Term: Total Harmonic Distortion (THD)  

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Any audio circuit adds some amount of distortion. The total harmonic distortion (THD) of a device or system is the sum of all the various distortion components compared to the original signal, typically expressed as a percentage. Pure audio amplifiers have a THD specification in the range of .001%. THD of 0.5% can be audible for pure sine waves, but for complex music waveforms, higher THD is acceptable. Professional loudspeakers may have THD specifications of between 0.1% and 5% measured within a specified frequency range.