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 Files
| SMPTE IMD Test 60Hz+7kHz |
The first file is the classic IMD test. 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.
| MULTI-TONE IMD Test 10 Octaves |
The second file is a multi-tone IMD. Essentially, it is a type of IMD test that more closely represents music by playing multiple tones crossing every octave. This test signal comprises of a series of 10 sine tones from 31.25 Hz up to 16 kHz, one tone for each octave in the hearing range. Their amplitudes are decreasing by 3dB per octave. The test signal itself is normalized to -6 dBFS. Play the signal through your audio system and capture the output using a USB audio interface. You're looking for any intermodulated harmonics; harmonics that were not present in the original signal.
Related pages
IMD Test Tone Generator
External Links
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