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So low they are given it away

Hi

“How low” distortion has to be, to be inaudible is highly dependent on frequency and a lesser degree on level (against a silent background).
The reasons are described in an earlier post about the equal loudness curve and masking.
The bottom line is that for example at the threshold of audibility at 20Hz, the 3rd harmonic of 7% has the SAME apparent loudness as the fundamental.
In the research Dolby Labs did on subwoofer performance, at the time, there were NO home subwoofers that could produce 20HZ audibly and not also produce significant audible distortion.
On the other hand how sensitive is one to the 3rd harmonic of 7KHz? (your hearing is already –75dB if your young).
Essentially, high levels of harmonic distortion are inaudible IF the fundamental is already a high enough in frequency. Conversely, it is essentially not possible to make a loudspeaker low enough in distortion to produce significant output at 20HZ without audible distortion. The range in between is, well in between.

If music is the program material, then odd harmonics are more annoying as they are dissonant.
If your listening to natural sounds (which are generally not harmonic), then either even or odd distortion is annoying.
Beginning with one of the Beetles albums, it has been common to electronically ADD even harmonics in the recording process, this adds a richness to the sound. In the beginning it was done with off biased tube circuitry, now days there are tons of processors and sweeteners to do this.

Also as I mentioned earlier, the higher the order the harmonic is, the easier it is to hear. It has been suggested that midband, about a -10dB per octave slope should be judged as “equal loudness” for the ascending harmonics.
Thoughts on the rest take too long.

Tom Danley
Danley Sound Labs

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