Measurement of Harmonic Distortion

Author: N.H. Crowhurst

The presence of these harmonics can be measured by using a wave analyzer. It is quite a complicated and expensive piece of measuring equipment that has a frequency selective amplifier that permits it to measure the amplitude of any particular frequency in a composite output waveform. By setting the frequency dial first to the fundamental frequency of a pure sinusoidal input and then to successive harmonics, the component of each harmonic in the output waveform can be measured to find out how much total distortion is produced.

Measuring the harmonic distortion using a wave analyzer

The use of the wave analyzer is rather a long-winded method, so a simple distortion measuring set is usually used to give the answer quite quickly. This method uses another kind of frequency-selective filter to eliminate the fundamental. Two positions are provided on the switch: one for measuring the amount of fundamental and the other for measuring the total amount of audio after the fundamental has been removed. This gives a quick and ready means of measuring the total distortion.

In the early days of audio amplifiers 5% harmonic was considered a good figure of distortion. At that time, tests were made which showed that human hearing could barely detect 5% of second harmonic. If the distortion is third harmonic, about 1.5% is just audible. At higher harmonics lower percentages become audible. Modern amplifiers produce harmonic distortion figures that are a fraction of 1%. According to the tests just described, this distortion should be completely inaudible.

Using a distortion meter