South Plymouth Elementary School
Classroom acoustics is an important but often-overlooked design element
in schools. Considering the amount of education that takes place
through verbal communication, accurate listening conditions are
critical. South Elementary School is introducing students with hearing
disabilities including the use of cochlear implants. Proper acoustics
are even more important for these students, as discussed in Classroom
Acoustics for Children with Normal Hearing and With Hearing Impairment,
(October 2000). DCC conducted acoustical testing on May 29, 2006 and
August 25, 2006 to quantify existing listening conditions as well as to
identify problem areas and potential acoustical treatments.
Reverberation in classrooms and other teaching spaces is essentially
the effect of sound reflecting off various hard surfaces within the
space. The net effect of reverberation, a combination of many echoes,
is perceived as a longer incoherent or 'blurry' echo that can interfere
with speech communication. Reverberation time is typically measured in
octave bands and is designated RT60, or the time it takes sound levels
to decay 60 decibels (dB). In general, the larger the room, the greater
the reverberation time.
The ANSI S12.60-2002 Classroom Acoustics Standard specifies that the
RT60 for classrooms should be at or below 0.6 for small classrooms and
0.7 seconds for mid-size classrooms, for the upper mid-frequencies
(500, 1000, and 2000 Hz). DCC measured the RT60 at a number of
locations within each space tested, and then averaged the results.
Reverberation time can vary both spatially and temporally because of a
number of factors including room geometry. Table 1 shows the results of these tests.
Table 1. Measured Reverberation Time (RT60 in seconds)
Measured reverberation times meet the ANSI standard for all classrooms tested.
Classroom Acoustics for Children with Normal Hearing and With Hearing Impairment,
(October 2000) suggests that the RT60 not be higher than 0.4 to 0.6 seconds for children with
hearing impairments. As can be seen from Table 1, there are several rooms
which would be slightly above this requirement and therefore would require the addition of
some acoustical absorption treatment in order to meet this criterion. The recommendations
section delineates the acoustical treatments needed for each room.
Background noise in classrooms can be problematic if background noise levels are sufficiently high.
High background noise levels can reduce the signal to noise ratio needed to properly understand
verbal communication. That is, if high enough, background noise can mask speech or cause it to be
unintelligible. The ANSI standard calls for background noise (e.g., caused by HVAC systems)
to be at or less than 35 dBA. As shown in , Classroom 233, 221, and the
Gymnasium meet this requirement. Since Classrooms 232 and 233 are essentially identical,
the reason that Classroom 232 was noisier than Classroom 233 is because at the time of the
testing a ventilation fan was on in Classroom 232 and the fan in Classroom 233 was not operating.
The background noise level in the cafeteria is just slightly above this requirement.
Table 2. Measured Background Noise
Classroom Acoustics for Children with Normal Hearing and With Hearing Impairment,
(October 2000) suggests a background noise level limit of 30 to 35 dBA for hearing impaired
children. For the purposes of this project, I recommend the 35 dBA threshold value since it
meets both hearing impaired and ANSI requirements.
Wall Noise Reduction
Noise Reduction, or NR, is the amount of sound reduction a wall or other partition provides.
For classrooms with corridors adjacent to the space, ANSI recommends a minimum
Sound Transmission Class (STC) of 45. Table 3 shows the measured results for
Table 3. Measured Sound Transmission Class
As can be seen from this table, the only room which meets the STC requirement is classroom 232.
However, since classroom 233 is essentially identical to classroom 232, they both likely meet
the STC requirement. The difference in the test results between these two rooms was very likely
the difference in proximity of the loudspeaker to the door. Since the wall construction in all
the rooms tested is fairly robust, leaks in the door openings caused the lower STC results.
Consequently, very simple and inexpensive means of sealing the doors in these rooms will
likely raise the STC to 45 or greater. The recommendations section provides details on how
to seal these doors.
The measured RT60 in this space is slightly above the criterion and in order to meet the
criterion, 200 square feet of 2" thick fiberglass should be mounted on the existing hard
reflecting surfaces of the walls. While this material is typically commercially available
in 5'x2' panels, the raw materials themselves are generally much less expensive. However,
should you decide to fabricate the panels in-house, please keep in mind that there can be
difficulties working with fiberglass. The 2" thick semi-rigid fiberglass is Owens Corning 703
fiberglass with the following octave band sound absorption coefficients.
(A regional representative of Owens Corning can be contacted at 678-947-5845.)
Table 4. Sound Absorption Coefficients of Acoustical Absorption Treatment
The panels should be covered with fireproof fabric which is ‘acoustically transparent.’
This means that sound can easily pass through the fabric which can be tested by blowing through
the fabric. A common fabric used for this application is manufactured by Guilford of Maine
(item no. FR701, 800-755-9234). Another acoustical absorption product which would be a suitable
substitute for fiberglass is Tectum. Tectum has the advantage over fiberglass of being more
durable, such as when hit by a ball. If you select Tectum, its acoustical properties should
match those shown in Table 4.
Based on the measurement data in Classroom 232 and 233, it appears that this room
would meet the background noise requirement if the fan was turned off.
This room meets the wall STC requirement, but because of the test results in Classroom 233,
I recommend that the door openings be sealed. This can be achieved by using rubber bulb
type weather stripping around the door perimeter, and a drop down metal and rubber acoustical
seal at the bottom of the door. A test of the adequacy of the seal is if any light
can be seen coming through the door perimeter when the room is completely dark, and with a
bright light on in the hall. Since the wall construction is robust, sealing the doors should
achieve STC 45.
The reverberation time measurements in this space varied slightly from Classroom 232, even
though the rooms are essentially identical. As in Classroom 232, 200 square feet of acoustical
absorption should be installed on the existing hard reflective walls.
Since this room meets the background noise criterion and the fan does not operate in this room,
no further noise reduction is needed in this space.
I recommend the same door sealing treatments for this classroom as for Classroom 232.
No sound absorption treatment or background noise reduction is needed in this space.
Because of the robustness of the walls, and the 'leaky' door comprising the main acoustical
path into the room, it is very likely that sealing the door will result in STC 45. Therefore,
I recommend the door sealing treatment discussed for the other classrooms.
Only the highest mid frequency octave band (2000 Hz) is above the reverberation time criteria
so less acoustical treatment is needed in this space. I recommend the installation of 100
square feet of 2" thick fiberglass on the hard reflective walls.
This space has background noise levels slightly higher than the criterion, and it is
uncertain at this time what remedies would be available to reduce HVAC noise in this
space. On the other hand, since acoustical absorption added to the room and sealing
the doors will likely reduce background noise levels somewhat, it is possible that
no other noise reduction measures will be needed once these treatments are installed.
I recommend the same door sealing treatments as previously discussed.
With respect to the reverberation time in this space, a substantial area of Tectum is
already installed, and the reverberation time is quite reasonable for a space with such a
large volume. Gymnasiums are not categorized as classrooms per se, and therefore the
classroom reverberation criterion technically does not apply.
This room meets the background noise requirement and therefore no other background
noise reduction is needed.
During the wall STC test I noticed a substantial amount of sound entering through the
gymnasium door, so like the other rooms tested, it is very likely that sealing the
doors into this space will result in STC 45. Consequently, I recommend the
previously discussed door sealing techniques be applied to the gymnasium.
Of the rooms tested, all essentially passed the reverberation time
test, all failed the wall STC test, and all failed the background noise
test. Since the background noise measurements were conducted with the
air conditioning on, it is probably worthwhile measuring the heating
system noise since the heating system is on for a greater portion of
the year. The poor acoustical performance of the walls dwarfs the
problems associated with HVAC noise. For spaces used for teaching
children with hearing impairment, the walls should be properly extended
to the deck with standard wall construction.