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In Reply to: RE: Ultrasonic Nonsense, speakers, etc. What do you guys think? posted by g_greenamyer@yahoo.com on March 11, 2012 at 20:48:58
Ultrasonic signal behave nonlinearly in air? that's right, two ultrasonic tones - say 100KHz and 101KHz produce an audible tone at 1KHz!
Air induced distortion - it is also dependent on level or the signals. That's right - you can produce a heterodyne loudspeaker that produces audio frequency sound "over there" but not here by using two ultrasonic loudspeakers.
Three most important things in Audio reproduction: Keep the noise levels low, the power high and the room diffuse.
Follow Ups:
"Ultrasonic signal behave nonlinearly in air? that's right, two ultrasonic tones - say 100KHz and 101KHz produce an audible tone at 1KHz!
Air induced distortion - it is also dependent on level or the signals."You're confusing two different things.
Yes, air is nonlinear - progressively distorting more and more as the SPL is raised.
But, your example is about "beat frequencies" - the periodic alignment/non-alignment of the pressure of two different tones. This can easily be demonstrated by simply singing a tone which is slightly different than the tone on a record, or by listening to the result of two musicians playing the same tone but slightly out of tune with each other.
Edits: 03/13/12
If one were to produce a signal at say 30KHz and another at say 30.1KHz, there IS a 100hz beat (amplitude modulation ) effect BUT only when there is non-linearity, does the 100Hz become audible.Non-linearity produces a sum and difference frequency when two signals add, a beat frequency is the rate of amplitude modulation of two periodically adding and canceling signals such as one note played on two nearly in tune guitar strings.
Air in not the only thing that can be non-linear too, ultrasonic energy within an audio system can cause other audible effects when the system has significant non-linearity.
Amplifiers for example normally have a small series choke at the output with the intention of DE-coupling the output stage from the speaker wire which can act like an antenna or parasitic capacitance.Fwiw, if i am not mistaken, the big guy is a hifi buff who happens to also be a rep for a VERY highly respected (as in world wide) line of acoustic and vibration measurement equipment.
Edits: 03/13/12
nt
The term "non-linear", as used here, refers to a component (such as air) in the chain reproducing the input signal as a disproportionately distorted signal as the input signal varies. For example, air more or less reproduces sound normally at typical day-to-day sound levels, but has disproportionally increasing distortion as the sound level increases.
This is one reason why most horns have "issues". The SPL in the throat can be extremely high.
hth
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).
"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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