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F.A.Q. Frequently asked questions.

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 given by 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 distorsion 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 60Hz, 7kHz and a 4:1 ratio (12dB).

Sound File

After the 3 beep tones, a 10s SMPTE IMD test tone (60Hz+7kHz@-12dB) slowly increasing from silence to 0dBFS.

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

Design your own IMD test tones using our Dual Frequency Test Tone Generator.

External Links

• IMD @ Wikipedia
• Rane note : audio specifications