Hacker News Reader

Comment by cop359 | Hacker News Reader

I don't really know much about this, but wouldn't the 22kHz sounds potentially create beats in the lower frequencies?

Acoustic "beat tones" aren't "real" tones— you hear them because of non-linearies in the ear-brain system, but you have to hear the initial tones first. (Well, unless you're talking >>130dB SPL levels where the air starts becoming non-linear, but then lower frequency recording would capture it fine)

If you could hear subharmonic beats from ultrasonics then it would be _very_ easy to demonstrate, alas.

Curious, what does non-linear mean in this context?

IIRC, linearity is when you put a sound wave frequency into the medium (air) a some point, you can predict the frequency of the sound wave at some other place using a linear function - meaning that there is no distortion. Non-linear is when the physics of the medium starts screwing with that function.

I believe that it means that the superposition principle (http://en.wikipedia.org/wiki/Superposition_principle) doesn't hold: the net response at a point is not just the weighted sum of the individual responses.

so without very high power sounds and the nonlinearity business the whole "Sound from Ultrasound" wouldn't work? Huh, I guess all this time I misunderstood it.

for those curious http://en.wikipedia.org/wiki/Sound_from_ultrasound

Well, what I can think of is that of course you need to sample at > 2*max frequency if you do uniform sampling to avoid aliasing (by Nyquist), but that's not the same as playback.

Yes, there will be inter-modulations from higher frequencies. There are also from the audible spectrum but if the amp is linear enough they will be low.

Reply on news.ycombinator.com