Page 54 - The Master Handbook Of Acoustics

29
SOUND LEVELS AND THE DECIBEL

RMS pounds per sq. in.
10 9 10 8 10 7 10 6 10 5 0.0001 0.001 0.01
120

100
Pounds per sq. in. (read top scale)
Sound pressure level - dB 60 Pascals (read bottom scale)
80

40

20

0
0.00001 0.0001 0.001 0.01 0.1 1 10 100 1000
RMS sound pressure - Pascals
FIGURE 2-1

The relationship between sound pressure in Pascals or pounds per square inch and
sound-pressure level (referred to 20 uPa) is shown in this graph. This is a graphical
approach to the solution of Eq. 2-2.

When a statement is encountered such as, “The sound pressure
level is 82 dB,” the 82-dB sound-pressure level is normally used in
direct comparison with other levels. However, if the sound pressure
were needed, it can be computed readily by working backward from
Equation 2-3 as follows:
p 1
82 = 20 log
20 µPa
p 1 82
log =
20 µPa 20
82
p 1
=10 20
20 µPa
x
The y button on the calculator (the Hewlett-Packard 41-C is assumed)
x
4.1
helps us to evaluate 10 . Press 10, enter 4.1 press y power button, and
the answer 12,589 appears.