Audio Engineering

74 Chapter 2

The second method is more useful. It uses a tunable wave analyzer. This instrument
allows measurement of the amplitudes of the fundamental and each harmonic, as well
as identifying the hum, the amplitude, and the noise spectrum shape ( Figure 2.20 ). Such
analyzers come in many different bandwidths, with a 1/10 octave unit allowing readings
down to 1% of the fundamental (it is  45 dB at 2 f ). By looking at Figure 2.20 , it is easy
to see that harmonic distortion appears as a spurious noise. Today, tracking fi lter wave
analysis allows nonlinear distortion behavior to be “ tracked ” or measured.

2.25 The Acoustical Meaning of Harmonic Distortion ...................................................

The availability of extremely wide-band amplifi ers with distortions approaching the
infi nitesimal and the gradual engineering of a limited number of loudspeakers with
distortions just under 1% at usable levels (90 dB SPL –100 dB SPL at 10–12 ft) brings up
an interesting question: “ How low a distortion is really needed? ”

Sound

system

loudspeaker

Sound

level

meter

Sound system

power

amplifier

Sinewave

oscillator

(^1) / 3 Octave
wave
analyzer
Graphic level
recorder
Use if available
Figure 2.19 : Measurement of harmonic distortion.
Response (dB)Ambient
noise
“Band rejection”
distortion analyzer
“Band pass”
wave analyzer
f 0 dB down
5 dB
3 f 25 dB down
2 f 36 dB down
Frequency (Hz)
500 2k 5 k
Figure 2.20 : Methods of measuring distortion.