Sound Card Inharmonic (Intermodulation) Distortion

Intermodulation distortion results when two or more different frequency components interact within a nonlinear system. The output will then contain not only harmonics of the original frequencies, but also components at sum and difference frequencies that typically aren't harmonics of either input. This is also known as "intermodulation" or "IM" distortion (IMD), since the result is similar to multiplying two sinusoids together.

You can demonstrate these sum and difference products by using the Generator setup described for clipping distortion. Set the two sine waves to different frequencies and set both amplitudes over 50%. Clipping will appear on the composite waveform, but not on every cycle... only where both waves are aligned.

For example, set both Levels to 60% and set the frequencies to 500 and 700 Hz. Notice that the most-positive waveform peaks are clipped, and that this happens about every 5 msec. (The same is true for negative peaks.) Now, 5 msec "just happens" to be the period of a 200 Hz wave, which is the difference between 500 and 700 Hz, and you will indeed see a 200 Hz peak in the spectrum, as well as one at the sum frequency of 1200 Hz.

In fact, you will see peaks at all multiples of 200 Hz. If we call the higher frequency f2 and the lower f1, then all these peaks represent "difference tones" at f2 - f1, 2f1 - f2, and all integer multiples Mf1 - Nf2. (Negative and positive frequencies are equivalent here.)

IM distortion is typically a problem when you are generating two tones from the same speaker or other source at high sound levels. Since any source is nonlinear at high levels, the sound output will include not only the two tones you desire (plus harmonic distortion products from each), but also intermodulation distortion products at inharmonic sum and difference frequencies. You must particularly minimize the intermodulation products if you are studying similar products that are generated by nonlinearities in the system under test... such as an ear or a microphone.

IMD is usually measured by applying two primary tones and looking at the distortion products. There are three different standards in common use, involving different primary frequencies and levels. Daqarta's IMD_Meter macro mini-app supports all three standards.

See also Creating Low IMD Acoustic Signals, Distortion - Theory And Measurement