Slough Library - Multi Use Hall as an Auditorium
Andrew Rickard
MACH Acoustics
81-83 Stokes Croft, Bristol, BS1 3RD
[email protected] mobile 0779 440 6553
Contents
1.0 Introduction
1.1 Report Format
1.2 Design Summary
1.3 Flexible Acoustics/Reverberation Time
1.4 Design Matrix
2.0 Acoustic Modelling
2.1 Acoustic Modelling – Results
Layout 1 – Dividing the Auditorium
Layout 2 – Council Space
Layout 3 – Speech, Theatre, Amplified Music
Layout 4 – Live Music
Layout 5 – Orchestral Music
2.2 Rear Wall Diffusion
Appendix A – Detailed Specification
Appendix B – Drawings of the Auditorium
Andrew Rickard
MACH Acoustics
81-83 Stokes Croft, Bristol, BS1 3RD
[email protected] mobile 0779 440 6553
MACH ACOUSTICS
1.0 Introduction
MACH ACOUSTICS
1.2 Design Summary
The auditorium within the Slough Library Development is to be designed as a flexible
space providing a performing arts facility for a range of activities including drama,
conferencing, amplified music events, film, orchestral music, dance as well as the facility
to a council chamber. This range of activities requires the acoustics of this space to be
changed depending upon the type of production taking place within the hall.
The various Hall Layouts described within this report will show that the majority of the
likely uses for the Hall will meet the specified design criteria such that they will function
correctly.
MACH Acoustics report ‘100325 Slough Library - Lecture Theatre – Room Acoustics’
dated 25th March 2010, provides general advice relating to the acoustic requirements of
multi propose hall design. This report intends to provide more specific design advice to
enable cost and detailed designs to be understood and adopted as required.
Currently the layout option for the use of Council Chambers provides a reverberation
time that is a little high than what is should be. However, as the layout of the room is not
known and how the space is likely to be used within Council proceedings, it is seen that
further design work will be required for this particular use.
1.1 Report Format
After summarising the design, this report starts by discussing reverberation and how it is
defined and most importantly what are its requirements with regard multiple uses.
Section 2 introduces the acoustic modelling undertaken for this hall. It discusses what
results are most important and summaries the design criteria for each room use. From
this point report starts to examine the findings for the various layout options and is
presented in the same order as in the Design Matrix.
Each Layout option will discuss the ‘Room Acoustics’ in relation to use and the findings of
the acoustic model. Where applicable design details will follow each Layout option, for
example ‘Ceiling Reflectors’, ‘Proposed Treatments’ or ‘Rear Wall Diffusion’.
The report contains two Appendices; Appendix A contains the detailed specification,
providing the absorption coefficients used and their areas, as well as diffusion and ceiling
reflector dimensions.
Appendix B – Contains the architects drawings from which the Hall dimension used within
the model were taken from.
Andrew Rickard
MACH Acoustics
81-83 Stokes Croft, Bristol, BS1 3RD
[email protected] mobile 0779 440 6553
The Second shortfall is with regard to the reverberation time when the Hall is to be used
for small orchestra performances. Currently the audience has been limited to 80
members covering a floor area approximately 32 m2. The modelling of this scenario
shows that the reverberation time is 1.6 seconds. Although this is short of the 1.8 – 2.0
second specification for this type of use, it is thought that 1.6 seconds is acceptable for
the use of a small orchestra in a Hall only 1100 m3
The Design Matrix following in section 1.3 provides as a quick reference to the room
layouts and their suited use. The images show areas of acoustic treatment in Green,
reflective areas Orange and diffuse areas Blue.
1.3 Flexible Acoustics/Reverberation Time
A Multi Use Hall requires that the reverberation time is altered depending on its use.
Cinemas would require this to be short, whilst for Orchestral music relatively longer.
Reverberation has two basic effects, high reverberation increases sound levels and aids
with the propagation of sound, hence the audience in the back row of seating will benefit
from high levels of reverberation. Reverberation also colours sound - in the case of
speech, high levels of reverberation tend to mask and blur the spoken voice.
Reverberation on the other hand, can enhance music tones and tends to mask minor
musical imperfections. A short reverberation time gives music definition or clarity, which
is not always desirable. An excessive reverberation time can lead to music sounding
muddled.
MACH ACOUSTICS
Reverberation time is linked to the room volume and to the acoustic characteristics of its
walls, floor and ceiling. The equation below provides a good estimation of the
reverberation time and shows the relationship between room volume, surface finishes
and reverberation time.
The graph below provides a useful guide for determining the optimum reverberation
time. It shows the relationship between room volume and reverberation time with regard
to different uses.
Equation 1
Rooms meant for speech require a relatively short reverberation time. For a small room,
a value of 0.5 seconds is appropriate. However for large auditoriums, a larger value is
necessary because in a large room, the audience is further away from the speaker and
hence a greater degree of reinforcement is required. Based upon the volume (1100m2)
of this hall, a reverberation time of 0.8 seconds is recommended.
Music requires a longer reverberation time, with the actual value depending on the type
of music. Slow and solemn music, such as church music, is best perceived in a space
with a long reverberation time. Quick rhythmic music (light concert) requires a shorter
reverberation time. Once again, a larger room is better served by a longer reverberation
time. Based of the volume of this hall a minimum reverberation time of 1.8 – 2 seconds
is recommended for classical music productions.
At this stage it is seen that providing audience levels are limited to 60 seats, a
reverberation time of 1.6 seconds can be achieved. With the bleacher seating extended
a total of 180+ seats occupied in the hall, reverberation times are reduced to 0.8
seconds.
Cinemas halls and amplified music venues require a very short reverberation time, since
the reverberation is built into the sound track of the film or can be electronically added to
the reproduced sound.
Late Reflections
Reverberation is a smooth decrease in energy as a result of successive reflections,
where reflections are not individually perceived. A late reflection on the other hand,
consists of one or more strong reflections which are detected separately. An additional
difference between reverberation and late reflection is that reverberation can be a
useful phenomenon. A late reflection simply reduces intelligibility for both speech and
music.
Such to prevent these late reflections, it is conventional and necessary to make the
back wall of an auditorium highly absorptive or diffusive.
Andrew Rickard
MACH Acoustics
81-83 Stokes Croft, Bristol, BS1 3RD
[email protected] mobile 0779 440 6553
1.3 Design Matrix – Acoustic Options
MACH ACOUSTICS
Amplified music &
Speech, Theatre
Hall Layout
Layout 1 - Two independent halls used as two conference spaces, dance room etc.
Rt = 0.8 sec
RASTI >0.7
Layout 2 - Council Chamber
Rt = 1.0 sec
RASTI >0.7
Layout 3 - Drama, conference, comedy and speech based productions
Rt = 0.8 sec
RASTI >0.7
Layout 4 - Live Music
Rt = 1.3 sec
Layout 5 - Orchestral Music
Rt = 1.6 sec
Andrew Rickard
MACH Acoustics
81-83 Stokes Croft, Bristol, BS1 3RD
[email protected] mobile 0779 440 6553
Light /Acoustic
Music
Orchestral
Music
2,0 Acoustic Modelling
To determine and assess the acoustic characteristics of this space, a CATT Acoustics
model of the auditorium has been built. A CATT model is a detailed computer simulation
of a space. This modelling technique works by sending thousands of rays out from a
simulated source position. The advantages of using this type of modelling are; its
accuracy; its potential to map acoustic properties across an audience and its ability to
predict speech intelligibility across that audience.
To assess the acoustic environment within the multi purpose hall, the room dimensions
and finishes have been taken from drawings provided by bblur. These have been
included in Appendix B – Drawing of the Auditorium.
Section 2 of this report looks at a range of different room layouts and scenarios. Within
these assessments, a calculated reverberation time and RASTI score where determined,
where RASTI provides an indication of speech intelligibility.
Andrew Rickard
MACH Acoustics
81-83 Stokes Croft, Bristol, BS1 3RD
[email protected] mobile 0779 440 6553
MACH ACOUSTICS
2.1 Acoustic Modelling - Results
MACH ACOUSTICS
Reverberation Time
RASTI
Reverberation time is defined by the time (seconds) it takes for sound to decay by 60 dB
after an abrupt termination.
RASTI is an acronym for RApid Speech Transmission Index and is a method for
calculating a value that represents the intelligibility of a space. Factors effecting
intelligibility include; background noise, reverberation, strength of source and number of
early reflections reaching the receiver and the effects of late reflections on the receiver. A
RASTI level varies between 0 and 1, alternatively RATSI levels are presented as a
percentage . The brackets are used to better understand these values.
Long reverberation times are undesirable in auditoriums for speech because
reverberation can mask and interfere with the speaker’s voice and consequently speech
intelligibility is impaired. Subjectively, speech in an auditorium with a longer than
desirable reverberation time will sound ‘muddy’ and blur.
Subjective Bracket
RASTI Value
Music however, prefers a longer reverberation time due to the qualities mentioned above
and blending all of the individual parts together.
Excellent
0.75 - 1.00
75% - 100%
Based on the information provided within MACH Acoustics previous report and above,
the following reverberation time targets have been chosen for each use.
Good
0.60 - 0.75
60% - 75%
Cinema, Amplified
Music
Speech & Theatre
Light Music
Orchestral
Fair
0.45 - 0.60
45% - 60%
(0.5 sec) 0.8 sec
0.8 sec
1.3 sec
1.8 - 2 sec
Poor
0.30 - 0.45
30% - 45%
Unsatisfactory
0.00 - 0.30
0% to 30%
It should be noted that the design criteria for cinemas and amplified music require an
RT of 0.5 seconds. It was found that to satisfy this, a large amount of acoustic
absorption would be required and would need to be placed on a greater number of
surfaces, including the ceiling. This treatment would then make it impossible to achieve
the high reverberation time required for music. As such, a minimum reverberation time
of 0.8 seconds has been adopted.
When designing auditoria for speech, MACH Acoustics typically uses a design target of
0.65 or greater. It is this value which will be used to design this Hall.
Maximising Reverberation and RASTI
MACH Acoustics previous report mentioned that achieving the ideal RT for a small
orchestra is limited by the volume of the hall. As such, soft finishes should not be
exposed for this use. However, late reflections caused by a reflection off the back wall will
reduce intelligibility for both speech and music. As such, the rear wall should be made
diffuse.
In addition to this, to maximise the intelligibility of the space, ceiling reflectors shall be
required. These two treatments have been used within the acoustic ray tracing models
and have been kept constant for each scenario. Further details provided in Section
‘Layout 3 – Speech and Theatre Music – Ceiling Reflectors’ and ‘Rear Wall Diffusion’
.
Andrew Rickard
MACH Acoustics
81-83 Stokes Croft, Bristol, BS1 3RD
[email protected] mobile 0779 440 6553
Layout 1 – Dividing the Auditorium – Room Acoustics
MACH ACOUSTICS
The hall is to have the option of providing two separate spaces which can function
independently from one another. This is proposed to be achieved by the use of two
folding partitions.
When designing an auditorium, soft finishes are typically applied to the rear half of the
auditorium and hard finishes are applied to the front half. There is therefore an inherent
problem with dividing an auditorium in two, since the front half of the hall will be hard.
This will result in space B sounding loud, harsh and unintelligible.
It is generally accepted that the upper limit for reverberation within a cellular space is 1.0
seconds, which is in line with guidance such as BB93 and BS8233.
A CATT Acoustics model was used to calculate the required area of Class A absorption
for these two spaces. A simple Sabine calculation was not used as this can sometimes
under predict the required treatment when considering large ceiling heights such as this
Hall.
Due to the reflective nature of space B, it is proposed to have one side of the folding wall
made acoustically absorbent. This option has been illustrated in the Image to the right.
Green represents absorption, Orange represents a reflective surface and Blue a diffuse
surface.
It is likely that the rear side walls, those in space A, will have fixed absorptive treatment
and the folding walls will cover this when required for other uses. However, it may be that
they are reflective. As such a number of scenarios have been calculated for.
This shows that both rooms can be sufficiently treated with an area equal to the folding
wall. Space A would also meet the specification if only the fixed absorptive treatment
were used.
Room
Location of Treatment
Area of
Class A
Reverberation
Time achieved
Room A
Rear, North side wall
25 m2
1.4 Sec
Room A
Folding wall
49 m2
1.0 Sec
Room A
Both rear side walls
67 m2
0.8 Sec
Room B
Folding Wall
49 m2
0.9 Sec
Andrew Rickard
MACH Acoustics
81-83 Stokes Croft, Bristol, BS1 3RD
[email protected] mobile 0779 440 6553
A
B
Layout 1 - Dividing The Auditorium – Sound Insulation
When the hall is divided into two, it is understood that two conferences may take place
simultaneously. It is highly likely that these events will contain amplified speech.
Alternatively, two dance or drama events may take place at the same time, within each
room. In each of these situations, a reasonably high level of sound separation is required
across the separating wall.
BB93 indicates that the acoustic separation between two dance spaces should achieve
at least 50 dB DnTw. This is a high level of sound reduction and one which is unlikely to
be achieved across a moveable wall. In MACH Acoustics experience, the maximum
practical level of sound reduction achieved on site across a folding wall is 47 dB DnTw.
This level of sound reduction is likely to reduce over time. The cost of this type of partition
is exceptionally high and these partitions are also difficult to move.
The proposed solution for Slough Library is to use two independent partitions, as shown.
These partitions are proposed to be rated as 45 dB R’W. These partitions will be lighter,
more flexible and potentially more cost effective solutions, than a single height
performance partition.
A second key advantage of this arrangement is that panels forming the two partitions can
be used to cover the acoustic soft finishes within the hall, enabling the acoustic
properties of this hall to be adjusted such to meet the requirements of theatre, drama and
musical productions.
Access
An important aspect to consider is that doors between the two halls should ideally be
lobbied. Lobbies will prevent noise from spilling between spaces when one door is open.
This is an important requirement for conferences, such to prevent distraction from an
adjacent dance event.
The illustration to the right indicates a possible arrangement.
Andrew Rickard
MACH Acoustics
81-83 Stokes Croft, Bristol, BS1 3RD
[email protected] mobile 0779 440 6553
MACH ACOUSTICS
Layout 2 - Council Chamber – Room Acoustics
MACH ACOUSTICS
The Hall will also provided as a use to the local council for large meeting, thought to take
place around a large table and areas for presentation with use of the projector. As the
use is similar to that of the Divided Auditorium Layout, the acoustic specification will be
the same. i.e. 1.0 second.
Initial modelling shows that with the raked seating away with acoustic treatment on the
rear side walls, which totals 73 m2, the reverberation time of 1.4 seconds is a little high
for this use. In addition the RASTI Level decreases across the room and reaches below
65% over a relatively short distance.
As the layout of this room is not known and how it is likely to be use is not understood,
further design work will be required, to ensure this use will function correctly.
s
3
Global reverberation time
%
20
Mean absorption coeff.
SabT
AbsC
EyrT
2
T-15
1
0
AbsCg
EyrTg
15
T-30
125
250
500
1k
2k
4k
Oct
125
250
500
1k
2k
4k
EyrT
1.27 1.41 1.28 1.22 1.17 1.12
EyrTg
1.34 1.54 1.41 1.33 1.27 1.18
Andrew Rickard
SabT
0.95 1.08 1.00 0.95 0.91 0.88
MACH Acoustics
81-83 Stokes Croft, Bristol, BS1 3RD
T-15
1.47 1.58 1.42 1.39 1.29 1.21
[email protected] mobile
0779 4401.46
6553
T-30
1.57 1.49 1.39 1.30 1.23
AbsC
14.62 13.11 14.22 14.73 14.83 13.58
10
s
s
s
s
s
%
125
250
500
1k
2k
4k
Oct
Trunc 1570.0 ms
Rays 12732 (used/oct)
0
(lost/oct)
0
(absorbed/oct)
Angle 1.80
degrees
Layout 3 - Speech, Theatre and Amplified Music – Room Acoustics
MACH ACOUSTICS
Through acoustic modelling and using the design criteria in the table below, the following
design proposal recommended within this space is to be used for Speech, Theatre,
Amplified Music and Cinema.
Target RT
Target RASTI
0.8 sec
=> 0.65
The image to the right indicates the location of the acoustic treatment used for this
particular scenario. Green areas represent Class A absorptive treatment, Orange
represents reflective surfaces and Blue represents diffuse surfaces.
Raked seating is used with additional flat, non staggered seating at the front. The
distance between the raked seating and source, i.e. the flat seating area, is no more than
8m.
It can be seen that the absorptive treatment is located on the rear side walls. A total area
of 73 m2 has been used.
Note; the south side is treated to the full height of the soffit, the north side has been
restricted by the balcony.
Measuring from the back wall, this treatment extends to 9.2m, approximately equal to the
width of the Hall. This allows the moveable walls to cover this area and allowing for
flexible acoustics.
The table below provides the calculated RT (T-30 in table). From this, the mid frequency
band RT (Tmf ) is 0.8 Seconds Tmf
Andrew Rickard
MACH Acoustics
81-83 Stokes Croft, Bristol, BS1 3RD
[email protected] mobile 0779 440 6553
The image below and to the left indicates the spread of the RASTI level in the Hall. It is
seen to have a minimum level of 0.70 meeting MACH Acoustics specification. The image
to the right shows the sound pressure level across the hall caused by the source, which
is at a typical level for speech. Both images show a good even spread resulting in no
areas of reduced performance.
Layout 3 - Speech, Theatre and Amplified Music – Proposed Treatments
The simplest way in making a wall absorptive, is to line the wall with 50 -100mm of Rw3
mineral wool, followed by 150 mm cavity, essential for low frequency absorption.
Finally covered by an acoustically transparent finish.
Examples of this type of material are; cloth, perforated wood, metal, aluminium, wood
in the form of batons and so on.
Cloth should be open weave and perforated wood/metal or mesh should have a
minimum 33% open area. Timber slats are also a possibility, providing a free area of
33% or more is achieved.
300mm Acoustic
Absorption
Concrete/Partition
Perforated
Covering
RW3 (60 kg / m2)
RW300
3 (60mm
kg /thick
m2)
300 mm thick
b
d
b
a
d
c
Concrete
Concreteb
d 40-60 % open area
Wooden Battens
Wooden Battens 40-60 % open area
Andrew Rickard
MACH Acoustics
81-83 Stokes Croft, Bristol, BS1 3RD
[email protected] mobile 0779 440 6553
RW3 (60 kg / m2)
300 mm thick
Protective
Protective
Concrete
Covering
Covering
Wooden Battens 40-60 % open area
Protective
Covering
MACH ACOUSTICS
Layout 3 – Speech, Theatre and Music – Ceiling Reflectors
Early reflections enhance speech intelligibility by increasing loudness at the audience.
These reflections also enhance clarity of music for all listeners.
Angling the side walls of a given auditorium is often difficult due to the inherent restraints
imposed by a building. Similarly in this case, the layout of Slough Library is fixed and
therefore the option of angling the side walls has not been explored further. The angle
and layout of the ceiling is now the critical factor affecting the acoustic performance of
the theatre.
The presence of a coffered ceiling limits the hall by reducing the useful reflections
reaching the rear of the hall, since sound will tend to enter the cavity and be reflected in
random directions rather than to the rear of the hall. As a minimum, a flat ceiling is
therefore required within this space.
A geometric method using symmetry of incident angles allows one to design reflectors
such that they direct sound to the required audience location. The results of this
assessment are provided within Appendix A - Detailed Specification which provide the
angle and dimensions of the proposed ceiling.
The image shows two reflectors coloured green, located at the front and rear of the hall.
The mid reflector has been incorporated such to eliminate the effects of the coffers.
The reflective panels are required to be formed from two layers of 15mm
SoundBloc or equivalent.
Andrew Rickard
MACH Acoustics
81-83 Stokes Croft, Bristol, BS1 3RD
[email protected] mobile 0779 440 6553
MACH ACOUSTICS
Layout 3 - The Stage & Raked Seating
MACH ACOUSTICS
Raked seating is important for visual and acoustics reasons. Raked seating enhances
both of these factors by increasing the line of sight between the audience and the stage.
Further improvements can also be gained by a raised stage.
Calculations which incorporate these options have been undertaken to provide design
advice on the seating and stage layout. Calculations have been based on stage heights
of 0m, 0.5m (as shown on drawings) and 1.05m (the ideal stage height). Calculations
have also been undertaken using staggered and non-staggered seating. Calculation
indicate the maximum depth of the flat seating area, which is where raked seating should
start. The riser height (R) for the raked seating is also calculated, and this has been
based on a seat to seat distance (T) of 0.9m, see the following figure for a definition of R
and T.
Stage Height (m)
Stager Seating
0
Yes
Max flat area (m)
7.2
0.5
Yes
16.2
1.05
Yes
26.1
0
No
3.6
0.5
No
8.1
1.05
No
13.1
The initial architectural drawings it show a stage height of 0.5m and seating is nonstaggered. This suggests that the maximum depth of flat area seating should be 8m. The
raked seating behind this area is to have a riser height of at least 111mm. The depth of
the flat seating shown on the drawings is approximately 7.5m and is within the 8 m limit.
MACH Acoustics has used a riser height of 220mm such to increase the flexibility of the
space. i.e. if performances were to use a larger floor space, which extended past the
stage and reduce the flat seating area. the stage height would effectively reduce thus
requiring a greater riser height to maintain intelligibility
Stage Height (m)
0
Stager Seating
Yes
Max flat area
7
Distance from source to
last seat (m)
17
Riser Height - R
70
0
0.5
Yes
16
17
1.05
Yes
26
17
0
0
No
4
17
335
0.5
No
8
17
111
1.05
No
13
17
30
Andrew Rickard
MACH Acoustics
81-83 Stokes Croft, Bristol, BS1 3RD
[email protected] mobile 0779 440 6553
When the retractable racked seating is stored away, a concern arises regarding its
absorptive / reflective nature. If the seats are upholstered and visible when stored, the
absorption they provide will significantly reduce the reverberation time and not meet the
ideal level for other uses such as music.
On the other hand, if housing of the seats can be closed and sealed such that the
absorption affects are minimised, late reflections will cause undesirable effects.
It is therefore required that when the seating is stored away, the exposed face of the
housing is non absorbent and diffuse.
Layout 4 - Live Music – Room Acoustics
Using the design criteria in the table below, the following option is proposed for the use of
light music/ acoustic music.
Target RT
1.3 sec
The images to the right indicate the location of the acoustic treatment used for this
particular scenario. Orange represents reflective surfaces and Blue represents diffuse
surfaces. It is seen that the rear of the auditorium is made reflective by placing/storing
the movable wall on either side of the auditorium.
It can be seen that the there is no absorptive treatment used in this scenario. The rear
wall is diffuse and non absorbent as is the housing for the retractable raked seating.
Raked seating has not been used. The audience has been modelled covering a floor
area of 68 m2. It is also important to note that the bleacher seating when closed
must not provide acoustic absorption.
The table below provides the calculated RT (T-30 in table). From this, the average global
RT is 1.3 Seconds
The reverberation time meets the target. In addition to this, the mapping of the area
shows an even spread of noise, suggesting no areas of reduced sound pressure level.
Andrew Rickard
MACH Acoustics
81-83 Stokes Croft, Bristol, BS1 3RD
[email protected] mobile 0779 440 6553
MACH ACOUSTICS
Layout 5 - Orchestral Music – Room Acoustics
Using the design criteria in the table below, the following option is proposed for the use of
a small orchestral performance.
Target RT
1.8 sec
The images to the right indicate the location of the acoustic treatment used for this
particular scenario. Orange represents reflective surfaces and Blue represents diffuse
surfaces.
It can be seen that there is no absorptive treatment used in this scenario. The rear wall is
diffuse and non absorbent as too the housing for the retractable raked seating.
The audience capacity has been reduced such that they cover a floor area of ~32m2.
The table below provides the calculated RT (T-30 in table). From this, the average global
RT is; 1.6 Seconds
The reverberation time has not quite reached the ideal target for this particular use,
although 1.6 seconds is very reverberant and for a venue this small should not be too
limiting for a small orchestra.
Andrew Rickard
MACH Acoustics
81-83 Stokes Croft, Bristol, BS1 3RD
[email protected] mobile 0779 440 6553
MACH ACOUSTICS
Rear Wall Diffusion
MACH ACOUSTICS
As explained on page XX, diffusers are to be used on the rear wall. This is to
stop the occurrence of late reflections which effect intelligibility levels and is
achieved by the irregularities of the surface scattering the incident sound wave
in many directions. This is an alternative to using absorptive treatment as it will
not reduce the reverberation time. This is important as the design is aiming to
provide a high reverberation time suitable for a small orchestra.
Any reflective surface with irregularities of size comparable to the wave length
will work as a diffuser. The greater the randomness in the surface irregularities
and size, the better the diffuser.
The images on this page provide some examples of diffusers. They can come
in many different shapes; cylindrical, domed, pyramids or most commonly a
series of different depth and width wells known as a Quadratic Residue
Diffuser based on a specific mathematical form.
The cylindrical and pyramid diffusers within these images are products named
Diffusorba by SoundSorba, whilst the quadratic residue diffusers shown in
these images are provided by RPG Diffusor Systems Inc.
For the purpose of the acoustic Ray tracing model, the spectral diffusion
coefficients provided below have been used.
Diffusion Coefficient used within MACH Acoustics Ray tracing model
125 Hz
250 Hz
500 Hz
1k Hz
2k Hz
4k Hz
70
77
89
92
92
94
Andrew Rickard
MACH Acoustics
81-83 Stokes Croft, Bristol, BS1 3RD
[email protected] mobile 0779 440 6553
Appendix A - Detailed Specification
MACH ACOUSTICS
Acoustic Treatment
This section will describe in detail the specification of all elements considered with the
Ray Tracing Model. This includes the maximum allowable background noise, noise
criteria for any plant/ services, absorption & diffraction coefficients of the materials used
and their corresponding area.
The acoustic properties of all materials used within MACH Acoustics Ray tracing model
are provided below. These include the absorption coefficients and diffusion coefficients.
Not all materials include diffusion properties, only those that would contribute significantly
were chosen.
Background Noise
All properties are typical of their specified material and were obtained from; Duncan
Templeton, ‘Detailing For Acoustics’, Third Edition, Spoon Press .
As mentioned in section 2.1, the background noise level effects the results of the RASTI
score. If the background noise is high, intelligibility will be reduced and vice versa.
Background noise due to external sources and occupancy within the building should not
exceed 35 dB(A). This will generally be achieved by the specified sound insulation
values provided within MACH Acoustics Sound Insulation report.
Reflectors
The following page provides a section through the hall indicating the positioning of the
reflectors.
The last consideration to make with regard to background noise is that which is caused
by services noise. MACH Acoustics therefore specifies that internal noise due to
mechanical and electrical services is to be no greater than NR30.
Surface Area of Materials Used, m2
Layout 1 – Layout 2 – Layout 3 – Layout 4 –
Space B
Council
Speech, etc. Live Music
Surface
Type
Layout 1 –
Space A
Audience
0
0
0
60
Carpet
0
0
51
Concrete
115
87
Diffuser
14
0
Door
6
Hard Floor
Glazing
Absorption Coefficient, a (Hz)
Diffusion Coefficient, (Hz)
Layout 5 –
Orchestral
125
250
500
1k
2k
4k
125
250
500
1k
2k
4k
95
48
0.51
0.64
0.75
0.8
0.82
0.83
0.30
0.40
0.50
0.60
0.70
0.80
60
51
51
0.05
0.08
0.2
0.3
0.35
0.4
-
-
-
-
-
-
220
220
220
220
0.01
0.01
0.01
0.02
0.02
0.03
-
-
-
-
-
-
14
14
14
14
0.1
0.1
0.1
0.1
0.1
0.1
0.70
0.77
0.89
0.92
0.92
0.94
11
19
19
19
19
0.14
0.1
0.06
0.08
0.1
0.1
-
-
-
-
-
-
84
60
50
50
88
124
0.01
0.01
0.01
0.02
0.02
0.02
-
-
-
-
-
-
22
17
43
43
43
43
0.02
0.06
0.03
0.03
0.02
0.02
-
-
-
-
-
-
Plasterboard
108
123
162
162
230
230
0.3
0.12
0.08
0.06
0.06
0.05
-
-
-
-
-
-
Reflector
32
34
66
66
66
66
0.1
0.1
0.1
0.1
0.1
0.1
-
-
-
-
-
-
Stage
0
13
13
13
13
13
0.4
0.3
0.2
0.17
0.15
0.1
-
-
-
-
-
-
Seat Housing
Class A
Absorption
40
0
55
55
40
40
0.4
0.3
0.2
0.17
0.15
0.1
0.70
0.77
0.89
0.92
0.92
0.94
49
At least 49
73
73
0
0
0.45
0.7
0.93
0.93
0.93
0.8
-
-
-
-
-
-
Andrew Rickard
MACH Acoustics
81-83 Stokes Croft, Bristol, BS1 3RD
[email protected] mobile 0779 440 6553
Appendix A - Detailed Specification
Andrew Rickard
MACH Acoustics
81-83 Stokes Croft, Bristol, BS1 3RD
[email protected] mobile 0779 440 6553
MACH ACOUSTICS
Appendix B – Drawings of the Auditorium
NB. not to scale
Andrew Rickard
MACH Acoustics
81-83 Stokes Croft, Bristol, BS1 3RD
[email protected] mobile 0779 440 6553
MACH ACOUSTICS