848 Chapter 29
Figure 29.3 shows the so-called mass law of sound insulation, which shows that sound
insulation increases by approximately 5 dB for every doubling of the mass. A number
of typical constructions and building elements have been drawn on for comparison
purposes.
It should be noted that the sound-insulating properties of a material are also affected
by the frequency of the sound in question. In fact the insulation generally increases by
5 dB for every doubling of frequency. This is good news as far as the insulation of high-
frequency sound is concerned, but bad news for the lower frequencies.
Generally, a single value of sound insulation is often quoted—this normally refers to the
average sound insulation achieved over the range 100 Hz to 3.15 KHz and is referred to as
the sound reduction index (SRI). Sound transmission class (STC) in the United States is
based on the average between 125 Hz and 4 KHz.)
An approximate guide to the performance of a material can be obtained from the
500-Hz value of insulation as this is often equivalent to the equated SRI. For
example, our 112-mm brick wall will tend to have an SRI of 45 dB and have a sound
insulation of 45 dB at 500 Hz, 40 dB at 250 Hz, and only 35 dB at 125 Hz, which
begins to explain why it is always the bass sound or beat that seems to be transmitted.
In practice, therefore, the studio designer or acoustic consultant fi rst considers
controlling break-in (or break-out) of the low-frequency sound components, as
overcoming this problem will invariably automatically sort out any high-frequency
problems.
Table 29.2 : Sound Absorption
Absorption coeffi cient Percentage absorbed
(%)Insulation produced
(dB)Incident sound energy
transmitted (%)
0.1 10 0.5 90
0.5 50 3 50
0.7 70 5 30
0.9 90 10 10
0.99 99 20 1
0.999 99.9 30 0.1
0.9999 99.99 40 0.01