Total Harmonic Distortion (THD) is the cumulative distortion found in a pure audio signal as it goes through an audio device. We already have introduced THD in our Total Harmonic Distortion test page. The standard test uses a 1kHz sine tone. This page uses a different frequency: 125Hz. You can change that frequency on the top of this page.
One kilo Hertz is located around the middle of our logarithmic hearing range; a spot where our ears exhibit a higher sensitivity. Therefore, it subjectively appear louder than any other frequency.
Carrying a THD test at 1kHz can be flattering: because of its apparent increased loudness, the fundamental (1kHz) will achieve a higher masking of the distortion that corrupts the signal. You now better understand why manufacturers are keen to use the 1000 Hz standard. And that is not the only reason. When a sine tone distorts, odd harmonics will appear (by clipping a sine tone, you get closer to the square wave which is characterized by a series of odd harmonics). At 1kHz, these harmonics are 3kHz, 5k, 7k, 9k, 11k, 13k, 15k, 17k and 19kHz: that's 9 harmonics across the audible spectrum. As you notice, these harmonics rapidly enter the ultra-high frequency range, a sonic spot that is only accessible to your ears if you have an excellent hearing. These are the reason why a 1kHz tone can accommodate distortion better than any other tone, its apparent loudness, its higher masking, the few and hard to hear harmonics.
At 1kHz, the standard THD test tolerates relatively high levels of distortion. By decreasing this frequency, we will shift the fundamental towards a frequency range where our hearing is not as sensitive anymore, and allow the harmonics produced by the distortion, to fall back into the frequency area where our sensitivity is maximal. Lower sine tone frequencies make distortion easier to hear.
In addition, the number of audible harmonics increases drastically as the frequency decreases. For example, if you perform the test at 100Hz, distortion will spread over 65 harmonics (300Hz, 500, 700, 900, ... 19500Hz) - to be compared with the 9 harmonics of the classic THD test (see above).
Each file plays a 125Hz sine tone (5s) at -6dBFS with increasing THD levels.
Start with the pure tone (O%) and then increase the THD until the tone coloration/distortion becomes audible to you.
Beware, your equipment's own THD will corrupt this listening test! You will not be able to discriminate THD figures lower than what your own audio equipment gives you access to.
If your headphones suffer from a 0.1% THD (e.g. the Sennheiser HD-600 reference headphones), a pure sine tone (0% THD) will reach your ears with 0.1% distortion. You won't be able to discriminate THDs lower than 0.1% under these conditions.
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