Page 136 - The Master Handbook Of Acoustics

111
SPEECH, MUSIC, AND NOISE

White and Pink Noise
References to white noise and pink noise are common and sometimes
confusing. What is the difference? White noise is analogous to white
light in that the energy of both is distributed uniformly throughout the
spectrum. In other words, white noise energy exhibits a flat distribu-
tion of energy with frequency (Fig. 5-18A).
White light sent through a prism is broken down into a range of
colors. The red color is associated with the longer wavelengths of
light, that is, light in the lower frequency region. Pink noise is noise
having higher energy in the low frequencies. In fact, pink noise has
come to be identified specifically as noise exhibiting high energy in
the low-frequency region with a specific downward slope of 3 dB per
octave (Fig. 5-18C). There is a practical reason for this specific slope.
These two colorful terms arose because there are two types of
spectrum analyzers in common use. One is the constant bandwidth

40
B
20 3 dB per octave

White noise
dB 0 A

20 3 dB per octave
Pink noise filter characteristic
C
40
20 Hz 100 Hz 1 kHz 10 kHz 20 kHz
Frequency - Hz
FIGURE 5-18
Random noise has constant energy per Hz. If the spectrum of random noise is measured
(white) with a wave analyzer of fixed bandwidth, the resulting spectrum will be flat
with frequency as in A. If measured with an analyzer whose passband width is a given
percentage of the frequency to which it is tuned, the spectrum will slope upward at 3
dB per octave, as in B. By processing the white noise spectrum of A with a filter that
slopes downward at 3 dB per octave, such as in C, a flat response results when constant
percentage bandwidth filters are used such as octave or one-third octave filters. In mea-
suring a system, pink noise is applied to the input and, if the system is flat, the read
out response will be flat if one-third octave filters, for example, are used.