Page 126 - Master Handbook of Acoustics

TABLE 5-3 Harmonic Distortion Products (Fundamental f = 1 kHz, 1.00 V amplitude)
0

The total harmonic distortion (THD) may be found from the expression:

where e , e , e , …, e =
n
4
3
2
voltages of second, third, fourth, etc., harmonics.
e =
o
voltage of fundamental

In Table 5-3, the harmonic voltages have been squared and added together. Using the equation:

A THD of 37.8% is a very high distortion that would make any amplifier sound poor on any type
of signal, but the example has served our purpose.
A simple adaptation of the THD method can also be used. Consider Fig. 5-18 again. If the f 0
fundamental were adjusted to some known value and then a notch filter were adjusted to f essentially
0
eliminating it, only the harmonics would be left. Measuring these harmonics together with an root-
mean-square (RMS) meter accomplishes what was done in the square root portion of Eq. (5-1).
Comparing this RMS measured value of the harmonic components with that of the fundamental and

expressing it as a percentage would give the total harmonic distortion (THD).
An undistorted sine wave is sent through an amplifier, which clips positive peaks, as shown in
Fig. 5-19. On the left, the flattening of the positive peaks with 5% THD is evident, and shown below
that is the combined total of all the harmonic products with the fundamental rejected. On the right is
shown the effect of greater clipping to yield 10% THD. Figure 5-20 shows a sine wave passing
through the amplifier and symmetrically clipped on both positive and negative peaks. The combined

distortion products for symmetrical clipping have a somewhat different appearance, but they measure
the same: 5 and 10% THD.