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South Plymouth Elementary School

CLASSROOM ACOUSTICS

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

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

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 each room.

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.

Recommendations

Classroom 232

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.

Classroom 233

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.

Classroom 221

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.

Cafeteria

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.

Gymnasium

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.

Conclusion

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.