Of all the contributors to audio fatigue, high-frequency distortion is the most insidious because it operates below the threshold of conscious identification. A listener will rarely say “this amplifier’s intermodulation distortion is fatiguing me.” They will say the system sounds harsh, aggressive, or tiring — without being able to identify a specific cause. Understanding the link between high-frequency distortion and fatigue provides a framework for both diagnosing the problem and resolving it.
Intermodulation distortion — IMD — occurs when two or more frequencies interact within a non-linear device to produce sum and difference frequency products that were not present in the original signal. Unlike harmonic distortion, which produces musically related overtones, intermodulation products are often inharmonic — not related to the original frequencies by simple integer ratios. They sit in between musical intervals, creating a background texture of dissonance that the auditory system finds difficult to process and impossible to ignore.
Music is particularly vulnerable to this because it is inherently multi-tonal. A piano chord, a string ensemble, a vocal with acoustic guitar accompaniment — all produce multiple simultaneous frequencies that interact within the amplifier, the speaker crossover, and even the drivers themselves. The more complex the signal, the greater the potential for intermodulation products, and the higher the distortion level at any given output power.
Class B amplifiers — which switch between two output devices at the zero crossing of the waveform — are particularly susceptible to crossover distortion, a form of distortion that produces high-order odd harmonics concentrated in the upper midrange and treble. Even at low levels, where the total THD figure may appear benign, crossover distortion injects a fine-grained roughness into the upper frequencies that accumulates into fatigue over extended listening.
High-frequency driver distortion — tweeter breakup, voice coil nonlinearity at high excursion levels, and ferrofluid resonances in compression drivers — adds further complexity to the distortion picture at the speaker end of the chain.
The presence of distortion in a system does not always manifest as an obvious, identifiable harshness. Sometimes it simply makes music less enjoyable to listen to without explanation. That is distortion at work.