tiled floor
4.18
64
Gypsum tile ceilings with 100% absorber
covered, tiled floor
65
5.1
Sorted results of all design stages
67
5.2
Reverberation time and value of %ALCons
in each design stages
71
xi
LIST OF FIGURES
FIGURE NO.
TITLE
PAGE
1.1
Example of meeting or conference room
3
2.1
Frequency ranges for hearing and for room acoustics
9
2.2
Sound in frequency domain
10
2.3
Relationship between STI (straight line) and
%ALCons (dotted line) obtained over a wide variety
of conditions comprising combinations of various
S/N ratios, reverberation times and echo-delay
times. The %ALCons score refers to the mean loss of
consonants in phonetically balanced monosyllabic (CVC)
nonsense words embodied in neutral carrier phases.
12
2.4
Relationship between STI and %ALCons
13
2.5
The concept of reverberation time
14
2.6
The preferred reverberation time values for various
Applications
2.7
15
The preferred ranges of reverberation time at mid-frequency
(average of reverberation at 500 and 1000 Hz) for a
variety of activities
2.8
16
Recommended reverberation times for different
listening rooms specified by the volume and kind of
usages of the room.
2.9
22
Sketch of frequency dependence of the sound absorption
coefficient for different materials
24
2.10
Sound absorption coefficient
26
2.11
An example of mounting of sound absorbing mineral
wool coated plates in the ceiling
26
xii
2.12
Illustration about achieving the audible improvement,
the absorption within a room has to be increased by
a factor of 2
2.13
29
Equal speech intelligibility contours for 300 m3 room and
reverberation time (RT) design goals
30
3.1
A snapshot of the CARA program
32
3.2
A snapshot of the CARA program
33
3.3
Optimum reverberation time for occupied instruction rooms
for the octave bands 500 Hz and 1000 Hz (above) and
band of tolerance for the recommended reverberation
time as a function of frequency (below) according to DIN
18041. RTo = optimal reverberation time according
to the upper diagram, RT = reverberation time.
34
3.4
Example of room properties
35
3.5
CARA program of furniture’s entry field
37
3.6
Gypsum tile ceilings manufactured by Rigips
38
3.7
Example of the graph of reverberation time over frequency 39
3.8
Example of the graph of noise reduction and
average absorption coefficient over frequency
40
3.9
Examples of the layout of the places in meeting room
45
4.1
Simulation graph of reverberation time, noise reduction
and average absorption coefficient of Design 1.1.1
4.2
Simulation graph of reverberation time, noise reduction
and average absorption coefficient of Design 1.1.2
4.3
52
Simulation graph of reverberation time, noise reduction
and average absorption coefficient of Design 1.2.3
4.7
51
Simulation graph of reverberation time, noise reduction
and average absorption coefficient of Design 1.2.2
4.6
50
Simulation graph of reverberation time, noise reduction
and average absorption coefficient of Design 1.2.1
4.5
49
Simulation graph of reverberation time, noise reduction
and average absorption coefficient of Design 1.1.3
4.4
48
53
Simulation graph of reverberation time, noise reduction
and average absorption coefficient of Design 2.1.1
54
xiii
4.8
Simulation graph of reverberation time, noise reduction
and average absorption coefficient of Design 2.1.2
4.9
Simulation graph of reverberation time, noise reduction
and average absorption coefficient of Design 2.1.3
4.10
63
Simulation graph of reverberation time, noise reduction
and average absorption coefficient of Design 3.2.2
4.18
62
Simulation graph of reverberation time, noise reduction
and average absorption coefficient of Design 3.2.1
4.17
61
Simulation graph of reverberation time, noise reduction
and average absorption coefficient of Design 3.1.3
4.16
60
Simulation graph of reverberation time, noise reduction
and average absorption coefficient of Design 3.1.2
4.15
59
Simulation graph of reverberation time, noise reduction
and average absorption coefficient of Design 3.1.1
4.14
58
Simulation graph of reverberation time, noise reduction
and average absorption coefficient of Design 2.2.3
4.13
57
Simulation graph of reverberation time, noise reduction
and average absorption coefficient of Design 2.2.2
4.12
56
Simulation graph of reverberation time, noise reduction
and average absorption coefficient of Design 2.2.1
4.11
55
64
Simulation graph of reverberation time, noise reduction
and average absorption coefficient of Design 3.2.3
65
5.1
Room no.1 with its acoustical properties
68
5.2
Room no.2 with its acoustical properties
69
5.3
Room no.3 with its acoustical properties
70
5.4
Reverberation time and value of %ALCons in each
design stages
72
xiv
LIST OF SYMBOLS
RT, T
-
Reverberation time
V
-
Volume
S, A
-
Total room surface
ά
-
Average absorption coefficient
∆L
-
Noise reduction
D
-
Critical distance
Q
-
Directivity
r
-
Distance
k
-
Constant
l
-
Length
b
-
Width
h
-
Height
CHAPTER 1
INTRODUCTION
This chapter will discuss briefly about problem statement, background study
and objectives of the project.
1.1
Problem Statement
Rooms such as meeting or conference room is intended for speech, but most
of them are often not designed to meet this intended use. Meeting room can be as
small as just consist of a few seats with a table in the center of the room with one
whiteboard in the front, and as large as consist of a few ten of seats with more tables,
projector, and some of them with sound reinforcement system. Conversation in a
small room is much more clear and ease to understand since the talker and listener
are seated face to face. They do not need to raise their voice when speaking to each
other.
In the case of larger meeting room, the installation of sound reinforcement
system is intended to having a louder, clear and bigger coverage of conversation in
the room, since speakers do not have to raise their voice to be heard. When the
acoustical design issues are ignored, inaccurate communication can result. Both the
excessive noise and inappropriate room acoustics can degrade the intelligibility of
speech in room, which will affect clarity of the speech.
2
1.2
Background Study
This project identifies the room and audio parameters that will affect the
degree of speech intelligibility. The room parameters that will discuss in this project
are regarding to the shape and size of the room and its room’s furnishes. This usage
of the room that will be analyzed is a meeting or conference room with rectangular
shape. Different sizes of the room will be analyzed since the volume and surface area
of the room are important parameters to determine the reverberation time. The
pictures on Figure 1.1 show some example of meeting or conference room.
3
Figure 1.1
1.3
Example of meeting or conference room
Objectives of Project
The objectives of this project are:
i. To identify parameters that influence speech intelligibility
ii. To select suitable audio and room parameters for analysis
iii. To manipulate these parameters so as to achieve optimum speech
intelligibility in the room
1.4
Scope of Project
The scope of this project included the design of proper meeting room to
achieve optimum speech intelligibility and fulfill the acoustical design requirements.
4
One of the important criteria when designing room acoustics is the reverberation
time. The reverberation time is influenced by room size and sound absorption.
Therefore, in this project, three different room size with small, medium and large
size are modeling with different absorption coefficient respectively. There are total
eighteen room models differ in dimension, surface material and furnishing.
The simulation is done by using CARA program. The reverberation time,
average absorption coefficient and noise reduction level are shown on simulation.
Based on the simulation results, the %ALCons is use as method to evaluate degree of
speech intelligibility, and also which kinds of room acoustical design much fulfill the
design requirements.
The limit of this project is room acoustical designs are analyzed by using the
components given in the CARA program only. Only simulation by computer but no
any experiments has been carried out for actual audio measurements. The
recommended requirement by the DIN 18041 standard, where stated in the program,
is not necessary to be met. Furthermore, the ventilation issue, lighting, cost of
acoustical design and etc. are not included in the project as well.
1.5
Layout of Thesis
The first chapter of this project thesis discusses the introduction and
background of the project. Problem statement and scope of the project also has been
mentioned.
Second chapter having the detailed researches on the theories of the room and
audio parameters that have been chosen. The equations of calculating reverberation
time and %ALCons also stated.
The more discussion about the usage of the CARA program to analyzed
speech intelligibility is located on the chapter three that is the project methodology.
The various room acoustical designs for analysis are also mentioned here.
5
The results of room acoustical design and analysis are discussed on chapter
four and chapter five. The conclusion and the recommendations for further study will
be made on the last chapter.
CHAPTER 2
LITERATURE REVIEW
This chapter give details on room characteristics, reverberation time, room
design for good acoustics, model studies and simulation and determine of
reverberation time.
2.1
Speech Intelligibility in Room
Speech intelligibility means how easily speech can be understood over a
distance. It is especially a critical issue in meeting or conference room. Good
acoustics conditions in meeting room affect and improve both concentration and the
communication between people. (Gemini, 2003)
The purposes of designing good acoustics in meeting room are to achieve
clarity of dialogue, smooth frequency response, smooth sound movement, wide
dynamic range, diffuse sound field, and every seat a good seat. There also must have
knowledge regarding the physical nature of sound and speech signal, how that sound
changes once it is trapped in a room, and how it can be controlled.
7
2.1.1 Room Acoustical Design
In room acoustical design, the shape and size of a room are importance to the
room acoustical planning process as well as the choice and positioning of interior
materials and furniture. In this project, acoustical design of meeting room with
rectangular shape has been chosen for analysis.
In this project has studied how it is possible to achieve good speech
intelligibility in a meeting room. It was found that as a first priority the optimum
reverberation time must be adhered to, for example by installing an absorbing
ceiling. Sound absorbing materials can be served to adjust the reverberation time of a
room. Through an optimum arrangement of the absorption a slight further
improvement can be accomplished. The typically recommended reverberation time
for general meeting room is less than 0.7 s for good intelligibility, as shown on Table
2.1.
On the other hand it is sometimes felt that the acoustics are unpleasant in
rooms where the reverberation time is too low. In this project, meeting room
acoustical design has target to lower and smooth the reverberation time curve. In
addition, depending on the room size, too much absorption can lead to a decrease in
speech intelligibility.
The acoustics of a room is also influenced by other factors: apart from the
shape of the room basically other features as floor, wall and ceiling coverings
determine the propagation of sound in a room. The size and positioning of sound
absorbing as well as sound reflecting surfaces decide whether the acoustics or a room
is judged as “good” or “bad”. In practice, one very often has only the possibility of
changing the ceiling and wall coverings to control the room acoustics.
When people are talking inside a room, the human ear not only hear the direct
sound, but also the sound reflected from walls and ceilings as well as any
background noise in the room. Reflected sound and background noise reduces the
quality and clarity of speech, as it mixes with the spoken words. The recommended
8
background noise level for meeting room does not exceed 35 dBA, as shown on
Table 2.1. While as the speech levels in the meeting room for normal situations can
be assumed approximately 65 dBA.
Therefore, by controlling background noise levels and room reverberation
time, good speech intelligibility can be achieved, measured by the signal-to-noise
ratio. Low background noise and short reverberation times contribute to positive
sound-to-noise ratios, maximal sound transmission indices, and high speech
intelligibility values.
Example Situations
Primary school classroom
Boardroom for elderly adults
Law court
High school classroom
General meeting room
Large lecture hall theatre
Table 2.1
Maximum noise
RT (s)
dBA
NC
30
23
0.5
30
23
0.5
35
28
0.7
30
23
0.7
Maximum ambient noise levels and optimum reverberation time (RT)
for good speech intelligibility. (Bradley, 2002)
2.1.2 Speech Signal
The music signal and speech signal are totally different. The ear / brain
processor can fill in a substantial amount of missing information in music, but
requires more detailed information for understanding speech. The speech power is
delivered in the vowels (a, e, i, o, u and sometimes y) which are predominantly in the
frequency range of 250Hz to 500Hz. The speech intelligibility is delivered in the
consonants (b, c, d, f, g, h, j, k, l, m, n, p, q, r, s, t, v, w) which requires information
in the 2,000Hz to 4,000Hz frequency range. People who suffer from noise induced
9
hearing loss typically have a 4,000Hz notch, which causes severe degradation of
speech intelligibility. Most telephones have a limited usable bandwidth of less than
4,000Hz, and one may have noticed that it can make it difficult to hear the difference
between the letters b, d, t, and v, and also between f and s, on the phone. Vowels
have more sound energy available as well, the consonants that give us the detail and
information in speech are quite low in level.
In this project a computer software CARA (Computer Aided Room
Acoustics) program is use to predict the acoustic conditions within a room.
Especially for rooms with high acoustic demands like meeting and conference
rooms, an exact planning of the room acoustics should be applied. To achieve good
room acoustics the planning would be carried out within the frequency range
between 100 Hz and 4000 Hz, where the listening range reaches from about 20 Hz to
20’000 Hz, as shown on Figure 2.1.
Figure 2.1
Frequency ranges for hearing and for room acoustics. (Nocke, 2002)
Room acoustics covers the frequency range between 100 Hz and 4’000 Hz;
for measurements and planning this range is split up into 18 one-third octave bands.
The Figure 2.2 shows the corresponding 1/3 octave band centre frequencies. A
simplified approach was carried out using the six octave band centre frequencies
between 125 Hz to 4000 Hz in the CARA program.
10
Figure 2.2
2.2
Sound in frequency domain. (Salameh, 2004)
Speech Intelligibility Evaluation
Speech intelligibility is a critical component of any space where the
communication through speech is the primary issue, especially for the meeting room.
The factors that influence speech intelligibility, reverberation time and background
noise, would be tested by using a CARA program in this project. By control the noise
levels, control the reverberation time and provide acoustical design to the meeting
room, optimum speech intelligibility can be achieved. Recommendation to improve
the room acoustical condition will be developed. In order to evaluate the degree of
speech intelligibility, a measurement of STI (Speech Transmission Index) or
%ALCons (Percentage Articulation Loss of Consonants) will be use.
2.2.1 STI (Speech Transmission Index)
Speech intelligibility was objectively quantified by the calculation of the
Speech Transmission Index (STI), which was hypothesized that can be related to the
scores of people taking live speech intelligibility tests.
11
Developed in the early 1970’s, the STI is a machine measure of intelligibility
whose value varies from 0 (completely unintelligible) to 1 (perfect intelligibility). In
STI testing, speech is modeled by a special test signal with speech-like
characteristics. Following on the concept that speech can be described as a
fundamental waveform that is modulated by low-frequency signals, STI employs a
complex amplitude modulation scheme to generate its test signal. At the receiving
end of the communication system, the depth of modulation of the received signal is
compared with that of the test signal in each of a number of frequency bands.
Reductions in the modulation depth are associated with loss of intelligibility.
The STI methods can be used to compare speech transmission quality at
various listening positions and under various conditions within the same listening
space. In particular it is useful for assessing the effect of changes in acoustic
properlies, including effects produced by the presence of an audience, changes in
room surface properties or changes in a sound systern. The methods are also able to
predict the absolute rating of speech transmission quality with respect to
intelligibility when comparing different listening spaces under similar conditions or
assessing a speech communication channel.
2.2.2 Percentage Articulation Loss of Consonants (%ALCons)
This machine measure of intelligibility is closely associated with the TEF
sound analyzer. It is computed from measurements of the Direct-to-Reverberant
Ratio and the Early Decay Time using a set of correlations defined by SynAudCon,
and is specified in percent.
%ALCons describes the percentage of consonants that will be misunderstood
or lost in conventional speech (assuming 25dB of signal to noise and a proficient
talker and listener). Since %ALCons expresses loss of consonant definition, lower
values are associated with greater intelligibility. The maximum acceptable %ALCons
for speech communication would be 15%, as long as there is a minimum of 25dB of
signal to noise ratio, and the spoken information is being delivered at a pace suitable
12
for the acoustic environment. A more suitable maximum %ALCons for familiar
speech would be 10% (this would also apply to life safety systems, voice warning or
fire evacuation page systems). For educational applications more stringent criteria of
5% should be considered. In this project, a %ALCons of less than 15% is target to
achieve reverberation time of 0.7s, has been evaluated as good speech intelligibility
for meeting room.
Figure 2.3
Relationship between STI (straight line) and %ALCons (dotted line)
obtained over a wide variety of conditions comprising combinations
of various S/N ratios, reverberation times and echo-delay times. The
%ALCons score refers to the mean loss of consonants in phonetically
balanced monosyllabic (CVC) nonsense words embodied in neutral
carrier phases. (Lam, 2000)
13
Figure 2.4
2.3
Relationships between STI and %ALCons. (Lam, 2000)
Reverberation
Reverberation defines as the persistence of sound in an enclosed space after
the original excitation sound has ceased. It consists of a series of very closely spaced
reflections, or echoes, whose strength decreases over time due to boundary
absorption and air losses.
Reverberation is so destructive of intelligibility, especially beyond critical
distance. Being itself caused by the speech, reverberation signals mimics the speech
spectrum, but generally with greater low-frequency energy. Sufficiently long reverb
and echoes can actually function like multiple distracter voices. And by its nature,
reverberant energy arrives from all angles, so it’s hard to separate from the speech
using directional clues.
Reverberation measures the amount of room echo that occurs between the
talker and the listener. Reverberation, which makes speech more difficult to
understand, is strongly affected by room characteristics (hard, reflective walls, floor,
and ceiling), room size, and the orientation between the speaker and the microphone.
If the microphone is not pointed at the speaker or is more distant, a greater
proportion of the sound picked up by the microphone will be reverberation instead of
direct speech, and the end result will be a decrease in intelligibility.
14
2.3.1 Reverberation Time
Reverberation time is the single most important parameter describing a
room's acoustic behavior. Reverberation time for different listening rooms can be
varied greatly across the frequency range depending on the type of room and usage.
Given this, it is not surprising that the overall or average reverberation times will be
different for different venues.
The standard method for specifying reverberation time, RT60 is the amount
of time it takes for the reverberant energy in an enclosed space to drop by 60 dB
from its initial, steady-state value after the original sound has ceased. In other words,
it is the time needed for a loud sound to be inaudible after turning off the sound
source. In simple terms this refers to the amount of time it takes for sound energy to
bounce around a room before being absorbed by the materials and air.
Figure 2.5
The concept of reverberation time. (Salameh, 2004)
Large rooms with hard, highly reflective surfaces (like cathedrals), or closed
spaces that don't have materials to absorb sound (concrete arenas, big rooms with
Gyproc walls, big water tanks etc.) have long reverberation times, while smaller
rooms with absorptive surfaces (like a movie theatre, or a carpet and drapery
showroom) have short reverberation times. Areas with a long reverberation time are
referred to as being ’live’, e.g. cathedrals and churches. Those with a short
reverberation time are referred to as being ’dead’, e.g. offices with thick carpets and
15
absorbent ceilings. Figure 2.6 shows the preferred RT60 values for various
applications.
Figure 2.6
The preferred reverberation time values for various applications.
(Jones, 2006)
2.3.2 Optimum Reverberation Time
The preferred ranges of reverberation time at mid-frequency (average of
reverberation at 500 and 1000 Hz) for a variety of activities are given on the Figure
2.7. The ranges are based on the experience of normal-hearing listeners in completed
spaces. Satisfactory listening conditions can be achieved in auditoriums which have
different reverberation times within the preferred range, provided other important
acoustical needs are fulfilled. In general, large rooms should be nearer the upper end
of the reverberation time ranges than smaller rooms of the same type.
16
Figure 2.7
The preferred ranges of reverberation time at mid-frequency (average
of reverberation at 500 and 1000 Hz) for a variety of activities. (Jones,
2006)
17
2.3.3 Sabine Equation
In 1920 Sabine published an article describing the relationship between
reverberation times, T, the volume of a room, V, and total sound absorption area, Sά.
In practice most people use Sabine’s equation to calculate the acoustics behavior of
rooms. As well as in this project, this equation as in Equation (2.1) is correlated to
the measurement of speech intelligibility. It is important to remember that the
equation is based on a diffused sound field, i.e. an evenly distributed sound field in a
room of equal proportions and not exceeding 2000 m3 in volume. (Stumpf, 1980)
Reverberation time,
(2.1)
where
V = volume of the room
S = total room surface area
= 2 * (l*b + l*h + b*h)
ά = average absorption coefficient
The total sound absorption area is given by the product of the sound
absorption coefficient and the surface area of the sound absorber. The calculation
may refer to the average sound absorption coefficient of all surfaces. Apart from the
sound absorption given by the surfaces of a room the absorption of furniture,
audience and damping in the air might be added. The reverberation time of a room
might be estimated from information concerning room size and furnishing with
sufficient accuracy. This estimation requires the description of the volume and the
materials of the room surfaces.
18
2.3.4 Critical Distance
The term “critical distance” refers to the distance from a loudspeaker in an
enclosed space at which the reverberation is equal in strength to the direct sound
from the speaker. In other words, critical distance is the distance between a sound
source and the listener where the direct sound from the sound source is equal in
sound level to the reverberant sound field. Beyond this distance, the reverberant
energy tends to mask the direct sound. The Equation (2.3) shows the formula to
determine critical distance. (Rossing, 1982)
In truth, because reflected sound loses energy to boundary absorption (and
also travels a longer path to the listener, thus incurring greater air absorption losses),
the reverberant energy from a discrete pulse sound stimulus can never equal the
direct sound on an instantaneous basis. In highly reflective environments, however,
the steady-state reverberation strength can easily exceed that of the direct sound at
many locations in the space. This degrades the signal-to-noise ratio and destroys
intelligibility.
2.3.5 Directivity
Directivity is a measure of the directional characteristic of a sound source. It
is often expressed as a Directivity Index in decibels, or as a dimensionless value of
Q. This is an important aspect of a sound source, especially in a reverberant field.
When a balloon is popped, it sends sound out in all directions equally (for all
practical purposes), and this would represent a Q value of 1. A person talking has a Q
value of approximately 2, which means that sound radiates in a hemispherical pattern
(half a sphere). Directivity is important because it helps indicate how much sound
will be directed towards a specific area compared to all the sound energy being
generated by a source. (Rossing, 1982)
19
2.3.6 Relationship between Reverberation Time (RT) and %ALCons
Reverberation can cause one syllable to mask the next one. In order to predict
the speech intelligibility, which takes account of the factor of critical distance and the
directivity of the source to the reverberation time, Equation (2.2) and (2.4) has been
established. Experimentally it was found that (Rossing, 1982):
For r < D
(2.2)
where
T = reverberation time
r = distance from the source
V = volume of the room
k = the constant for each listener that indicates listening ability
(1.5% for the best listener to 12.5% for the poorest)
D = critical distance
(2.3)
Q = directivity factor of the source
= 1, for spherical (all directions) source or nondirectional source (a
person speaking)
= 2, for hemispherical source
= 4, for corner source (one-quarter of a sphere)
= 8, for source at a three-surface corner
For r > D
(2.4)
20
2.4
Room’s Acoustical Treatment
Acoustical treatment can be looked as a solution to acoustical problems that
exist within a listening environment. The shape of a room, the materials that
comprise it, the room's layout with respect to windows, posts, doors, etc. are all
factors that will increase or decrease the overall quality of a space's sound.
The shape of a room has a lot to do with sound. A square room is most likely
to cause problems with sound waves because they will bounce back at themselves
and, depending on their wavelength, could cancel themselves out. Rooms that are
paneled and have hardwood floors can also be problematic. Sound waves reflect off
of walls and floors and cause unclear dialogue and distorted sounds.
From experience low pile floor coverings, conventional upholstered furniture,
blinds and curtains are not well suited to effective reverberation time reduction. Floor
coverings that are soft to walk upon, however (e.g. rugs, fitted carpets), avoid the
corresponding noise generation and therefore contribute significantly to the reduction
of the total noise pressure level.
Ideally, an acoustic-friendly room would include a room with walls of
differing length, carpeting and padded furniture. However in most situations are
going to require some kind of essential acoustical room treatment.
2.4.1 Acoustical Comfort
It is important to take into consideration all of the surfaces within a room to
ensure that the correct balance between absorption and reflection are achieved. Too
much absorption in a room can be as bad as not enough. As the ceiling generally
represents a large proportion of the room’s surface area it can be a major influence
on the acoustic performance.
21
It is recommended that, with new construction, the acoustical treatments can
be designed and integrated into the room as part of the construction, thereby saving
money and creating an acoustical-comfort pleasing environment. Properly designed
rooms can incorporate absorption, diffusion, traps, and many other acoustical devices
into the walls, ceilings, and even floors using conventional finish materials such as
glass and mirror, fabrics, carpet and wood.
2.4.2 Ergonomics and Room Layouts
Ergonomics is how we arrange the equipment, furniture and any other
element in our rooms and how these elements affect the usefulness and comfort of
the room. Basically - "where do we put everything in the room?" There is no one
perfect way to organize equipment and furniture in the room. These layout varieties
depend on the room’s exact usage, room size, budget, final use, etc. As the acoustic
performance of the meeting room is important, but the equipment layout, general
comfort, workability, and the ergonomics of the room should be considered first.
The shape of the room should be such that there is no focusing, i.e., no
sharply domed ceilings or curved rear walls. The room should be shaped to give
beneficial reflection if possible. In general, the sound should be diffused and not
strongly focused at given points in the room. A compromise between clarity and an
adequate intensity level may be important in determining the reverberation time for a
give room. The size of the room and the reverberation time has to be considered in
obtaining good acoustics according to the use made of the room.
2.4.3 Reverberation Time and Room Acoustics
The reverberation time in a room depends on the material's ability to absorb
sound waves, the physical size of the surfaces, the interaction between surfaces of
different materials, the position of the surfaces and the total size of the room. The
22
ways to improve the acoustics are therefore different from room to room. Internal
surfaces and furnishes should be selected so that reverberation times (for frequencies
between 125 Hz and 4k Hz) are with in the limits. In this project has given some
general guidelines that help in designing acoustical of meeting room, as shown in
Figure 2.8.
Key
A is for private offices, meeting rooms, consulting rooms.
B is for operating theatres, single and multi-bed wards, waiting rooms, wash rooms,
kitchens, recreation rooms, general offices.
C is for lecture theatres.
Figure 2.8
Recommended reverberation times for different listening rooms
specified by the volume and kind of usages of the room. (Jones, 2006)
2.5
Surface Applied Acoustic Treatments
There are three types of surface applied acoustic treatments can be applied to
the room's walls, ceilings, and floors that come in a number of styles and flavors.
They can be categorized by the way they alter the acoustic qualities in the room:
23
1) Applied absorption treatment does not change the reflection pattern but
reduces the level of harsh reflection. Reflections stay the same, they are
simply lowered in amplitude.
2) Applied diffusions treatment does not change the level of the reflections,
but changes the pattern. Reflections are scattered throughout the room,
but total energy stays the same.
3) Applied reflection treatment, energy is re-directed, but without scattering.
2.5.1 Sound Absorption and Absorbers
There are two basic sound absorbing materials:
• porous sound absorbers and
• resonant type sound absorbers.
At the surface of a porous sound absorber the incident sound wave enters the
material. The sound energy is converted to heat energy by friction in the pores of the
material. This mechanism reduces the reflected sound energy. Sound is absorbed.
Resonant type sound absorbers consist of a system that may vibrate, e.g. a
wooden plate or a column of air. The excitation works very well at the resonance
frequency of the system. The vibration converts incident sound energy to vibration
energy and thus lowers the reflection of sound.
The sound absorbing effect of a material or set-up of materials is given by the
sound absorption coefficient α. The sound absorption coefficient of porous sound
absorbers increases with increasing frequency. Resonant type sound absorbers show
a maximum sound absorption at a certain, mostly narrow frequency range. The
typical sound absorption characteristics of both types are sketched in Figure 2.9.
24
Figure 2.9
Sketch of frequency dependence of the sound absorption coefficient
for different materials. (Nocke, 2002)
2.5.2 Sound Diffusion and Diffusers
Sound in an enclosure can be described as a diffused if the intensity of the
sound energy is equal in every location of the room, or the sound energy flows
equally in every direction. Many different factors can enhance the diffused sound.
These factors include geometrical irregularities, absence of focusing surfaces, the
distribution of absorptive and reflective elements randomly scattered through the
space, and the existence of diffusing objects (furniture) or panels (diffusers).
The proper acoustic room treatment will minimize sound distortion and
increase perception of sound. An ideal diffusing surface for a particular room,
25
scatters sound arriving from any direction uniformly into all observation directions
for all frequencies. Room furnishings, such as bookcases and furniture, etc. and
acoustical treatments cause sound to scatter or reflect in a random way. Sound
diffusers scatter sound so that it eventually reaches absorbing surfaces in the room.
Small rooms require a very efficient surface to diffuse interfering wall reflections
and provide a diffuse sound field.
Different building materials produce differing sound reflections. A flat
concrete wall will produce a distinct echo when clap hands and that sound are
reflected off of it. However a brick wall, even though it is somewhat reflective, will
tend to diffuse the sound reflections and produces a much less distinct echo. This is
due to the surface of the brick, the mortar between the bricks and the varying edge of
the brick mortar area. Proper diffusion of sound can make a small room sound larger.
Diffusers are used to redirect reflected sound when additional absorptive
devices would be too much or make the room acoustically dead. Essentially, it is
absorbers and diffusers working together that will produce the optimal reverberation.
2.5.3 Reverberation Time and Sound Absorption
Sound absorption is the reduction of sound energy. The sound absorption
coefficient provides the relationship between absorption and reflection of sound.
Porous materials (materials that can blow air through) such as acoustical ceiling tiles,
carpets, or curtains and drapes tend to absorb sound best at mid and higher
frequencies depending on the thickness and other material properties. Thin panels
and other resonant systems generally absorb most at particular lower frequencies.
Proper choice of the amounts and distributions of these classes of absorbers can tailor
the behavior of the reverberation time with frequency to obtain almost any desired
acoustic environment.
A value of 0 represents total reflection and a value of 1 represents total
absorption. Multiplying the sound absorption coefficient by 100 provides the
26
percentage of sound energy being absorbed, e.g. a sound absorption coefficient of
0.75 stands for a sound absorption of 75% and a sound reflection of 25%, as shown
on Figure 2.10.
Figure 2.10
Sound absorption coefficient. (OWA, 2005)
2.5.4 Meeting Room Acoustical Treatment
2.5.4.1 Offices
Most offices have reasonable good acoustics and but if it has few furniture
then it will find it advantages to add some sort of sound absorbent material. This is
particularly true if using a conference phone in the office. Recommend to mount
mineral wool coated plates in the ceiling covering about 50% of the ceiling. It can
further improve the acoustics by using thick textiles hanging approximately 5 to 10
cm from the wall, e.g. deep fold curtains and wall rugs. Floor carpets on the other
hand, do not have much impact of the acoustics in the room.
Figure 2.11
An example of mounting of sound absorbing mineral wool coated
plates in the ceiling. (Gemini, 2003)
27
2.5.4.2 Conference Rooms
For smaller conference rooms (6-8 persons), it is recommend to have a good
sound absorbing ceiling in the form of mineral wool coated plates. If changing to the
ceiling is not possible or if needed to further improve the room, it is recommend
having the same wall decoration as in offices (deep fold curtains and wall rugs). In
rooms where glass is heavily applied, the room may have an unfortunate reflection
between parallel surfaces. Therefore it is recommend using sound absorbing plates
on the wall as well as the ceiling.
Larger conference rooms may prove more difficult. In addition to adding
sound absorbing material in the ceiling and on the walls, also recommend installing a
horizontal mounted sound-absorbing surface above the conference table. Required
alteration will vary much more from room to room than when dealing with smaller
conference rooms.
2.6
Background Noise
Noise refers to any unwanted, introduced signal or sound in a
communications system or speaking environment. Background noise refers to the
proportion of ambient noise that is picked up along with speech. The sources of noise
are many, and can be both acoustical (HVAC, street sounds, crowd noise,
reverberation and echoes, etc.) and electronic (thermal noise or hiss, hum, etc.).
Room noise, such as air conditioning and projector fan noise, can be easily heard by
microphones, and it plays a significant role in decreasing the intelligibility of speech.
In a meeting or conference room, experience shows that the noise from
heating and air conditioning systems is generally quite low. However, equipment like
video beamers often produce annoying noise levels, although models are available
which definitely can be installed in the same room. If the noise is a problem, then
separate booths must be furnished to house the devices. Sometimes components of
28
inferior sound systems also generate high noise levels (noise from fans as well as
electronic noise such as hiss, hum or clicks).
DIN 18041 specifies three categories of noise limits as shown on Table 2.2.
These are a function of distance between the talker and listeners, the makeup of the
listeners and the type of instruction (for hearing impaired, difficult and foreign
language texts). For meeting room noise limits of 35 dB(A) or 30 dB(A) are
recommended.
Table 2.2
Requirements
Maximum Noise Level
low
40 dB(A)
middle
35 dB(A)
high
30 dB(A)
Maximum allowable background noise levels in accordance with
DIN 18041. (Eggenschwiler, 2005)
2.6.1 Noise Reduction
Attenuating background noise is the easiest way to increase the dynamic
range capability and increase the signal-to-noise ratio. The signal-to-noise ratio is the
relationship between the loudness of the message and the background noise it must
overcome to be heard and understood. A significantly positive signal-to-noise ratio is
necessary for optimum performance where room sound levels are high.
Background noise interferes with our perception of loudness. It masks lowlevel signals and it makes speech intelligibility more difficult. High background
noise values across the frequencies of speech (500 to 2000 Hz) require louder speech
signals to overcome. A high background noise reduces the quality and clarity of the
speech as it mixes with the spoken words.
29
Rooms and spaces can be protected from unwanted exterior sound by mass,
insulation, and isolation in wall and slab construction and minimizing (or sound
protecting) openings. Low background noise and short reverberation times contribute
to positive sound-to-noise ratios, maximal sound transmission indices, and high
speech intelligibility values.
2.6.2 Noise Reduction by Sound Absorption
The average noise level in a room is dependent on the sound source and the
sound absorption. Increasing the absorption within the room will generally reduce
the noise level, in practice by approximately 3 to 10 dB.
To achieve an audible improvement the absorption within a room has to be
increased by a factor of 2. Therefore, an increase of the absorption of the ceiling
from 20% to 40% or from 40% to 80% is advisable, an increase from 70% to 80 %
will show very little, if any, noticeable improvement.
Figure 2.12
Illustration about achieving the audible improvement, the absorption
within a room has to be increased by a factor of 2. (OWA, 2005)
30
2.6.3 Ambient Noise Level and Reverberation Time Design Goals
Figure 2.13
Equal speech intelligibility contours for 300 m3 room and
reverberation time (RT) design goals. (Bradley, 2002)
Figure 2.13 plots contours of equal speech recognition scores for
combinations of ambient noise levels and reverberation times. These were derived
from speech studies in many rooms and from measured useful-to-detrimental sound
ratios. The 99% contour is used to determine a design goal for normal use. The 99%
speech intelligibility contour indicates a range of combinations of ambient noise
levels and reverberation times can be expected to lead to the same speech
intelligibility score. The point marked with an open square represents a desirable
reverberation time design goal. Following the same 99% contour indicates that
rooms with higher reverberation times would require much lower ambient noise
levels, which would be very costly and difficult to achieve. A lower RT goal on the
same contour would require more added sound-absorbing material for minimal
additional benefit. Therefore the reverberation time point marked by the open square
symbol can be considered optimum because it minimizes both the need to add soundabsorbing material and the need to reduce ambient noise levels.
CHAPTER 3
METHODOLOGY
CARA (Computer Aided Room Acoustics) is a software to compute and
optimize the room acoustical design for various type of room. An online JAVA
Applet CARA program is use in this project to simulate the room acoustical
condition and evaluate room acoustics effects, and calculate the reverberation time,
absorption coefficient and noise reduction to suit the room acoustics for the room.
3.1
Introduction to CARA Program
To use the program, just have to enter some data for the room and its
furnishings. In this project, only meeting/conference room will be analyzed, and the
Germany DIN 18041 standard is choosing as reference. The tolerance range for the
reverberation time (grey curves) is determined according to the volume of the room
and its intended use based on the Germany DIN 18041 standard (Acoustical quality
in small to medium-sized rooms). When changing a value in one of the entry fields,
all arithmetic results are updated automatically. Figure 3.1 and Figure 3.2 are the
snapshots of the program.
32
Figure 3.1
A snapshot of the CARA program
33
Figure 3.2
A snapshot of the CARA program
34
3.1.1 Reverberation Time in DIN 18041 Standard
In the revised DIN 18041 standard (Acoustical quality in small to mediumsized rooms), the recommended reverberation times are given according to the latest
knowledge, see Figure 3.3. This standard describes measures for ensuring sufficient
acoustical quality (speech communication) with and without electro acoustical
equipment and with varying background noise levels for rooms with volumes up to
5000 cubic metres. This standard does not apply to studio-quality rooms. For the
unoccupied room the reverberation time should not exceed the recommended values
by more than 0.2 seconds.
Figure 3.3
Optimum reverberation time for occupied instruction rooms for the
octave bands 500 Hz and 1000 Hz (above) and band of tolerance for
the recommended reverberation time as a function of frequency
(below) according to DIN 18041. RTo = optimal reverberation time
according to the upper diagram, RT = reverberation time.
(Eggenschwiler, 2005)
35
3.1.2 Room Usage, Shape and Size of Room
There are four selections of room shape available in this CARA program, but
in this project, meeting/conference room with rectangular shape has been chosen.
The length, width and height’s data are input by user and the volume and
surfaces area will be calculated automatically. For example in Figure 3.4, according
to the data, results are shown in Table 3.1.
Figure 3.4
Example of room properties
36
Table 3.1
Parameter
Value
Length, l
12 m
Width, b
8m
Height, h
3m
Volume
288 m3
Bare ceiling area
96 m2
Bare floor area
96 m2
Floor coverings area
96 m2
Walls area
100 m2
Windows area
20 m2
Surface Area
312 m2
Example of room properties
Remark
Assuming all floor areas have been covered
Assuming 15% of wall’s area is constructed by
windows
bare ceiling area + bare floor area + walls area
+ windows area
3.1.3 Ceiling, Floor and Walls
There have two choices of bare ceiling where massive or timber construction
can be choose. In this project, massive type of ceiling construction has been chosen.
Absorbers are mounting on the ceiling and covering parts or all the ceiling area
depend to the design specifications.
Also, there have three types of bare floor construction for chosen. There are
floating composite floor, compound composite floor and timber construction. For
this project, only compound composite floor has been chosen.
For this CARA program, it gives two out of four selections of floor coverings
and its covering area. User can select floor covering with carpet, parquet/laminate,
linoleum and tiled floor. In this project, the compound composite floor is covered by
either carpet or tiles with assumption all floor’s area have been covered.
37
Similarly, user can select either massive type or lightweight type of walls
construction or both at the same time. In this project, massive type construction of
walls is selected for all the walls area. Further more, the windows area is assumed to
cover about 15% of the wall’s area, that means only one side of the wall constructed
by windows. In most meeting room, there are always having curtains or blinds. In the
windows selection field, user can select no curtains or blinds on the window, curtains
with lightweight, middleweight or heavyweight type and lastly windows with blinds
only. For the ease of analysis in this project, curtains with lightweight type have been
chosen. Further study shown that, either curtains or blinds will have small portion of
effect to the reverberation time, since curtains or blinds act as a sound absorber or
diffuser.
3.1.4 Room Furnishes and Absorber Type
There are various selections of furniture and its quantities available in this
CARA program as shown on Figure 3.5. In this project, one speaker is assumed
occupied with three different quantities of places in meeting room.
Figure 3.5
CARA program of furniture’s entry field
There are eight types of absorber can be selected as mounting on the ceiling
area. Up to three types of absorber can be selected at the same time in the CARA
program to evaluate the effect of absorption to reverberation time. The total
absorber’s absorption coefficients with its covering area affect the reverberation time
as stated in the Sabine equation. Various absorbers with its respective absorption
coefficient, manufacturer and product specifications are listed as it has been chosen.
38
In this project, three types of absorbers are selected, there are mineral wool
ceiling, gypsum board ceiling and gypsum tile ceiling. As the ceiling represents a
large portion of the room’s surface area, it has major impact to the acoustic
performance. For example, the absorption coefficient of Rigips’ gypsum tile ceilings
is shows in Figure 3.6.
Figure 3.6
Gypsum tile ceilings manufactured by Rigips
3.1.5 Graph of CARA Simulation
Figure 3.7 shows an example of the graph of reverberation time over
frequency. The graph of reverberation time is plotted over frequency where
frequency is split up to six octave band centre frequencies between 125 Hz to 4000
Hz. The red line indicates the reverberation time without absorbers, furniture and
people. The blue line indicates the reverberation time with absorbers and furniture
39
but without people. While the dark blue line shows the reverberation time when
absorbers, furniture and people are considered. This dark blue line will become the
reference curve of the result.
The tolerance limit for the reverberation times (gray lines) is determined
according to the volume of the room and its intended use based on the DIN 18041
standard. For this project, this tolerance limit is not necessary to be met, since they
have other factors influence reverberation time. All the designs are attempt to meet
the requirement as stated on chapter of literature review and analysis. The average
reverberation time is obtained over the six-octave frequency band.
Figure 3.7
Example of the graph of reverberation time over frequency
40
Figure 3.8 shows an example of the graph of noise reduction and average
absorption coefficient over frequency. The yellow line indicates the noise reduction
that can be reduced through the selected absorbers and furniture over the frequency
band. The red line indicates the average absorption coefficient without absorbers,
furniture and people, that means it only considered the surface’s treatment of an
unoccupied room. The blue line indicates the average absorption coefficient with
absorbers and furniture but without people occupied inside the room. The dark blue
line shows the average absorption coefficient with absorbers, furniture and people all
occupied together inside the room. This dark blue line will become the reference
curve of the result. Similarly, the overall noise reduction and average absorption
coefficient are taken over the six-octave frequency band.
Figure 3.8
Example of the graph of noise reduction and
average absorption coefficient over frequency
41
3.2
Room Design by CARA Program
In order to evaluate the effects of room and audio parameter to the speech
intelligibility, several simulation models have been planed and designed. In these
simulation models, three types of ceiling’s absorbers are selected as to assess the
effectiveness of absorber’s covering surface area. Either all ceiling areas or half of it,
and without absorbers, the reverberation time is differing in every case. Similarly to
the floor coverings by carpet or tiles, they also contribute to the sound absorption
coefficient. Furthermore, three room models having different dimension, absorbers
and furnishes are analyzed as to interpret the reverberation to the score of speech
intelligibility.
3.2.1 Absorbers Type
Three types of absorbers where they are mounting on the ceiling are selected
for simulation, as shown in Table 3.2. This three absorbers will mount on the ceiling
according to all ceiling area has been covered (100%), half of the ceiling area has
been covered (50%) and no absorber mounted on the ceiling. The purpose of this
design is to simulate the sound absorption to the reverberation time, as the ceiling
represent a large portion of room surface area, it has major impact to the acoustic
performance. The total absorption coefficient is determined by the absorbers’
absorption coefficient and their respective covered surface area.
42
Table 3.2
Absorber
Three types of absorbers use in simulation
Description
Type
Mineral
Manufacturer : OWA
wool
Model : Plain
ceilings
Material : mineral wool
Dimensions : 300-625 mm x
600-2500 mm x 15(20) mm
Mounting : absorption
coefficient valid for 200 mm
depth of suspension
Gypsum
Manufacturer : Rigips
board
Model : 8/18 Q (200 mm)
ceilings
Material : gypsum board
with acoustic tissue
Dimensions : maximum
2988 mm x 1188 mm x 12.5
mm
Mounting : absorption
coefficient valid for 200 mm
depth of suspension
Absorption Coefficient αp
43
Gypsum
Manufacturer : Rigips
tile
Model : Point 14 (45 mm)
ceilings
Material : gypsum board
with acoustic tissue
Dimensions : 600 (625) mm
x 600 (625) mm x 12.5 mm
Mounting : absorption
coefficient valid for 45 mm
depth of suspension
3.2.2 Floor Coverings
There are two types of floor coverings, which are either carpet or tiles. It is
assumed that carpet or tiles have covered all the floor areas. Similarly to the ceiling
portion, floor area also contribute to a large portion of room surface, it will affect the
44
absorption coefficient and the reverberation time. Furthermore, carpet and tiles floor
have different ability to the noise reduction.
3.2.3 Room Size and Furnishes
In order to simulate the effect of room size to the reverberation time, there are
three different room layouts with their furniture and people occupied inside are
carried out. Table 3.3 shows the room dimension and the number of places in the
meeting room. With a larger meeting room, the number of places that can be
occupied inside also increased. The number of places is assumed as each place
consumed 10% of room volume. An unoccupied room has lower absorption
coefficient value than room occupied by people. Since people also act as an sound
absorption material.
Table 3.3
Room
Room dimension and number of places occupied of simulation model
Length, Width, Height,
Surface
Volume,
Number of places
no.
l (m)
b (m)
h (m)
Area, A (m2)
V (m3)
meeting room
1
4
4
3
80
48
5
2
8
6
3
180
144
15
3
12
8
3
312
288
30
45
Figure 3.9
Examples of the layout of the places in meeting room
3.2.4 Reverberation Time, Noise Reduction and Average Absorption
Coefficient
According to Table 2.1 for general meeting room, the optimum reverberation
time is 0.7s and the allowable maximum ambient noise level is 35dBA. However,
every room’s layout and its furnishes occupied inside will developed different
reverberation time according to different situation. The ideal reverberation time is
taken from the tolerance limits according to DIN 18041 standard as reference. Mean
value of reverberation time, noise reduction level and average absorption coefficient
is calculated.
3.2.5 Simulation Models of Room Design
By designing three types of room size, with each room having one kind of
absorber covered three different ceiling surface’s area, and also that room is
differentiate by carpet and tiles floor coverings. There are total eighteen design
stages constructed for simulation. The first digit on design stage means the room
number. There are three room models for the simulation. The second digit of design
46
stage indicates number 1 for carpet floor coverings and number 2 for tiles floor
coverings. Whereas the third digit on the design stage, 1 indicates ceiling without
absorber, 2 indicates half of ceiling area covered by absorber, and 3 indicates all the
ceiling areas covered by absorber.
Table 3.4
Room
no.
1
2
3
Absorber’s Type
Mineral Wool
Ceilings
Gypsum Board
Ceilings
Gypsum Tile
Ceilings
Simulation models of room design
Absorber’s Surface Area
%
m2
0
0
8
50
16
100
0
0
8
50
16
100
0
24
48
0
24
48
0
50
100
0
50
100
0
48
96
0
48
96
0
50
100
0
50
100
Floor
Coverings
Carpet
Tiles
Carpet
Tiles
Carpet
Tiles
Design
Stage
1.1.1
1.1.2
1.1.3
1.2.1
1.2.2
1.2.3
2.1.1
2.1.2
2.1.3
2.2.1
2.2.2
2.2.3
3.1.1
3.1.2
3.1.3
3.2.1
3.2.2
3.2.3
CHAPTER 4
RESULTS
By input all the acoustical data, CARA program shows different graphs
simulate the effect of room size, absorbers and other acoustic parameters. The graphs
show the average absorption coefficient of all the acoustical materials, the noise
reduction level that can be achieved in each design stage, and the reverberation time
of simulated result. Table 4.1 to Table 4.18 and Figure 4.1 to Figure 4.18 show the
results of each design stage. In total, there have 18 stages are being analyzed. Ideal
reverberation times (RT Ideal) as shown on the tables are taken from the
recommended tolerance limits. Mean values of simulated reverberation times (RT
Simulated), noise reduction level and average absorption coefficient over the
frequency range are taking into accounts for diagnosis.
48
Table 4.1
Mineral wool ceilings without absorber, carpet floor
Room no. 1
Length, l
=4m
Width, b
=4m
Height, h
= 3m
Surface Area, A = 80 m2
Volume, V
= 48 m3
Number of places = 5
Frequency
(Hz)
125
250
500
1000
2000
4000
Figure 4.1
Floor coverings = carpet
Absorber’s type = mineral wool ceilings
Absorber’s surface area = 0 m2
Design 1.1.1
RT Ideal (s)
RT Simulated
(s)
0.25 –0.45
0.30 – 0.45
0.30 – 0.45
0.30 – 0.45
0.30 – 0.45
0.25 – 0.45
Mean Value
0.89
0.74
0.57
0.46
0.40
0.37
0.572
Noise
Reduction, ∆L
(dB)
1.6
1.5
1.5
1.6
1.5
1.5
1.53
Average
Absorption
Coefficient, ά
0.10
0.14
0.17
0.20
0.24
0.25
0.183
Simulation graph of reverberation time, noise reduction and average
absorption coefficient of Design 1.1.1
49
Table 4.2
Mineral wool ceilings with 50% absorber covered, carpet floor
Room no. 1
Length, l
=4m
Width, b
=4m
Height, h
= 3m
Surface Area, A = 80 m2
Volume, V
= 48 m3
Number of places = 5
Frequency
(Hz)
125
250
500
1000
2000
4000
Figure 4.2
Floor coverings = carpet
Absorber’s type = mineral wool ceilings
Absorber’s surface area = 8 m2 (50%)
Design 1.1.2
RT Ideal (s)
RT Simulated
(s)
0.25 –0.45
0.30 – 0.45
0.30 – 0.45
0.30 – 0.45
0.30 – 0.45
0.25 – 0.45
Mean Value
0.75
0.66
0.55
0.45
0.39
0.36
0.527
Noise
Reduction, ∆L
(dB)
2.4
2.0
1.8
1.9
1.9
1.8
1.97
Average
Absorption
Coefficient, ά
0.12
0.15
0.17
0.21
0.25
0.26
0.193
Simulation graph of reverberation time, noise reduction and average
absorption coefficient of Design 1.1.2
50
Table 4.3
Mineral wool ceilings with 100% absorber covered, carpet floor
Room no. 1
Length, l
=4m
Width, b
=4m
Height, h
= 3m
Surface Area, A = 80 m2
Volume, V
= 48 m3
Number of places = 5
Frequency
(Hz)
125
250
500
1000
2000
4000
Figure 4.3
Floor coverings = carpet
Absorber’s type = mineral wool ceilings
Absorber’s surface area = 16 m2 (100%)
Design 1.1.3
RT Ideal (s)
RT Simulated
(s)
0.25 –0.45
0.30 – 0.45
0.30 – 0.45
0.30 – 0.45
0.30 – 0.45
0.25 – 0.45
Mean Value
0.65
0.61
0.52
0.41
0.37
0.35
0.485
Noise
Reduction, ∆L
(dB)
3.0
2.4
2.0
2.1
2.0
2.0
2.25
Average
Absorption
Coefficient, ά
0.14
0.15
0.18
0.24
0.25
0.27
0.205
Simulation graph of reverberation time, noise reduction and average
absorption coefficient of Design 1.1.3
51
Table 4.4
Mineral wool ceilings without absorber, tiled floor
Room no. 1
Length, l
=4m
Width, b
=4m
Height, h
= 3m
Surface Area, A = 80 m2
Volume, V
= 48 m3
Number of places = 5
Frequency
(Hz)
125
250
500
1000
2000
4000
Figure 4.4
Floor coverings = tiles
Absorber’s type = mineral wool ceilings
Absorber’s surface area = 0 m2
Design 1.2.1
RT Ideal (s)
RT Simulated
(s)
0.25 –0.45
0.30 – 0.45
0.30 – 0.45
0.30 – 0.45
0.30 – 0.45
0.25 – 0.45
Mean Value
0.91
0.83
0.67
0.60
0.55
0.50
0.677
Noise
Reduction, ∆L
(dB)
2.0
2.1
2.3
2.5
2.6
2.5
2.33
Average
Absorption
Coefficient, ά
0.10
0.11
0.15
0.16
0.17
0.19
0.147
Simulation graph of reverberation time, noise reduction and average
absorption coefficient of Design 1.2.1
52
Table 4.5
Mineral wool ceilings with 50% absorber covered, tiled floor
Room no. 1
Length, l
=4m
Width, b
=4m
Height, h
= 3m
Surface Area, A = 80 m2
Volume, V
= 48 m3
Number of places = 5
Frequency
(Hz)
125
250
500
1000
2000
4000
Figure 4.5
Floor coverings = tiles
Absorber’s type = mineral wool ceilings
Absorber’s surface area = 8 m2 (50%)
Design 1.2.2
RT Ideal (s)
RT Simulated
(s)
0.25 –0.45
0.30 – 0.45
0.30 – 0.45
0.30 – 0.45
0.30 – 0.45
0.25 – 0.45
Mean Value
0.79
0.76
0.65
0.55
0.52
0.48
0.625
Noise
Reduction, ∆L
(dB)
2.6
2.5
2.4
2.7
2.9
2.6
2.62
Average
Absorption
Coefficient, ά
0.12
0.12
0.15
0.17
0.18
0.19
0.155
Simulation graph of reverberation time, noise reduction and average
absorption coefficient of Design 1.2.2
53
Table 4.6
Mineral wool ceilings with 100% absorber covered, tiled floor
Room no. 1
Length, l
=4m
Width, b
=4m
Height, h
= 3m
Surface Area, A = 80 m2
Volume, V
= 48 m3
Number of places = 5
Frequency
(Hz)
125
250
500
1000
2000
4000
Figure 4.6
Floor coverings = tiles
Absorber’s type = mineral wool ceilings
Absorber’s surface area = 16 m2 (100%)
Design 1.2.3
RT Ideal (s)
RT Simulated
(s)
0.25 –0.45
0.30 – 0.45
0.30 – 0.45
0.30 – 0.45
0.30 – 0.45
0.25 – 0.45
Mean Value
0.68
0.70
0.63
0.53
0.49
0.46
0.582
Noise
Reduction, ∆L
(dB)
3.3
2.7
2.5
3.0
3.1
3.0
2.93
Average
Absorption
Coefficient, ά
0.14
0.14
0.16
0.18
0.20
0.21
0.172
Simulation graph of reverberation time, noise reduction and average
absorption coefficient of Design 1.2.3
54
Table 4.7
Gypsum board ceilings without absorber, carpet floor
Room no. 2
Length, l
=8m
Width, b
=6m
Height, h
= 3m
Surface Area, A = 180 m2
Volume, V
= 144 m3
Number of places = 15
Frequency
(Hz)
125
250
500
1000
2000
4000
Figure 4.7
Floor coverings = carpet
Absorber’s type = gypsum board ceilings
Absorber’s surface area = 0 m2
Design 2.1.1
RT Ideal (s)
RT Simulated
(s)
0.35 – 0.62
0.41 – 0.62
0.41 – 0.62
0.41 – 0.62
0.41 – 0.62
0.35 – 0.62
Mean Value
1.10
0.88
0.69
0.55
0.47
0.44
0.688
Noise
Reduction, ∆L
(dB)
2.3
2.0
2.0
2.0
1.9
1.8
2.00
Average
Absorption
Coefficient, ά
0.12
0.15
0.19
0.24
0.27
0.30
0.212
Simulation graph of reverberation time, noise reduction and average
absorption coefficient of Design 2.1.1
55
Table 4.8
Gypsum board ceilings with 50% absorber covered, carpet floor
Room no. 2
Length, l
=8m
Width, b
=6m
Height, h
= 3m
Surface Area, A = 180 m2
Volume, V
= 144 m3
Number of places = 15
Frequency
(Hz)
125
250
500
1000
2000
4000
Figure 4.8
Floor coverings = carpet
Absorber’s type = gypsum board ceilings
Absorber’s surface area = 24 m2 (50%)
Design 2.1.2
RT Ideal (s)
RT Simulated
(s)
0.35 – 0.62
0.41 – 0.62
0.41 – 0.62
0.41 – 0.62
0.41 – 0.62
0.35 – 0.62
Mean Value
0.80
0.55
0.44
0.43
0.37
0.35
0.490
Noise
Reduction, ∆L
(dB)
3.6
4.0
3.9
3.0
2.9
2.5
3.32
Average
Absorption
Coefficient, ά
0.15
0.24
0.29
0.30
0.34
0.35
0.278
Simulation graph of reverberation time, noise reduction and average
absorption coefficient of Design 2.1.2
56
Table 4.9
Gypsum board ceilings with 100% absorber covered, carpet floor
Room no. 2
Length, l
=8m
Width, b
=6m
Height, h
= 3m
Surface Area, A = 180 m2
Volume, V
= 144 m3
Number of places = 15
Frequency
(Hz)
125
250
500
1000
2000
4000
Figure 4.9
Floor coverings = carpet
Absorber’s type = gypsum board ceilings
Absorber’s surface area = 48 m2 (100%)
Design 2.1.3
RT Ideal (s)
RT Simulated
(s)
0.35 – 0.62
0.41 – 0.62
0.41 – 0.62
0.41 – 0.62
0.41 – 0.62
0.35 – 0.62
Mean Value
0.62
0.41
0.32
0.35
0.31
0.30
0.385
Noise
Reduction, ∆L
(dB)
4.7
5.3
5.2
3.9
3.7
3.3
4.35
Average
Absorption
Coefficient, ά
0.20
0.30
0.39
0.36
0.40
0.40
0.342
Simulation graph of reverberation time, noise reduction and average
absorption coefficient of Design 2.1.3
57
Table 4.10
Gypsum board ceilings without absorber, tiled floor
Room no. 2
Length, l
=8m
Width, b
=6m
Height, h
= 3m
Surface Area, A = 180 m2
Volume, V
= 144 m3
Number of places = 15
Frequency
(Hz)
125
250
500
1000
2000
4000
Figure 4.10
Floor coverings = tiles
Absorber’s type = gypsum board ceilings
Absorber’s surface area = 0 m2
Design 2.2.1
RT Ideal (s)
RT Simulated
(s)
0.35 – 0.62
0.41 – 0.62
0.41 – 0.62
0.41 – 0.62
0.41 – 0.62
0.35 – 0.62
Mean Value
1.19
1.04
0.87
0.75
0.70
0.62
0.862
Noise
Reduction, ∆L
(dB)
2.5
2.5
2.6
3.0
3.1
2.9
2.77
Average
Absorption
Coefficient, ά
0.10
0.12
0.15
0.17
0.18
0.20
0.153
Simulation graph of reverberation time, noise reduction and average
absorption coefficient of Design 2.2.1
58
Table 4.11
Gypsum board ceilings with 50% absorber covered, tiled floor
Room no. 2
Length, l
=8m
Width, b
=6m
Height, h
= 3m
Surface Area, A = 180 m2
Volume, V
= 144 m3
Number of places = 15
Frequency
(Hz)
125
250
500
1000
2000
4000
Figure 4.11
Floor coverings = tiles
Absorber’s type = gypsum board ceilings
Absorber’s surface area = 24 m2 (50%)
Design 2.2.2
RT Ideal (s)
RT Simulated
(s)
0.35 – 0.62
0.41 – 0.62
0.41 – 0.62
0.41 – 0.62
0.41 – 0.62
0.35 – 0.62
Mean Value
0.84
0.62
0.51
0.55
0.51
0.49
0.587
Noise
Reduction, ∆L
(dB)
4.0
4.8
4.9
4.5
4.5
4.0
4.45
Average
Absorption
Coefficient, ά
0.15
0.20
0.25
0.24
0.25
0.26
0.225
Simulation graph of reverberation time, noise reduction and average
absorption coefficient of Design 2.2.2
59
Table 4.12
Gypsum board ceilings with 100% absorber covered, tiled floor
Room no. 2
Length, l
=8m
Width, b
=6m
Height, h
= 3m
Surface Area, A = 180 m2
Volume, V
= 144 m3
Number of places = 15
Frequency
(Hz)
125
250
500
1000
2000
4000
Figure 4.12
Floor coverings = tiles
Absorber’s type = gypsum board ceilings
Absorber’s surface area = 48 m2 (100%)
Design 2.2.3
RT Ideal (s)
RT Simulated
(s)
0.35 – 0.62
0.41 – 0.62
0.41 – 0.62
0.41 – 0.62
0.41 – 0.62
0.35 – 0.62
Mean Value
0.65
0.45
0.36
0.42
0.40
0.40
0.447
Noise
Reduction, ∆L
(dB)
5.1
6.3
6.4
5.4
5.5
4.8
5.58
Average
Absorption
Coefficient, ά
0.20
0.29
0.35
0.30
0.33
0.33
0.300
Simulation graph of reverberation time, noise reduction and average
absorption coefficient of Design 2.2.3
60
Table 4.13
Gypsum tile ceilings without absorber, carpet floor
Room no. 3
Length, l
= 12 m
Width, b
=8m
Height, h
= 3m
Surface Area, A = 312 m2
Volume, V
= 288 m3
Number of places = 30
Frequency
(Hz)
125
250
500
1000
2000
4000
Figure 4.13
Floor coverings = carpet
Absorber’s type = gypsum tile ceilings
Absorber’s surface area = 0 m2
Design 3.1.1
RT Ideal (s)
RT Simulated
(s)
0.40 – 0.74
0.49 – 0.74
0.49 – 0.74
0.49 – 0.74
0.49 – 0.74
0.40 – 0.74
Mean Value
1.22
0.95
0.75
0.59
0.51
0.45
0.745
Noise
Reduction, ∆L
(dB)
2.6
2.3
2.2
2.2
2.0
1.9
2.20
Average
Absorption
Coefficient, ά
0.12
0.15
0.20
0.25
0.29
0.32
0.222
Simulation graph of reverberation time, noise reduction and average
absorption coefficient of Design 3.1.1
61
Table 4.14
Gypsum tile ceilings with 50% absorber covered, carpet floor
Room no. 3
Length, l
= 12 m
Width, b
=8m
Height, h
= 3m
Surface Area, A = 312 m2
Volume, V
= 288 m3
Number of places = 30
Frequency
(Hz)
125
250
500
1000
2000
4000
Figure 4.14
Floor coverings = carpet
Absorber’s type = gypsum tile ceilings
Absorber’s surface area = 48 m2 (50%)
Design 3.1.2
RT Ideal (s)
RT Simulated
(s)
0.40 – 0.74
0.49 – 0.74
0.49 – 0.74
0.49 – 0.74
0.49 – 0.74
0.40 – 0.74
Mean Value
1.14
0.75
0.50
0.40
0.44
0.44
0.612
Noise
Reduction, ∆L
(dB)
3.0
3.1
3.7
3.6
2.6
2.1
3.02
Average
Absorption
Coefficient, ά
0.12
0.19
0.30
0.36
0.35
0.35
0.278
Simulation graph of reverberation time, noise reduction and average
absorption coefficient of Design 3.1.2
62
Table 4.15
Gypsum tile ceilings with 100% absorber covered, carpet floor
Room no. 3
Length, l
= 12 m
Width, b
=8m
Height, h
= 3m
Surface Area, A = 312 m2
Volume, V
= 288 m3
Number of places = 30
Frequency
(Hz)
125
250
500
1000
2000
4000
Figure 4.15
Floor coverings = carpet
Absorber’s type = gypsum tile ceilings
Absorber’s surface area = 96 m2 (100%)
Design 3.1.3
RT Ideal (s)
RT Simulated
(s)
0.40 – 0.74
0.49 – 0.74
0.49 – 0.74
0.49 – 0.74
0.49 – 0.74
0.40 – 0.74
Mean Value
1.06
0.64
0.37
0.31
0.37
0.40
0.525
Noise
Reduction, ∆L
(dB)
3.2
3.9
5.1
4.8
3.3
2.4
3.78
Average
Absorption
Coefficient, ά
0.14
0.24
0.40
0.47
0.40
0.36
0.335
Simulation graph of reverberation time, noise reduction and average
absorption coefficient of Design 3.1.3
63
Table 4.16
Gypsum tile ceilings without absorber, tiled floor
Room no. 3
Length, l
= 12 m
Width, b
=8m
Height, h
= 3m
Surface Area, A = 312 m2
Volume, V
= 288 m3
Number of places = 30
Frequency
(Hz)
125
250
500
1000
2000
4000
Figure 4.16
Floor coverings = tiles
Absorber’s type = gypsum tile ceilings
Absorber’s surface area = 0 m2
Design 3.2.1
RT Ideal (s)
RT Simulated
(s)
0.40 – 0.74
0.49 – 0.74
0.49 – 0.74
0.49 – 0.74
0.49 – 0.74
0.40 – 0.74
Mean Value
1.31
1.16
0.96
0.83
0.76
0.68
0.950
Noise
Reduction, ∆L
(dB)
3.0
2.9
3.0
3.3
3.4
3.1
3.12
Average
Absorption
Coefficient, ά
0.11
0.13
0.15
0.18
0.20
0.21
0.163
Simulation graph of reverberation time, noise reduction and average
absorption coefficient of Design 3.2.1
64
Table 4.17
Gypsum tile ceilings with 50% absorber covered, tiled floor
Room no. 3
Length, l
= 12 m
Width, b
=8m
Height, h
= 3m
Surface Area, A = 312 m2
Volume, V
= 288 m3
Number of places = 30
Frequency
(Hz)
125
250
500
1000
2000
4000
Figure 4.17
Floor coverings = tiles
Absorber’s type = gypsum tile ceilings
Absorber’s surface area = 48 m2 (50%)
Design 3.2.2
RT Ideal (s)
RT Simulated
(s)
0.40 – 0.74
0.49 – 0.74
0.49 – 0.74
0.49 – 0.74
0.49 – 0.74
0.40 – 0.74
Mean Value
1.22
0.88
0.57
0.51
0.59
0.63
0.733
Noise
Reduction, ∆L
(dB)
3.3
3.9
5.2
5.5
4.5
3.5
4.32
Average
Absorption
Coefficient, ά
0.12
0.17
0.25
0.29
0.25
0.24
0.220
Simulation graph of reverberation time, noise reduction and average
absorption coefficient of Design 3.2.2
65
Table 4.18
Gypsum tile ceilings with 100% absorber covered, tiled floor
Room no. 3
Length, l
= 12 m
Width, b
=8m
Height, h
= 3m
Surface Area, A = 312 m2
Volume, V
= 288 m3
Number of places = 30
Frequency
(Hz)
125
250
500
1000
2000
4000
Figure 4.18
Floor coverings = tiles
Absorber’s type = gypsum tile ceilings
Absorber’s surface area = 96 m2 (100%)
Design 3.2.3
RT Ideal (s)
RT Simulated
(s)
0.40 – 0.74
0.49 – 0.74
0.49 – 0.74
0.49 – 0.74
0.49 – 0.74
0.40 – 0.74
Mean Value
1.14
0.71
0.42
0.36
0.49
0.56
0.613
Noise
Reduction, ∆L
(dB)
3.6
4.8
6.7
7.0
5.4
4.0
5.25
Average
Absorption
Coefficient, ά
0.13
0.21
0.35
0.40
0.30
0.25
0.273
Simulation graph of reverberation time, noise reduction and average
absorption coefficient of Design 3.2.3
CHAPTER 5
ANALYSIS OF RESULTS
Analyses are done on finding the average absorption coefficient of all the
acoustical materials, the noise reduction level that can be achieved in each design
stage, and the reverberation time of simulated result. Table 4.1 to Table 4.18 and
Figure 4.1 to Figure 4.18 show the results of each design stage. The first digit on
design stage means the room number. There are three room models for the
simulation. The second digit of design stage indicates number 1 for carpet floor
coverings and number 2 for tiles floor coverings. Whereas the third digit on the
design stage, 1 indicates ceiling without absorber, 2 indicates half of ceiling area
covered by absorber, and 3 indicates all the ceiling areas covered by absorber. In
total, there have 18 stages are being analyzed. Notes that the reverberation times are
compared with the ideal range recommended by DIN 18041 standard, but in this
project, the reverberation time is not necessary to be met with the ideal value. Mean
values over the frequency range are taking into accounts for diagnosis.
There are two summaries from analysis. Summary 1 compares the effect of
sound absorption coefficient to the reverberation time and noise reduction level. The
most significant acoustical property of a room is its reverberation time.
Reverberation time is determined by the size and sound absorption coefficient of the
room. Summary 2 shows the relationship between reverberation time and the
corresponding %ALCons, where the degree of speech intelligibility is evaluated. In
this project, %ALCons of less than 15% is considered acceptable.
67
5.1
Summary 1
The mean values of reverberation time, noise reduction level and average
absorption coefficient are sorted and rearranged as shown on Table 5.1. The ideal
reverberation time is taken as recommended by DIN 18041 standard. Three room
models with different size are as shown.
Table 5.1
Sorted results of all design stages
0.4
0.4
0.4
0.4
0.4
0.4
RT
Simulated,
RTM (s)
0.6
0.5
0.5
0.7
0.6
0.6
Noise
Reduction,
∆LM (dB)
1.53
1.97
2.25
2.33
2.62
2.93
Average
Absorption
Coefficient, άM
0.20
0.20
0.20
0.15
0.15
0.15
144
144
144
144
144
144
0.5
0.5
0.5
0.5
0.5
0.5
0.7
0.5
0.4
0.9
0.6
0.4
2.00
3.32
4.35
2.77
4.45
5.58
0.20
0.30
0.35
0.15
0.25
0.30
288
288
288
288
288
288
0.6
0.6
0.6
0.6
0.6
0.6
0.7
0.6
0.5
1.0
0.7
0.6
2.20
3.02
3.78
3.12
4.32
5.25
0.20
0.30
0.35
0.15
0.20
0.25
Design
Stage
Volume,
V (m3)
RT Ideals,
RTI (s)
1.1.1
1.1.2
1.1.3
1.2.1
1.2.2
1.2.3
48
48
48
48
48
48
2.1.1
2.1.2
2.1.3
2.2.1
2.2.2
2.2.3
3.1.1
3.1.2
3.1.3
3.2.1
3.2.2
3.2.3
Refer to Figure 5.1, Figure 5.2 and Figure 5.3, above part is the chart plotting
the simulated reverberation time and corresponding noise reduction level in each
design stage. The red line indicates the ideal reverberation time recommended by the
DIN 18041 standard as shown in the simulation. Below part is the graph showing the
average absorption coefficient and corresponding noise reduction level in both the
carpet and tiles floor coverings.
68
From these three figures, in all cases, the room without absorber mounting on
the ceiling having the longer reverberation time as compared to room with absorber
covered all the ceiling’s area. The longer reverberation time is corresponding to
lower sound absorption coefficient. This is due to the absorber on the ceiling area
represents a large portion of average absorption coefficient. When the absorber’s
area is increasing, the average absorption coefficient also increased. When more
sound has been absorbed, the reflected sound has decreased. Rooms with more
absorptive surfaces, and hence higher sound absorption coefficient value, have
shorter reverberation time.
Figure 5.1
Room no.1 with its acoustical properties
69
According to these three figures, it is clearly shown that, increasing the
absorption within the room will generally increasing the noise reduction levels.
Within this project analyses, floor covered by carpet has higher absorption
coefficient value as compared to tiled floor. Porous materials such as carpets, convert
acoustic energy into heat, and reduce the reflected sound. But tiled floor can reduce
more noise as compared to carpet. One can decide to choose tiled floor if ambient
noise has to be reduced without changing the ceiling materials. In other words, by
select tiled floor, and various choices of ceiling absorbers, it can generally reduce the
noise level and at the same time shorten the reverberation time.
Figure 5.2
Room no.2 with its acoustical properties
70
Figure 5.3
5.2
Room no.3 with its acoustical properties
Summary 2
In order to evaluate the relationship between the reverberation times to the
degree of speech intelligibility, value of %ALCons in each case has been calculated.
Refer to Table 5.2, reverberation time from CARA simulation result has been
compared to the reverberation time calculated from Sabine Equation (2.1). The
71
%ALCons value has calculated using Equation (2.4) by taken the reverberation time
from simulation. The results are plotted in the Figure 5.4.
Table 5.2
Reverberation time and value of %ALCons in each design stages
1.1.1
1.1.2
1.1.3
1.2.1
1.2.2
1.2.3
48
48
48
48
48
48
Surface
Area,
A (m2)
80
80
80
80
80
80
2.1.1
2.1.2
2.1.3
2.2.1
2.2.2
2.2.3
144
144
144
144
144
144
180
180
180
180
180
180
0.20
0.30
0.35
0.15
0.25
0.30
0.7
0.5
0.4
0.9
0.6
0.4
0.6
0.5
0.4
0.8
0.6
0.4
13
11
10
15
12
11
3.1.1
3.1.2
3.1.3
3.2.1
3.2.2
3.2.3
288
288
288
288
288
288
312
312
312
312
312
312
0.20
0.30
0.35
0.15
0.20
0.25
0.7
0.6
0.5
1.0
0.7
0.6
0.7
0.5
0.4
0.9
0.7
0.5
14
13
12
16
14
13
Design Volume,
Stage
V (m3)
Average
Absorption
Coefficient, άM
0.20
0.20
0.20
0.15
0.15
0.15
RT
Simulated,
RTM (s)
0.6
0.5
0.5
0.7
0.6
0.6
RT
Sabine,
RTS (s)
0.5
0.5
0.5
0.7
0.6
0.6
%ALCons
(%)
12
12
11
13
13
12
Refer to Figure 5.4, the Sabine’s reverberation times are a bit lower than
reverberation times taken from CARA simulation. It is due to in the Sabine’s
reverberation time calculation, mean value of average absorption coefficient and total
room surfaces are considered. For more precise result, the total of absorption
coefficient of different surfaces of the room and their respective area should be
considered. However, these results reflected the efficiency and effectiveness of
CARA program to simulate the reverberation time base on its materials database.
For a maximum degree of speech intelligibility, %ALCons of less than 15%
would be acceptable for speech communication, as long as there is a minimum of
25dB of signal to noise ratio, and the spoken information is being delivered at a pace
suitable for the acoustic environment. In this project, a %ALCons of less than 15%
72
and achieve reverberation time of less than 0.7s, has been evaluated as good speech
intelligibility for meeting room.
The red line in Figure 5.4 shows the 15%ALCons. In ⅔ of all design stages,
they are lower than 13%. They are ⅓ of all design stages having %ALCons of less
than 12%. Design 2.1.3 having the lowest value, that is about 10%. From the graph,
it can be seen that smaller room having better speech intelligibility, but a well
acoustical design room have the best speech intelligibility.
Figure 5.4
Reverberation time and value of %ALCons in each design stages
CHAPTER 6
CONCLUSION AND RECOMMENDATION
In most room acoustical design cases, the dimensions of the room are fixed
due to the overall layout structure after the construction of the building. The
conclusions can be derived from this project, are by proper selection of acoustical
materials for the surfaces of ceiling, wall and floor in these meeting room models
provide optimum acoustical properties and meet the design requirements.
In conclusion, through out all the design stages, design 2.1.3 having the
lowest value of %ALCons which is about 10%. The noise reduction level is about
4.3 dB. It has the shortest reverberation time among all stages, which is about 0.38s.
In the other hand, if the background noise has to be considered as the main
issue in acoustical design, then design 2.2.3 having the most noise reduction levels,
which is about 5.5 dB. The reverberation time is about 0.44s and a value of
11%ALCons, which is the second best.
The parameters that have been set in both the design are as:
i. room volume is 144 m3 with total surface area 180 m2
ii. there are 15 places in that meeting room
iii. the gypsum board absorber is covering all the ceiling areas
The different between design 2.1.3 and 2.2.3 is that in design 2.1.3 floor is
covered by carpet while in design 2.2.3 floor is covered by tiles. The reverberation
time curve in design 2.2.3 is much more smooth over the whole frequency range.
74
Therefore, it can be concluded that this design stage could be considered as achieve
the optimum speech intelligibility.
6.1
Recommendations for Future Work
Follows are proposed recommendations for future work.
i. Speech privacy can be measured in terms of the intelligibility of speech,
that is, the percentage of words that can be understood. When a very low
percentage of speech is intelligible, we have conditions with relatively
high speech privacy. However, in some cases even zero intelligibility is
not good enough. For future work, it should strive to attain conditions
where speech is not even audible outside the room.
ii. Whether speech from an adjacent meeting room is intelligible or even
audible depends on three factors: (a) the sound isolation characteristics of
meeting room boundaries, (b) the levels of speech sounds in the meeting
room, and (c) the levels of ambient noise at listener positions outside the
meeting room. For future work, further study could be done on these three
factors, so that when the transmitted speech sounds are loud enough
relative to the ambient noise at some listener position, the speech will be
audible and sometimes also intelligible.
iii. Speech security can be defined as a condition where transmitted speech is
likely to be either intelligible or audible for no more than some specific
very small percentage of the time. For future work, high degree of speech
security should be achieved in meeting rooms and offices. Meeting rooms
and offices are said to be speech secure when it is very difficult for an
eavesdropper outside the room to understand speech from inside the
room.
75
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