Text: Distortion, in playback, is any change in the recorded sound that takes place in the playback system. At its worst, distortion can make a violin sound like a trolley car screeching around a curve. More frequently, however, distortion is quite subtle and barely perceptible at first. But in prolonged and attentive listening it causes a sense of discomfort known as listener fatigue. In recent years, improvements in audio design have reduced distortion in quality components to such a low level that it does not obtrude upon the listener even after many hours of concentrated listening. Distortion exists in two principal forms: harmonic distortion, which falsifies tonal nuances, and intermodulation distortion (usually called IM), which results from the interaction of various frequencies within the playback components and produces a harsh, grainy sound. Precise numerical statement in the manufacturer's specifications of both these types of distortion is a hallmark of trustworthy sound equipment. In high-fidelity components, distortion is usually expressed as a percentage of the total sound at a certain level of output power. High-quality amplifiers should have less than 2 percent IM and harmonic distortion when operating at full-rated output.

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