Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics.

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Re: Good points on harmonic spectrum.

Posted by Jon Risch (B) on October 7, 2003 at 14:28:54

In Reply to: Good points on harmonic spectrum. posted by Tony Montana on October 3, 2003 at 12:44:19:

Depends on a lot of things.

Lets look at how a distortion component could actually affect the primary amplitude response first.

It is possible to have a non-flat response due to distortion. Add-in additional signals at a level of approx. -20 dB, and the amplitude response versus frequency can vary UP TO +/-1 dB. Added signals at levels of -40 dB can affect the amplitude response up to +/-0.1 dB. However, some measurement systems will not show this, due to frequency tracking or other forms of signal averaging/processing.

So your assumption of a flat response when there is distortion present may already be in jeopardy.

Now on to your original question. Whether or not one can hear a change in distortion spectra depends on several things: the absolute levels of distortion, the order of the distortion, the type of distortion, as well as how it is being measured.

All of these will affect whether or not a given change in distortion spectra levels could be detected audibly.

If the distortion is 5th or 7th order harmonic distortion, and the 1 dB toggles it across the threshold of detection, then the obvious answer is that, yes, a 1 dB change could be detected.

Another example would be classic power amplifier output stage notch distortion. Many of the THD meters/software-hardware systems use heavy signal averaging to deri9ve the THD figure. The power amp notch distortion get averaged down to a very small percentage, say the meter reads 0.1% THD, yet the peak values of the notch distortion could actually be reaching several percent or worse.

By the time the averaged reading came up 1 dB (from 0.1% to 0.12%), the peak levels might have risen another percent ot two, and it would clearly be audible as an even more frazzled sounding amp.

Jon Risch

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