Page 193 - The Master Handbook Of Acoustics

168 CHAPTER EIGHT

attenuated only about 2 dB in 100 ft. Sound travels in steel about 20
times as far for the same loss! Although joints and cross-bracing mem-
bers increase the transmission loss, it is still very low in common
structural configurations.

Noise Transmitted by Diaphragm Action
Although very little airborne sound energy is transmitted directly to a
rigid structure, airborne sound can set a wall to vibrating as a
diaphragm and the wall, in turn, can transmit the sound through the
interconnected solid structure. Such structure-borne sound might then
cause another wall at some distance to vibrate, radiating noise into the
space we are interested in protecting. Thus two walls interconnected
by solid structure can serve as a coupling agent between exterior air-
borne noise and the interior of the listening room or studio itself.
Sound-Insulating Walls

For insulating against outside airborne sounds, the general rule is the
heavier the wall the better. The more massive the wall, the more diffi-
cult it is for sound waves in air to move it to and fro. Figure 8-2 shows
how the transmission loss of a rigid, solid wall is related to the density
of the wall. The wall weight in Fig. 8-2 is expressed as so many pounds
per square foot of surface, sometimes called the surface density. For
example, if a 10 × 10 ft concrete block wall weighs 2,000 lb, the “wall
weight” would be 2,000 lb per 100 sq ft, or 20 lb per sq ft. The thick-
ness of the wall is not directly considered.
From Fig. 8-2 you can see that the higher the frequency, the greater
the transmission loss, or in other words, the better the wall is as a bar-
rier to outside noises. The line for 500 Hz is made heavier than the
lines for other frequencies as it is common to use this frequency for
casual comparisons of walls of different materials. However, don’t for-
get that below 500 Hz the wall is less effective and for frequencies
greater than 500 Hz it is more effective as a sound barrier.
The transmission losses indicated in Fig. 8-2 are based on the mass
of the material rather than the kind of material. The transmission loss
through a layer of lead of certain thickness can be matched by a ply-
wood layer about 95 times thicker. But doubling the thickness of a con-
crete wall, for instance, would increase the transmission loss only
about 5 dB.