Environmental Engineering FOURTH EDITION

428 ENVIRONMENTAL ENGINEERING

80

70

40

30

Over-all 63 125 250 500 1000 2000 4000 8000

Octave band center frequency

Figure 223. 'Qpical analysis of a machine noise with background noise.

where P and PEf are the pressure and reference pressure, respectively. If we define

P,,f = 0.00002 N/m2, the threshold of hearing, we can define the SPL as

(22.5)

Although it is common to see the sound pressure measured over a full range of
frequencies, it is sometimes necessary to describe a noise by the amount of sound
pressure present at a specific range of frequencies. Such afrequency analysis, shown
in Fig. 22-3, may be used for solving industrial problems or evaluating the danger of
a certain sound to the human ear.
In addition to amplitude and frequency, sound has two more characteristics of
importance. Both may be visualized by imagining the ripples created by dropping a
pebble into a large, still pond. The ripples, analogous to sound pressure waves, are
reflected outward from the source, and the magnitude of the ripples is dissipated as
the ripples get farther away from the source. Similarly, sound levels decrease as the
distance between the receptor and the source is increased.
In summary, then, the four important characteristics of sound waves are as follows:

0 Sound pressure is the magnitude or amplitude of sound.

0 The pitch is determined by the frequency of the pressure fluctuations.

0 Sound waves propagate away from the source.

0 Sound pressure decreases with increasing distance from the source.

The mathematics of adding decibels is a bit complicated. The procedure may be

simplified a great deal by using the graph shown as Fig. 22-4. As a rule of thumb,