GENERAL DESIGN CONSIDERATIONS
DG 1110-3-112 May 1979
3-4 BUILDING DESIGN (cont'd)
(c) Sound Quality. This relates to the type of response a room should make to the noise generated
within. A "live" room should have a low average absorption coefficient with hard surfaces to reflect
most of the sound. Conversely, a "dead" room should have a high absorption coefficient with surfaces
to absorb sound. Values given in Chapter 4, Table 4-1 for sound quality are abbreviated as follows: L =
-
live, ML = medium-live, AVE = average, MD = medium dead, D = dead.
(d) Sound Reduction. Building design should allow a reduction in sound between ACES spaces as
indicated in Table 3-1, Sound Reduction Goals. Each group of spaces have similar acoustic
requirements. Reduction is achieved by a combination of interposed distance and barriers.
(e) Maximum Sound Level. Loud and sustained noise can be a hazard to hearing. The safe limit for
an unprotected ear is approximately 135 dB. At 150 dB even short-term exposure may cause damage.
These facts have important implications for the design of shop areas, where high intensity noise is
frequently a problem.
(f) Effect of Distance and Interposing Barriers. Sound dissipates over distance. Doubling the
distance from a sound source reduces the level of sound received by 6 dB. A similar reduction occurs
for each doubling of distance between source and receiver. The operation of heavy armor or
transportation equipment produces between 80 and 120 dB of sound at a distance of 20 feet. Aircraft
may produce in excess of 140 dB. Assuming 100 dB at 20 feet, such sound would diminish to 94 dB at
L
40 ft., 88 dB at 80 ft., 82 dB at 160 ft., 76 dB at 320 ft., on up to 34 dB at 40,000 ft. (7 1/2 miles), etc.
Since 35 decibels is the maximum desired ambient noise level for classrooms and study areas,
distance alone generally is not a practical solution to reducing sound between the classroom
environment and the outside environment or other related environments containing high noise
producing sources. However, if such sources can be located at a distance remote enough to allow
reduction of the noise to an ambient noise level around classrooms of 65 decibels, normal construction
barriers can be reasonably designed to further reduce the noise to acceptable ambient levels within
the classroom.
(9) Background Noise. Background noise is most distracting when the frequency range of the
desired audio stimuli and the background noise are similar. For example, voice noise of 35 dB is more
disruptive than mechanical ventilation noise of 35 dB.
(h) Reflective and Absorptive Surfaces. To reinforce an instructor's voice and help eliminate
distracting reverberations in classrooms and other similar rooms, the ceiling, the wall behind the
instructor's station, and the upper half of the side walls should be provided with sound reflective
surfaces as shown in Figure 3-7. The remaining surfaces of the room should be sound absorptive so
that noise generated close to the floor, e.g. dropping objectives, scuffling of shoes, or the moving of
chairs, is reduced.
3-17
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