Article
Background Noise Levels and
Reverberation Times in Unoccupied
Classrooms: Predictions and Measurements
Heather A. Knecht
The Ohio State University, Columbus, OH
Peggy B. Nelson*
University of Maryland, Baltimore
Gail M. Whitelaw
Lawrence L. Feth
The Ohio State University, Columbus, OH
Classrooms are often filled with deterrents
that hamper a child’s ability to listen and
learn. It is evident that the acoustical
environment in classrooms can be one such
deterrent. Excessive background noise and
reverberation can affect the achievement and
educational performance of children with
sensorineural hearing loss (SNHL) and
children with normal hearing sensitivity who
have other auditory learning difficulties, as
well as elementary school children with no
verbal or hearing disabilities. The purpose of
this study was to evaluate the extent of the
problem of noise and reverberation in schools.
To that end, we measured reverberation times
and background noise levels in 32 different
unoccupied elementary classrooms in eight
public school buildings in central Ohio. The
results were compared with the limits
A
coustical standards for classrooms in the United
States had never been formalized until the recent
adoption of ANSI S12.60 –2002. The absence of
national standards has resulted in highly variable acoustical conditions for U.S. classrooms (Pekkarinen & Viljanen, 1991). In contrast, the standards that are in place
for classrooms in Sweden mandate strict performance criteria for classroom background noise levels and reverberation times. According to Swedish standards, background
noise levels in an unoccupied classroom should not exceed 35 dB(A), and reverberation times should fall within
*Currently affiliated with the University of Minnesota Department of
Communication Disorders, Minneapolis.
recommended in the American National
Standards Institute standard for acoustical
characteristics of classrooms in the United
States (ANSI S12.60 –2002). These
measurements were also compared to the
external and internal criteria variables
developed by Crandell, Smaldino, & Flexer
(1995) to determine if a simple checklist can
accurately predict unwanted classroom
background noise levels and reverberation.
Results indicated that most classrooms were
not in compliance with ANSI noise and
reverberation standards. Further, our results
suggested that a checklist was not a good
predictor of the noisier and more reverberant
rooms.
Key Words: classroom acoustics, noise,
reverberation, educational audiology
the limits of 0.6 to 0.9 s. In the absence of official standards, the American Speech-Language-Hearing Association (ASHA, 1995) has recommended that unoccupied
classroom noise levels should not exceed 30 dB(A) and
that reverberation times should not exceed 0.4 s. Recently
an ANSI Working Group on classroom acoustics has recommended maximum background noise levels of 35
dB(A) and maximum reverberation times of 0.6 s (Nelson, 2000).1 The ANSI recommendations, recently
1
Although several authors suggest that reverberation times should not
exceed 0.4 s for optimum speech recognition (i.e., Crandell & Bess,
1986; Finitzo-Hieber & Tillman, 1978), the members of the Working
Group hypothesized that if unoccupied reverberation times are 0.6 s or
less, then occupied rooms will have acceptably shorter reverberation
times because of the absorption of sound provided by the children
themselves.
American Journal of Audiology ● Vol. 11 ● 65–71 ● December 2002 ● © American Speech-Language-Hearing Association
1059-0889/02/1102– 065
http://professional.asha.org/resources/journals/aja
65
adopted as ANSI S12.60 –2002 may evolve into a national building standard for American classrooms. Unfortunately, many classrooms today do not meet any of the
above criteria. Reverberation times varying from 0.3 s to
greater than 1.5 s (Crandell & Bess, 1986; Finitzo-Hieber,
1988) and background noise levels ranging from 30
dB(A) to 47 dB(A) have been reported for “typical” unoccupied classrooms in the United States.
For critically evaluating how effectively an individual
can hear and understand speech in the classroom setting,
three well-established factors have been identified: background noise level, reverberation time, and signal-to-noise
ratio (SNR). Background noise commonly refers to any
undesired sound that impedes what a child wants, or
needs, to hear (Crandell, Smaldino, & Flexer, 1995). Examples of background noise include noise generated by
heating, ventilating, and air conditioning (HVAC) systems; external noise such as outdoor traffic flow, noise
from halls and other rooms adjacent to classrooms; and
noise that is generated by the children themselves within
the classroom and/or school building.
Reverberation refers to the persistence or prolongation
of sound within a space as sound waves reflect off hard
surfaces in a room (Kurtovic, 1975). In highly reverberant
rooms, speech signals are delayed and overlap with the
direct sound, often masking the intended message of the
speaker. The longer the reverberation time (RT), the
greater the negative effect on speech intelligibility. Reverberation increases linearly with room volume and is inversely related to the amount of sound absorption in an
environment. RT is a function of the physical properties
of a room and can be calculated by the Sabine formula
(RT60 ⫽ k*V/A) if the volume (V), surface area, and surface absorbencies (the equivalent area of perfect absorption, A) are known. (The constant k is 0.049 for English
units and 0.161 for metric units.) The smaller the volume
of the room and the more absorptive material the room
possesses, the lower the RT. SNR may be defined as the
“relationship between the intensity of the signal and the
intensity of the ambient noise at the child’s ear” (Crandell
et al., 1995, p. 340). The signal in a typical classroom
may vary from moment to moment and may be the teacher’s voice, a fellow student’s voice, or a multimedia presentation. As the distance between the signal and listener
increases, the intensity of the speech signal decreases,
reducing the effective SNR. Obviously, classrooms with
higher background noise levels and/or long RTs will have
poor SNRs. Evidence has shown that reduced SNR results
in reduced understanding and learning, especially for children with hearing loss or other special needs (see Nelson
& Soli, 2000, for a review).
Often reverberation and background noise levels of
classrooms are too high for optimum speech recognition
to occur for children (e.g., Crandell et al., 1995). A national effort is underway to set guidelines for maximum
noise and reverberation in new classrooms (Sorkin, 2000).
In the near future, school construction projects may consider noise levels and reverberation times during their
planning and construction. It is therefore important to
understand current classroom acoustic conditions and their
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relationship to factors such as room size, window construction, and method of HVAC. Thus, the purpose of
this study was to describe representative classroom acoustics by measuring reverberation and background noise
levels in 32 different unoccupied elementary classrooms
in eight different public school buildings in central Ohio.
The goals were to obtain acoustic measures in classrooms, to compare those to ASHA and ANSI recommendations, and to determine if a simple checklist can help
school officials predict classroom noise and reverberation.
The schools chosen represented urban, suburban, and rural schools so that an adequate cross-section of school
buildings was obtained. Unoccupied noise levels were
measured because they are controlled primarily by architects and school planners. Although occupied noise levels
are also of interest, they are controlled more by teachers
and students in the rooms. Thus the focus of this article
and of the ANSI standard is noise levels in unoccupied
rooms. The background noise level measurements and
reverberation values measured in each classroom were
compared with the external and internal criteria developed
by Crandell et al. to determine whether certain observable
building characteristics predicted classroom background
noise levels or reverberation times. Data were analyzed
for overall background noise level; reverberation time;
and correlation between noise, reverberation time, and the
presence or absence of the internal and external criteria.
Methods
Classrooms located in three school districts were selected at random with the aid of the Office of Professional Practices in the College of Education at The Ohio
State University. A total of 32 unoccupied elementary
school classrooms from the three different school districts
were chosen at random, in order to obtain measurements
from a representative sample of rooms. These included 12
classrooms in newer suburban schools, 12 classrooms in
older urban schools, and 8 classrooms in rural schools. A
description of the classrooms, their ages, and volumes can
be found in Table 1.
Description of Measurement Procedures
Identical procedures were used in each of the 32 classrooms to obtain the needed measurements. First, each
classroom’s length, width, and height were measured, and
the room volume was calculated. Next, the classroom’s
internal and external criteria were determined and noted
using the checklist developed by Crandell et al. (1995;
see Table 2). The HVAC system status was not under our
control, and thus we noted whether it was on or off.
Subsequently the noise and reverberation measurement
equipment was set up in the unoccupied classroom. Five
different points were marked on the floor in each classroom using a tape measure and tape. The locations of the
five points were designed to make certain that they were
not regular locations in any potential standing wave pattern. Hence, standing wave patterns occurring in the
classroom would not have significantly altered the data. A
TABLE 1. Description of classrooms, including school age
and room volume, for suburban, rural, and urban school
rooms.
TABLE 2. Sample checklist for a) external and b) internal
criteria variables (after Crandell et al., 1995).
a) External criteria variables:
Classroom
Age
(yr)
Room
Volume
(m3)
Suburban
1a
1b
1c
1d
2a
2b
2c
2d
3a
3b
3c
3d
37
37
37
37
63
63
63
63
50
50
50
50
186
193
186
186
190
187
204
790
206
227
199
199
Off
Off
Off
Off
On
On
On
Off
On
On
On
On
Classroom
Age
(yr)
Room
Volume
(m3)
HVAC
(on/off)
77
77
77
77
77
77
77
77
296
306
306
306
232
294
240
246
Off
Off
Off
Off
Off
Off
Off
Off
Age
(yr)
Room
Volume
(m3)
HVAC
(on/off)
80
80
80
80
92
92
92
92
37
37
37
37
250
250
257
262
261
261
295
295
217
208
319
319
Off
Off
Off
Off
Off
Off
Off
Off
Off
Off
On
On
Rural
1a
1b
1c
1d
2a
2b
2c
2d
Classroom
Urban
1a
1b
1c
1d
2a
2b
2c
2d
3a
3b
3c
3d
HVAC
(on/off)
sound level meter was placed above each point marked
on the floor of the classroom. For each classroom the
same five placement points were used.
The amplifier and speaker were placed in the front
left-hand corner (Figure 1) for all of the classrooms while
obtaining measurements. The speaker was placed on the
floor with the output generated upward to simulate an
omni-directional speaker system.
Classrooms
#1
#2
#3
#4
#2
#3
#4
Located away from noise sources
Exterior walls free of cracks
Windows properly installed
Proper landscaping
Concrete barriers
b) Internal criteria variables:
Classrooms
#1
Fiberglass sheets if applicable
Double wall construction
Acoustical ceiling tile in hallways
High mass per unit doors
Lined heating/cooling ducts
Permanently mounted
blackboards
School personnel would be instructed to indicate “⫹” for items present,
“⫺” for items absent in target classrooms.
Description of Materials
A Bruel & Kjaer 2260 Investigator sound level meter
(SLM) was used to measure reverberation times and
background noise levels in the unoccupied classrooms.
The SLM was supplied with the excitation signal to the
EV Dynacord 7 100 Stereo Amplifier and Radio Shack
Realistic Speaker Minimus 7.2 For each measurement of
reverberation time and background noise levels in the
classroom, the SLM was calibrated both internally and
externally using the Bruel & Kjaer Sound Level Calibrator Type 4231.
The sound level meter was preprogrammed to allow
10 s for the investigator to leave the room so that the
measurements obtained were accurate for an unoccupied
classroom. A measurement was then made of the room’s
background noise levels and reverberation times at each
of the five different positions. After the acoustic measurements were obtained for each classroom, the data were
then extracted using Bruel & Kjaer Qualifier Type 7830
building acoustics software.
Background noise levels were first extracted in linear
form and then equalized to provide data expressed in
dB(A) to facilitate comparisons with those in the literature. A-weighted sound levels better approximate human
hearing by simulating the sensitivity of an average human
ear. Reverberation times are reported for 0.5, 1, and 2
2
An omni-directional speaker system was the best choice for these measurements of room acoustics, but an omni-directional speaker was not
available at the time these measurements were made.
Knecht et al.: Background Noise and RT
67
FIGURE 1. Diagram of the five points for typical classroom
placement of the Bruel & Kjaer 2260 sound level meter and
sound source placement. The five sound level meter
placement positions were as follows: 3ⴕ ⴛ 13ⴕ, 11ⴕ ⴛ 7ⴕ, 5ⴕ ⴛ
15ⴕ, 13ⴕ ⴛ 21ⴕ, 7ⴕ ⴛ 21ⴕ.
rooms had background noise levels below 35 dB(A).
Only one room met or exceeded the more conservative
criterion of 30 dB(A) suggested by ASHA. Overall, background noise levels for the 32 classrooms appeared to be
5 to 15 dB higher than recommended (see Figure 2).
The status of the HVAC system was not under experimental control. The experimenter merely noted the status
and type of the HVAC system at the time of measurement.
Overall, the quieter classrooms in this study had the HVAC
system turned off during testing. The noisier classrooms had
room HVAC systems on during testing (these are noted with
an asterisk * in Figure 2) and also had items in the classroom that produced a noise source. The largest single source
of extraneous noise in this study was a fish tank that produced an overall background noise level of 65.9 dB(A).
Reverberation Times
kHz because room reverberation is typically reported as
an average of these three frequencies.3
Results
Background Noise Levels
Noise levels for the 32 classrooms ranged from 34.4
dB(A) to 65.9 dB(A). Of the 32 classrooms, only four
3
The reverberation data had to be manually altered because when they
were extracted, the 500-Hz measurement appeared to contain artifact.
This was attributed to the use of a speaker that may not have provided
adequate low-frequency power for the purpose of obtaining reverberation measurements.
Reverberation time measurements ranged from 0.2 to
1.27 s. Of the 32 rooms, 13 exceeded the ANSI maximum
recommended reverberation time of 0.6 s. Only 6 of the 32
unoccupied classrooms met the more conservative ASHA
criterion of 0.4 s, with the remainder exceeding the recommended reverberation time for classrooms (see Figure 3).
The least reverberant rooms had on average smaller
volumes; conversely the rooms with larger volumes had
longer reverberation times. Reverberation times were
longest in rooms with high ceilings, as expected. Classrooms in one suburban school were approximately 20⬘ ⫻
30⬘ ⫻ 11⬘. Reverberation times for the four classrooms in
this building ranged from 0.17 to 0.75 s. Four classrooms
in a school in the urban district were approximately 21⬘
⫻ 30⬘ ⫻ 13⬘. The four classrooms in one rural school
were approximately 21⬘ ⫻ 35⬘ ⫻ 12⬘. Reverberation
FIGURE 2. Background noise levels in dB(A) for the 32 classrooms. Those classrooms with HVAC systems on at the time of
testing are noted with an asterisk (*).
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FIGURE 3. Reverberation times (RT 60) at 0.5, 1, and 2 kHz for the 32 classrooms.
times were quite high for these classrooms with higher
ceilings, with most of the measurements at or exceeding
1.0 s for all three frequencies (500 Hz, 1 kHz, and 2
kHz). All rooms with ceilings of 10⬘ or less had acceptable reverberation times.
Room volume was calculated for all of the 32 classrooms in the study and each room’s volume was then
compared to reverberation time measurements (averaged over frequencies of 1 and 2 kHz). Those rooms
with the longest reverberation times were those with
the largest volumes. The actual reverberation time as a
function of room volume is shown in Figure 4. For
example, room 19 had a long reverberation time of
1.16 s and a room volume of 257 m3, whereas, room 2,
which had a short reverberation time of 0.35 s, had a
room volume of 193 m3. There was one notable classroom out of the 32 rooms that did not follow this pattern. (This outlier, room 16, was eliminated from Figure 4 because of its extremely large volume.) Room 16
had the largest room volume of all of the rooms at 790
m3 and possessed a reverberation time of 0.89 s. Possible explanations that may account for this measurement
are unknown but may be related to the use of proper
sound absorbing materials on the room’s ceiling and
walls.
Internal and External Criteria
Rooms in this investigation met between 0 and 4 of
the 5 criteria for external variables and between 1 and 4
of the 6 internal variables proposed by Crandell et al.
(1995). Examination of the tallied results suggested no
clear relationship between number of criteria met and
measured noise or reverberation. It did not appear that
satisfying more criteria from Crandell’s checklist necessarily meant lower background noise levels. Some classrooms met only 2 or 3 of the criteria and yet had low
background noise levels. One classroom had a background noise level of 50.3 dB(A) even though it met 8 of
the 11 criteria.
Certain specific criteria, however, did seem to be related to the acoustic measurements. For example, the
classrooms that had the best reverberation and background noise level measurements had new window installation. The windows in these classrooms had double
panes of glass with blinds located between the sheets of
glass. All four rooms from suburban district A, building 1
had acceptable reverberation times and background noise
levels between 26.5 dB(A) and 40.4 dB(A). Conversely,
the rooms in older schools that had poor window installation had longer reverberation times and higher background noise levels. The windows in these classrooms
were single panes of glass and appeared to be rather old.
The relationship between the windows and the background noise levels could be coincidental rather than
causal. It seems logical, however, that the better quality
windows would attenuate noise from outside most
successfully.
Therefore, it seems that simply assessing the overall
acoustical learning environment according to the general
checklist developed by Crandell et al. (1995) may not
provide an adequate estimate of noise levels and reverberation times. Some items, however, may be more predictive of background noise and reverberation times than
others.
Overall Results
Of particular interest is the fact that only one classroom
(Classroom 2, Suburban 1b) met both acceptable noise and
reverberation criteria based on the ASHA criteria. Of the 32
classrooms assessed, the rooms with both the lowest reverberation measurements and background noise levels were the
classrooms from building 1, one of the newer schools from
Knecht et al.: Background Noise and RT
69
FIGURE 4. Average reverberation time (0.5, 1, and 2 kHz) by
room volume for the 32 classrooms. The solid line indicates
the best-fit regression line of our own measurements.
the suburban district. These classrooms were approximately
27⬘ ⫻ 29⬘ ⫻ 9⬘. The background noise levels in three of the
four classrooms fell below 35 dB(A). [The fourth had a
background noise level of 40.4 dB(A).] The reverberation
time measurements were less than 0.4 s, with only one measurement in one classroom at 1 kHz exceeding the conservative specification established by ASHA (1995).
Rooms in other buildings in that district and in others
were less favorable. For building 3 in the same suburban
district A, all four of the classrooms exceeded the recommended background noise level limit with measurements
ranging from 46.1 dB(A) to 52.1 dB(A).
Rooms with the HVAC system turned on had average
noise levels of 49.7 dB(A). In contrast, those with HVAC
off averaged noise levels of 39.8 dB(A). The single highest noise level measured, 66 dB(A), appeared to arise
from a noisy fish tank in one classroom.
Discussion
In order to improve the listening, learning, and teaching environment in the classroom setting, it is obvious
that a consensus regarding specifications for classroom
acoustics is needed. Background noise levels, reverberation times, and SNRs all impact critical communication in
the classroom. Children in the classroom setting may not
hear information essential to learning if these acoustical
factors are not properly controlled. When students cannot
listen effectively in school, their comprehension abilities
may be compromised, directly affecting the child’s
progress or lack of progress in the classroom, which may
result in poor learner outcomes. Children in the public
school setting may also have difficulty staying on task,
resulting in discipline and cooperation issues for the
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teacher. The teacher is adversely affected by poor classroom acoustics because he or she has to make vocal adjustments in order to project his or her voice and maintain
classroom control (Ray, 1985).
In order for students to listen effectively, the American
Speech-Language-Hearing Association (1995) has suggested that the noise level for an unoccupied classroom
should not exceed 30 dB(A), and for an occupied classroom the noise level should be no higher than 40 to 50
dB(A). The ANSI Working Group on classroom acoustics
has recommended maximum background noise levels of
35 dB(A). Background noise level measurements obtained
in this study are in agreement with those of previous
studies (Bess and McConnell, 1981; Crandell, 1991;
Finitzo-Hieber and Tillman, 1978), which all suggest that
ambient noise levels in unoccupied classrooms usually
exceeded 35 dB(A).
In this study, only 4 of the 32 classrooms examined had
background noise levels at or below 35 dB(A). Overall, there
was a wide range of background noise levels, ranging from 28
to 67 dB(A). In light of the new acoustics standard, some of the
results may be seen as encouraging. Several of the newer rooms
met the maximum allowable noise levels without special effort
toward that goal. This finding suggests that the background
noise goals of the Working Group are achievable with a reasonable school design.
On the other hand, none of the rooms with HVAC systems on met the standard’s noise limits. In fact, only one
room was within 5 dB of the standard, and most were approximately 15 dB higher than the limits. This finding suggests that achieving quiet HVAC systems may be the largest
challenge facing schools in the future. Many HVAC systems
(especially window or room units) are frequently very noisy,
as shown in the current survey. It seems that simple attenuators and bafflles will be insufficient to control HVAC noise
in existing systems. According to Schaeffer (1999), attenuation can be expected to reduce HVAC noise by a maximum
of 6 to 8 dB, when in reality we need systems that are 15
dB quieter than current room units. These findings suggest
that only central ducted HVAC systems can meet the background noise goals described in the standard. As an additional complication, one building principal reported that the
blowers in the current HVAC system will have to be increased in order to circulate more air. This increased circulation of air will obviously contribute to an increase in background noise levels in the classroom setting.
Reverberation time goals also seem achievable. The
rooms in the current study that were the least reverberant
had on average smaller volumes; conversely the rooms
with larger volumes had a longer reverberation times. All
rooms with ceilings of 10⬘ or less met the desired reverberation times. Thus it appears that dropped ceilings of
appropriate height will allow classrooms to attain the desired RT values.
Unfortunately, a simple checklist may not fully predict
a room’s acoustic characteristics. Crandell et al. (1995)
had suggested the use of an internal and external criteria
variable checklist to predict appropriate classroom acoustics. It was hypothesized that if these external and internal
criteria variables were identified and measured, one would
be able to effectively conclude which intervening variables had the most impact on background noise levels and
reverberation times. In the current study, there was no
strong relationship between the number of criteria met
and the level of the background noise or reverberation
time. Therefore, it seems that simply assessing the acoustical environment using the checklist approach may not
provide an adequate estimate of noise levels or reverberation times.
Suggestions to improve the listening conditions of
classrooms with poor acoustics have included the use of
signal control without amplification [i.e., installing carpet
and placement of absorbent paneling (Berg, Blair, & Benson, 1996)] as well as specifications for classroom mechanical equipment (HVAC). In fact, according to Seep et
al. (2000), the installation of carpeting has very little effect on sound absorption in classrooms. In addition, in
today’s classroom environment, carpeting does not seem
to be a priority health issue within the school. Many
classrooms in this study, particularly those from suburban
district A, are going to be required by The Ohio State
Board of Health to remove carpeting from all classrooms
to lessen children’s potential risk.
Other suggestions to improve acoustics in the learning
environment involve individual amplification systems as
well as sound field amplification. These two strategies
enable students to hear the teacher across the classroom
and under poor acoustical conditions (Berg et al., 1996).
Amplification strategies are more complicated when considering classrooms in which active peer-centered learning
occurs. The target signal may vary quickly as students
and teachers participate in learning activities. For these
scenarios, maximizing the room’s SNR through improving overall acoustics seems essential. One might postulate
that acoustical modifications combined with sound field
amplification would result in benefit for children with
sensorineural hearing loss and children with normal hearing sensitivity who have other auditory learning difficulties as well as all elementary school children.
Poor classroom acoustics have been a long-standing
problem in America. In part this is probably due to the
complexity of the factors involved and the issues underlying these factors. Excessive classroom noise and reverberation appears to be common in American schools. There
does not seem to be a simple way to identify or treat the
problem of poor classroom acoustics. As school districts
enter into future school construction projects, a combination of several strategies seems necessary to meet the recommended guidelines. Audiologists and speech-language
pathologists can help to make school administrators aware
of the common problems and the upcoming standards. A
collaborative approach will be needed to allow new
schools to meet the recommended guidelines so that all
students can hear and learn at their maximum potential.
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Received November 12, 2001
Accepted August 12, 2002
First published (online) February 18, 2003
http://professionals.asha.org/resources/journals/aja
D.O.I: 10.1044/1059 – 0889 (2002/009)
Contact author: Peggy B. Nelson, Department of Communication
Disorders, University of Minnesota, 164 Pillsbury Drive SE,
Minneapolis, MN 55455.
Knecht et al.: Background Noise and RT
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