Page 142 - The Master Handbook Of Acoustics

117
SPEECH, MUSIC, AND NOISE

In Fig. 5-21 an undistorted sine wave is
sent through an amplifier, which clips Undistorted sine wave
positive peaks. On the left, the flattening
of the positive peaks with 5% THD is evi-
dent, and shown below is what the com-
bined total of all the harmonic products
look like with the fundamental rejected.
On the right is shown the effect of greater Symmetrically clipped
5% THD 10% THD
clipping to yield 10% THD. Figure 5-22
shows what happens when the sine wave
passing through the amplifier is symmetri-
cally clipped on both positive and nega-
tive peaks. The combined distortion
products for symmetrical clipping have a This is what the harmonics look like on the cathode
somewhat different appearance, but they ray oscilloscope to the same scale as above.
measure the same 5% and 10% THD.
In all of this exercise keep in mind that
consumer-type power amplifiers commonly
have specifications listing total harmonic
distortion nearer 0.05% rather than 5% or FIGURE 5-22
10%. In a series of double-blind subjective
Cathode ray oscillograms show a pure, undistorted
tests Clark found that 3% distortion was sine wave, which is applied to the input of an
11
audible on different types of sounds. With amplifer which clips both positive and negative peaks
carefully selected material (such as a flute in a symmetrical fashion. The appearance of the
clipped sine wave is shown for 5% and 10% total har-
solo) detecting distortions down to 2% or
monic distortion. The appearance of the harmonics
1% might be possible. A distortion of 1% alone, with the fundamental filtered out, is also
with sine waves is readily audible. shown.

Endnotes

1
Flanagan, J. L, Voices of Men and Machines, J. Acous. Soc. Am., 51, 5, Part 1, May 1972,
1375–1387.
2 Kuttruff, Heinrich, Room Acoustics, London, Applied Science Publishers, Ltd., 1979, 16, 17.
3
Hutchins, Carleen M., and Francis L. Fielding, Acoustical Measurement of Violins, Physics
Today, July 1968, 35–41. Contains extensive bibliography.
4
Fletcher, Harvey, The Ear As a Measuring Instrument, J. Audio Eng. Soc., 17, 5, October
1969, 532–534.
5
Sivian, L.J., H.K. Dunn, and S.D. White, Absolute Amplitudes and Spectra of Certain Musi-
cal Instruments and Orchestras, J. Acous. Soc. Am., 2, 3, January 1931, 330–371.