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D-Tools MVP member Steve Feinstein of Atlantic Technologies sets us straight on the concept of distortion.

The term “distortion” is used pretty frequently in audio, but it’s not understood particularly well. There are different kinds of distortion, and some is more audibly bothersome than others.

By pure definition, if the output of an audio device differs in any way from the input, that difference is “distortion,” because theoretically, the device’s output should be a perfect mirror of the input signal. Nothing more, nothing less.

Well, unfortunately, this is the real world, and except for former Heavyweight Champion Rocky Marciano’s 49-0 record, nothing is perfect. Audio equipment produces distortion, and this distortion falls into two main categories-harmonic distortion and intermodulation distortion. We’ll explain both kinds (in simple terms, don’t worry!), and then you’ll know more than your friends.

Harmonic Distortion

This distortion is defined as unintended signal artifacts generated by an audio product that are whole number multiples of the original signal. For example, if an audio device is asked to reproduce a 40Hz signal, and instead produces 40Hz and a small amount of 80Hz + 120Hz + 160Hz + 200Hz, the 80Hz + 120Hz + 160Hz + 200Hz artifacts are called harmonic distortion. Small amounts of distortion close in multiples to the original signal (“lower-order harmonics,” like 80Hz) are barely audible; larger amounts of distortion in greater multiples far away from the original signal (“upper-order harmonics,” like 200Hz) are grossly objectionable to the human ear. The sum total of all harmonic distortion artifacts is expressed as a percentage of the original signal, or % Total Harmonic Distortion (% THD).

The human ear is most sensitive to midrange frequencies, so small amounts of distortion (less than 1%) in the 300-2000Hz range are clearly audible. We’re pretty insensitive to harmonic distortion in the bass range, so even 5-10% THD in the 20-60Hz region is usually not objectionable, as long as the device is not exhibiting other forms of misbehavior as well (mechanical scraping, buzzing, etc.). (See figure 1.) THD in the treble range is not usually an issue, because the harmonics are usually too high in frequency to hear: the first harmonic of 8kHz is 16kHz, which is beyond the audible range for most men, and only barely audible to women and children.

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Intermodulation Distortion

This is a bad one, because-unlike THD-it’s not harmonically-related to the original signal at all, so even very small amounts are audibly objectionable. IM distortion is when the distortion products occur at frequencies that are sums and differences of the original input signal. For example, if the input is 500 Hz and 2200 Hz, then the IM distortion products will occur at 1700 Hz and 2700 Hz. These frequencies have no musical relationship to the original frequencies at all, so the sound is very harsh and discordant. To be inaudible, IM distortion should always be less than 1% or so. (See figure 2.)

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Now when you read those THD and IM distortion specifications, you’ll know what they mean.

© 2009 Steve Feinstein, Atlantic Technology