Creating Low IMD Acoustic Signals via Sound Card

To measure distortion in acoustic systems, you need a good microphone with known low distortion of its own. But to measure the harmonic distortion of a microphone, you would need a pure acoustic wave with no distortion. Yet how would you know the acoustic wave's distortion, unless you already had a known microphone?

The solution is to use intermodulation measurements instead of harmonic distortion measurements, since harmonic distortion of the individual sources does not compromise the results.

The best way to get two tones with no intermodulation is to create them separately via two different loudspeakers or other drivers and let them combine acoustically. Since air is much more linear than any driver, there will be very little intermodulation. Some small amount may arise, particularly if the sound must be mixed in a closed system, due to sound pressure from one source moving the other.

Using the Lines mode of the Daqarta Generator to create the two tones will insure that they fall exactly on spectral lines, along with all harmonic and inharmonic distortion products, to allow simple spectral measurements without leakage "skirts".

Note that this technique does nothing to reduce harmonic distortion, which is generated separately by each source.

The dual-source method is also useful when studying the "difference tone" distortion products generated by the ear itself. These cause tiny "oto-acoustic emissions" of sound from the inner ear, which can be measured by a sensitive microphone in the ear canal for clinical diagnosis of certain conditions. The procedure is often referred to as "DPOAE" for Distortion Product Oto-Acoustic Emissions.

See also Distortion - Theory And Measurement