Page 111 - The Master Handbook Of Acoustics

86 CHAPTER FOUR

0 6 dB

Relative sound-pressure level, dB 10 Distance

20
double

30

1 2 4 8 16 32
Distance units

FIGURE 4-2
The inverse square law of sound intensity becomes the inverse distance law for sound
pressure. This means that sound-pressure level is reduced 6 dB for each doubling of
the distance.

Solution:
20 log 10/15 3.5 dB; the level is 80 3.5 76.5 dB.
What is the sound-pressure level at 7 ft?
Solution:
20 log 10/7 3.1 dB, and level is 80 3.1 83.1 dB.
All this is for a free field in which sound diverges spherically, but
this procedure may be helpful for rough estimates even under other
conditions.
If a microphone is 5 feet from an enthusiastic soprano and the VU
meter in the control room peaks +6, moving the microphone to 10 feet
would bring the reading down approximately 6 dB. The word
“approximately” is important. The inverse square law holds true only
for free field conditions. The effect of sound energy reflected from