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24.13.230.11
In Reply to: RE: By non-linear do you mean that compression and rarifaction are not exact mirrors of one another? nt posted by viridian on March 13, 2012 at 13:27:52
Air follows the “gas law” which means it with a large change in pressure, it is not a linear “spring”.
In the case of what we call airborne sound we reach 194dB when the negative pressure =0 and the positive 2X ambient pressure. It is possible to have a greater positive pressure however but then it’s called a shock wave. In a normal sine wave, the non-linearity can be seen once the intensity is in the high 150’s or above (dB). Because the positive pressure is slightly warmer, it travels faster and over many wavelengths, the sine wave processes into a saw tooth wave.
In audio, one has horn throat distortion which is related to the high frequency sound intensity and how slowly the horn area expands (the more slowly, the worse it is).
Follow Ups:
"In audio, one has horn throat distortion which is related to the high frequency sound intensity"Horn throat distortion, because of the high SPLs up in there, affects ALL frequencies produced by the horn.
Edits: 03/13/12
Actually it is the same effect as the "shocking up" (progression of a high intensity sine wave into a saw tooth wave), if sound were traveling though a pipe, then it is the intensity and number of wavelengths that govern this, the greater either is the greater the issue.
In a horn, the pressure falls with increasing area and so one finds for a given initial throat intensity, the problem is worse with increasing frequency and decreasing expansion rate. At the low frequency end of most horns response the driver linearity is by far dominant issue.
Best,
Tom
Hey Tom, thanks for the clarification/correction. I was over-generalizing my point.
As an aside... My long lost friend John Meyer, back in the early 70s, hit on the idea of using electronics to "pre-distort" the signal in basically the opposite manner that the horn distorts it, effectively cancelling a large percentage of the distortion and reducing it by a significant amount, resulting in a much cleaner sound quality. I thought that was pretty novel.
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