NOISE IN RELOCATABLE CLASSROOMS–A CASE STUDY
 Noise and speech intelligibility are well studied issues. A lot of information is available on the subject, both from the hearing aid industry and various acoustical groups. Very little in the way of recommended practical information seems to be available on best practices to achieve the maximum speech intelligibility given existing construction standards.
The standard for determining what level of acoustic interference creates speech intelligibility problems seems to be too complicated to have a practical standard. However, some practical mileposts exist that indicate the envelope of the problem.
DEFINITIONS:
dB - stands for decibel, and when used with sound, is a dimensionless quantity that indicates the "loudness" or volume of the sound. For the purposes of this paper, the science of the decibel is not required. The numbers should be viewed as relative.
dB(A) - a weighted decibel curve, developed to reflect the fact that the human ear does not respond to all frequencies equally. The A-weighted curve is used in almost all noise regulations. An HVAC unit may produce 50 dB(A) at 10 feet, while its unweighted decibel production may be 65 to 70 dB. This indicates that a lot of the unit's sound production is in the lower frequencies.
0 dB - the lower threshold of hearing.
3 dB - the amount of change in the volume of sound that can be perceived by the average person when two sources are identical in tonal content or pitch of the sound. When the tonal content or pitch of two sources is different, smaller changes in sound volume can be detected.
6 dB - the decrease in the volume of sound when measured as the distance is doubled for a person standing out of doors. The reduction in volume will be approximately this value in larger rooms when a person moves from close to the source to double the distance, but less when you are farther from the source and then double the distance. For example, this could be true when moving from 3 to 6 feet from the teacher for a larger classroom.
10 dB - change in the volume of sound is perceived as "twice as loud" when the tonal content of the sounds is identical.
25 dB(A) - the background sound that would be measured in a "quiet" room, in church nave without people or in a graveyard at night without people or major roadways nearby.
35 dB(A) - the background sound level recommended for classrooms.
40 dB(A) - a recommended background sound level for a private office.
47 dB(A) - the recommended upper background sound level for an open plan office area.
50 dB(A) - the loudness of public areas of a building such as a lobby where speech privacy and interference are not a concern.
65 dB(A) - the sound level of someone talking in a "lecture" voice at close range, about 2 feet.
Figure 1
If the classroom is treated with acoustical materials such as carpet, tack boards and ceiling tile, some sound will be absorbed and this will generally lower the noise level in the room. The effect is less the closer you are to the noise source such as the HVAC unit. However, in the frequency region of the voice, this may not be beneficial. Depending on the frequency content of the background noise, the possibility exists that the teacher will need to "speak louder" in order to make up for the absorption. This will be tiring for the teacher as they are forced to speak loudly for extended periods.
Using those measurements one can start to see the problem in a relocatable classroom.
A typical wall mounted heating, ventilating and air-conditioning (HVAC) unit in a relocatable classroom will produce about 50 dB(A) at a point 10 feet from the unit. At 20 feet, or approximately the middle of a group seating area, the level will be 44 to 48 dB(A). The recommended background sound level for a classroom is 35 dB(A). The 50 dB(A) HVAC unit is 32 times louder than the recommended level when you are 10 feet from the unit and 10 times higher when you are in the middle of the classroom.
Figure 2
1. HVAC noise propagates directly at the teaching center of the room.
2. The closer a student is to the HVAC unit and the farther from the teacher, the less is the speech intelligibility.
3. The HVAC generated noise at approximately 50 dB(A) is very high compared to a recommended 35 dB(A) requirement for a classroom.
4. The teacher (65 dB(A)) vs. the HVAC (50 dB(A)) is very close in loudness.
A classroom with no HVAC noise and "quiet" students should have a background sound level no higher than 35 dB(A). A teacher lecturing at the opposite side of the room, 30 feet from the HVAC unit, is assumed to be speaking at 65 dB(A). Assuming that the teacher's sound level is measured at a 2 feet distance, at a 12 foot distance the loudness is 49 dB(A) in the worse case of perfect sound absorption in the space, but more likely in the range of 55 to 58 dB(A). (if you assumed 6 dB drop per doubling of distance, the sound level at 12 feet would only be 49 dB(A). Because of sound reflecting off of walls and floors, the drop will be less than this). Since HVAC unit is closest to the students who are farthest away from the teacher, they will hear more background noise and less of the sound from the source of interest, the teacher. Speech interference will prevent any meaningful speech intelligibility. In the center of the room, the HVAC noise and the teacher generated speech levels are getting very close in sound level and speech intelligibility will be impaired.
Unknowns:
- The directional pattern of the HVAC noise.
- The frequency (tonal content or pitch) content of the HVAC noise.
- The amount of noise energy that is transmitted into the wall. When this energy is transferred to the wall, it can act as a "loudspeaker driver", increasing the total noise.
- The classroom configuration of teacher and students.
Potential solutions:
- Mandate quieter HVAC units
- Modify the installation of the existing types of HVAC to mitigate the noise problems.
- Install a "T" on the front of the unit with two short pieces of ductwork with registers at the ends. The ductwork must be lined with a minimum of 1 inch thick duct liner.
- Decouple the unit from the wall (depending on the noise contribution)
- Change HVAC system types.
In any case, there will be some cost impact. The least expensive and easiest to achieve would be to modify the HVAC installation. Some in-situ sound testing and experimenting with this solution in a typical relocatable classroom is worth trying.
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