Page 139 - The Master Handbook Of Acoustics

114 CHAPTER FIVE

a pure tone is impressed on the ear, harmonics can be heard. If two
loud tones are presented simultaneously, sum and difference tones
are generated in the ear itself; and these tones can be heard as can
their harmonics. A cross-modulation test on an amplifier does
essentially the same thing. If the amplifier (or the ear) were perfectly
linear, no sum or difference tones or harmonics would be generated.
The production within the component of frequency elements that
were not present in the input signal is the result of nonlinear distor-
tion.
Transient distortion Strike a bell and it rings. Apply a steep wave-
front signal to an amplifier and it might ring a bit too. For this reason,
signals such as piano notes are difficult to reproduce. Tone burst test
signals are an attempt to explore the transient response characteristics
of equipment, as are square waves. Transient intermodulation (TIM)
distortion, slew induced distortion, and other sophisticated measuring
techniques have been devised to evaluate transient forms of distortion
in systems.

Harmonic Distortion

The harmonic distortion method of evaluating the effects of circuit
nonlinearities is probably the oldest and the most universally
accepted method. It certainly is the easiest to understand. In this
method the device under test is driven with a sine wave of high purity.
If the signal encounters any nonlinearity, the output waveshape is
changed, i.e., harmonic components appear that were not in the pure
sine wave. A spectral analysis of the output signal is made to measure
these harmonic distortion products. The most revealing method is to
use a wave analyzer having a constant passband width of, say, 5 Hz,
which can be swept through the audio spectrum. Figure 5-20 shows
illustrated results of such a measurement. The wave analyzer is first
tuned to the fundamental, f 1 kHz, and the level is set for a conve-
o
nient 1.00 volt. The wave analyzer is then tuned to the 2 kHz region
until the 2f second harmonic is found. The voltmeter, which is a part
o
of the analyzer, reads 0.10 volt. The third harmonic at 3 kHz gives a
reading of 0.30 volt, the fourth a reading of 0.05 volt and so on up the
frequency scale. Beyond 6f 6 kHz no measurable components were
o
found after diligent search. The data are then assembled in Table 5-2.