Page 137 - The Master Handbook Of Acoustics

112 CHAPTER FIVE

analyzer, which has a passband of fixed width as it is tuned through-
out the spectrum. One well-known analyzer of this type has a band-
width of 5 Hz. If white noise with its flat spectrum were measured
with a constant-bandwidth analyzer, another flat spectrum would
result because the fixed bandwidth would measure a constant energy
throughout the band shown in Fig. 5-18A.
Another very popular and convenient spectrum analyzer is the
constant percentage bandwidth analyzer. In this instrument the band-
width changes with frequency. An example of this is the one-third-
octave analyzer, commonly used because its bandwidth follows
reasonably well with the critical bandwidth of the human ear through-
out the audible frequency range. At 100 Hz the bandwidth of the one-
third-octave analyzer is only 23 Hz but at 10 kHz the bandwidth is
2,300 Hz. Obviously, it intercepts much greater noise energy in a one-
third octave band centered at 10 kHz than one centered at 100 Hz.
Measuring white noise with a constant-percentage analyzer would
give an upward-sloping result with a slope of 3 dB/octave, as shown in
Fig. 5-18B.
In audio-frequency measurements, the desired characteristic of many
instruments, rooms, etc. is a flat response throughout the frequency
range. Assume that the system to be measured has a characteristic almost
flat with frequency. If this system is excited with white noise and mea-
sured with the very convenient constant-percentage analyzer, the result
would have an upward slope of 3 dB/octave. It would be far more desir-
able if the measured result would be close to flat so that deviations from
flatness would be very apparent. This can be accomplished by using a
noise with a downward slope of 3 dB/octave. By passing white noise
through a filter, such as that of Fig. 5-19, such a downward sloping exci-
tation noise can be obtained. Such a noise, sloping downward at 3
dB/octave, is called pink noise. A close-to-flat system (amplifier, room)
excited with this pink noise would yield a close-to-flat response, which
would make deviations from flatness very obvious. For such reasons
pink noise is here to stay.

Signal Distortion

Our discussion of the various signals encountered in audio is
incomplete without at least an acknowledgment of what can happen