Page 128 - Master Handbook of Acoustics

FIGURE 5-20 Oscillograms show an undistorted sine wave, which is applied to the input of an
amplifier which clips both positive and negative peaks in a symmetrical fashion. The appearance of
the clipped sine wave is shown for 5 and 10% THD. The appearance of the harmonics alone, with
the fundamental filtered out, is also shown.

Consumer power amplifiers commonly have specifications listing THD nearer 0.05% rather than 5
or 10%. In a series of double-blind subjective tests, Clark found that 3% distortion was audible on

different types of sounds. With carefully selected material (such as a flute solo), detecting distortions
down to 1 or 2% might be possible. A distortion of 1% with sine waves is readily audible.

Resonance

The amplitude of vibration of any resonant system is maximum at the natural or resonant frequency f 0
and is less at frequencies below and above that frequency. A modest excitation signal at that
resonance frequency will result in high amplitude. As shown in Fig. 5-21, the amplitude of vibration

changes as the frequency of excitation is varied, going through a peak response at the frequency of
resonance. Perhaps the simplest example of a resonant system is a weight on a spring.