Page 194 - The Master Handbook Of Acoustics

169
CONTROL OF INTERFERING NOISE

50 4,000 Hz 2,000 Hz 1,000 Hz 500 Hz
250 Hz

Transmission loss - dB 40 100 Hz

30

20
1 2 5 10 20 50 80
Wall weight - lb/sq ft
FIGURE 8-2
The mass of the material in a barrier rather than the kind of material determines the transmission loss of sound
going through the barrier. The transmission loss is also dependent on frequency although values at 500 Hz are
commonly used in casual estimates. The wall weight is expressed in pounds per square foot of wall surface.

A discontinuous structure such as bricks set in lime mortar con-
ducts sound less efficiently than a more homogeneous material like
concrete or steel. Unbridged air cavities between walls are very effec-
tive in sound reduction, but completely unbridged cavities are unat-
tainable. Only in the case of two separate structures, each on its own
foundation, is this unbridged condition approached.

Porous Materials

Porous materials such as fiberglass (rock wool, mineral fiber) are
excellent sound absorbers and good heat insulators, but they are of
limited value in insulating against sound. Using fiberglass to reduce
sound transmission will help to a certain extent, but only moderately.
The transmission loss for porous materials is directly proportional to
the thickness traversed by the sound. This loss is about 1 dB (100 Hz)
to 4 dB (3,000 Hz) per inch of thickness for a dense, porous material
(rock wool, density 5 lb/cu ft) and less for lighter material. This direct
dependence of transmission loss on thickness for porous materials is