Comment by Anechoic
14 years ago
For example, the human hear will hear a 30kHz frequency if it's fundamental is 10kHz
You are going to have to provide me with a citation to back that up because that goes against everything I've learned and experience in 17 years of working in acoustics.
Here is a Wikipedia article on the subject: http://en.wikipedia.org/wiki/Sound_from_ultrasound
Basically, if you produce two ultrasonic frequencies, they will create an interference pattern at a much lower frequency than either of the individual frequencies. Modulate a signal on the difference between two signals, and you can create a directional speaker, since ultrasonic sounds tend to be highly directional (so long as the diameter of the transducer is greater than 1/2 wavelength, which is almost guaranteed with ultrasonic signals). This is how the "sound cannons" that are being deployed for crowd control work.
That article describes hetrodyning which happens because ultrasonic frequencies at high amplitudes interacts nonlinearly with air. You are not going to see that effect with sound waves generated near the audible spectrum, and normal loudspeakers are going to generate ultrasonic sound waves.
Yes, but the effects of interference patterns between multiple ultrasonic frequencies is the same, and definitely does affect the audible spectrum. This is why we must filter the square wave that comes out of a DAC. And the limitations of filters (phase shifts and roll-off) are why modern CD players oversample the signal--so that the filtering can be performed well beyond the audible spectrum.
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Recording music is supposed to be a snapshot (with room for interpretation) of the composition at play.
Trying to record an edge case like this is the same as recording in a room with bad acoustics. So you end up with some weird (but not faithful) representation of the sound which is a snapshot of the microphone's characteristics and directionality of the ultrasonic tones. It's not reasonable to assume any microphone will behave exactly like a human ear. Even if you could, you're going to have to mimic the tiny random movements a normal person would make listening to a sound, movements which would definitely impact the perception of the sound, because microphones are much more stationary than any human would be.
The "different sounding" argument two posts above is silly, because sound is almost never that monochromatic, and if it is, it's usually boring. Also I don't understand how missing out on an odd order harmonic would be a bad thing :) The reality is none of these arguments are based in a reality of what people would hear, and because of that, the arguments aren't practical.
In reality, 20 bits at 48kHz (or 64kHz) would be more than acceptable for even the most discerning of ears and probably the most practical in terms of space and fidelity, but it'd be a weird format to distribute in.
That's very cool, but it requires pretty high-intensity ultrasound to be noticeable. I doubt that will be the case with ordinary music.
> Basically, if you produce two ultrasonic frequencies, they will create an interference pattern at a much lower frequency than either of the individual frequencies.
So the interference pattern will be made up of one low frequency sound and higher frequency harmonics. Once again the higher frequency harmonics are redundant, because you only need to record the lower frequency sound.
The only possible way ultrasound can be picked up by the ear is if the ear has a non-linear response to the input sound. Going by the information in the article linked, it is highly unlikely that any significant non-linearity exists in the ear.
It's definitely possible for two sounds to be indistinguishable when played separately, but when played together it is revealed that they are in fact different (see link below). Whether this applies for sounds with frequencies above 20kHz I don't know. I'd like to see a citation as well. Doesn't seem like it would be the hardest experiment to set up either.
http://en.wikipedia.org/wiki/Cent_(music)#Sound_files
Me and my brother would sing at each other in certain tones such that we created harmonics in both our ears. It wasn't pleasant, but it was interesting. Regardless, I'd smash my equipment if it made harmonics like that.
Humans will hear the impact > 20kHz frequency has on the lower frequencies, not the 30kHz frequency itself. That's been proven a million times.
If that is true, surely in your up thread example of recording a triangle, the "impact on lower then 20kHz frequencies" would already have happened during the recording process in between the triangle and the microphone, and would have been captured perfectly on recording equipment that's proven capable of capturing everything below 20kHz? So we'd "hear" the effect as part of the recording instead of requiring it to happen in our listening room…
>That's been proven a million times.
Then you should be able to provide at least one citation.
If you're not going to hear the frequency, then there's no reason to record it, so I don't see what you're objection is.
Yes, but if you sample the frequency to create a step wave, then neglect to filter the results, you will end up reproducing tons of high frequencies. That is why we need to filter the output for signals >20KHz...to remove these harmonics that result from reproducing the square wave.
Of course, filters aren't perfect, and result in phase shift and roll-off. So we over-sample the signal to create a signal with a much higher frequency than 20KHz, so that the filtering occurs well outside the audible band, allowing us to filter out all of these harmonics without affecting the desired signal.
Basically, the end result is that by sampling the signal, you are introducing high frequency content that must be removed prior to playback. This high frequency content is one of the reasons old CD players from the 80s and 90s cause "listener fatigue", although I have no sources to back up that last statement.
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In other words, your theory is that the superposition principle doesn't hold for sound waves.
Well, the superposition principle only holds in linear media. Sound waves can propagate in linear media, but they can also propagate in nonlinear media, and any medium that can carry sound will go nonlinear at sufficiently high amplitudes.
Note the lack of citation
http://en.wikipedia.org/wiki/Overtones
I don't know about the physics of the speaker itself generating the overtone (in cabinet), but it could certainly resonate a wine glass in the room, for example.
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"A/B or GTFO," I believe is the parlance of our times.
Are they talking about "beat frequency" type effects?
Yes. A 40KHz tone and a 41KHz tone will interfere with each other and can create 1KHz tones that are audible. Edited to correct error, thanks anechoic.
No, Holosonics is not creating sound from beating, they are using heterodyning, which takes advantage of how high-amplitude ultrasonic sound waves interact with the atmosphere, that's different from beating.
They don't— the air is linear (except at insane sound pressures) so there is no interference. While the ear is not linear, it doesn't respond at those frequencies.
If it really worked that way it would be trivial to demonstrate. Alas, it doesn't.