Intermodulation Distortion (IMD) Sound Test

Background

Intermodulation distortion (IMD) measures the level of unwanted combinations of different frequencies found in the input signal. IMD adds components to the sound that are not found in the original signal. This effect results from non-linearities in your audio system.

For example, two frequencies, f1 and f2 may produce new frequencies such as f2-f1; f1+f2; f2-2f1; f2+2f1 or any other intermodulation component:

m*f2 ± n*f1

where m and n are integers. When a complex musical passage is the source, the IMD distortion can be quite extraordinary!

While a small amount of IMD will often be difficult to hear, it has always been desirable to reduce intermodulation distortion to the absolute minimum.

Standardized IMD Test

IMD measurements use a two sine wave test signal consisting of a large amplitude low frequency tone linearly mixed with a high-frequency tone at much lower amplitude. The most common standard is SMPTE/DIN which uses 60 Hz, 7 kHz and a 4:1 ratio (12 dB).

The Sound File

SMPTE IMD Test
60Hz+7kHz

After the 3 beep tones, you will hear a 10 s SMPTE IMD test tone, slowly increasing from silence to 0 dBFS.

As the level of the test tone increases, both frequencies should remain pure and clean. Any warbling of the highest frequency denotes the presence of excessive IMD.

Related pages

IMD Test Tone Generator

External Links

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