Total Harmonic Distortion (THD) is used to measure the current noise intensity produced by the device and may be the most common audio metric other than frequency response. During the test, a single sine wave frequency with known harmonic purity passes through the test device and enters the distortion measuring instrument. Based on the reference measurement level, the instrument calculates the frequency used for the test and then passes the residual signal through a set of band limiting filters that are adjusted according to the bandwidth (usually 20 Hz-20 kHz). The last thing left is noise, which includes the humming or disturbing sound of AC wires, and all the harmonic distortions produced by the device.
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