Page 191 - The Master Handbook Of Acoustics

166 CHAPTER EIGHT

Noise Sources and Some Solutions

Protecting a room from street traffic noise is becoming more difficult
all the time. It is useful to remember that doubling the distance from a
noisy street or other sound source reduces the level of airborne noise
approximately 6 dB. Shrubbery and trees can help in shielding from
street sounds; a cypress hedge 2 ft thick gives about a 4 dB reduction.
The level of noise that has invaded a room by one means or another
can be reduced by introducing sound-absorbing material into the stu-
dio. For example, if a sound level meter registers a noise level of 45 dB
inside a studio, this level might be reduced to 40 dB by covering the
walls with great quantities of absorbing materials. Going far enough in
this direction to reduce the noise significantly, however, would proba-
bly make the reverberation time too short. The control of reverberation
must take priority. The amount of absorbent installed in the control of
reverberation will reduce the noise level only slightly, and beyond this
we must look to other methods for further noise reduction.
Reducing the noise output of the offending source, if accessible
and if possible, is the most logical and profitable approach. Traffic
noise on a nearby street or airplanes overhead may be beyond con-
trol, but the noise output of a ventilating fan might be reduced 20 dB
by the installation of a pliant mounting or the separation of a metal
air duct with a simple canvas collar. Installing a carpet in a hall
might solve a foot traffic noise problem, or a felt pad might reduce a
typewriter noise problem. In most cases working on the offending
source and thus reducing its noise output is far more productive
than corrective measures at or within the room in question.
As for terminology, a wall, for example, must offer a given trans-
mission loss to sound transmitted through it, as shown in Fig. 8-1. An
outside noise level of 80 dB would be reduced to 35 dB by a wall hav-
ing a transmission loss of 45 dB. A 60 dB wall would reduce the same
noise level to 20 dB if no “flanking” or bypassing of the wall by other
paths is present. The wall “attenuates” the sound or it “insulates” the
interior from the outside noise. The walls, floor, and ceiling of the
sound-sensitive area must give the required transmission loss to out-
side noises, reducing them to tolerable levels inside the room.
Noise can invade a studio or other room in the following ways: air-
borne, transmitted by diaphragm action of large surfaces, transmitted
through solid structures, or a combination of all three.