# Audible frequency amplitude modulation of 20kHz carrier for diffraction demo.

I am attempting to build an audio projector that takes advantage of the low diffraction of high frequency audio waves. Audible range frequencies diffract more than, say 20kHz, so I want to modulate a 20kHz carrier wave by an audible range source, hoping that the carrier wave will be maintained for longer distances (like a beam of sound), and that my ear will ultimately perceive the audible input being delivered on the carrier wave.

My question is: Should I be aiming to produce an AM Radio type modulation that has an envelope of the input both above and below, mirrored across the x axis? Or should I try to make it so the final signal has the same shape as the source wave, just filled in with 20kHz carrier?

Also, is a square wave carrier sufficient, or will it need to be a sine wave?

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6 years ago
I don't think you want AM. I can't see how your ears will demodulate it. I think you need to look at beating the 20K and the audio together, which will give you something you might hear.
laserjocky (author)  steveastrouk6 years ago
That's my intuition. I'm worried that the speaker may have low response at 20k and I'll end up with just a superposition of the frequencies in the end anyway. Maybe a piezo?
6 years ago
Yes, piezo is probably a good way to do it. How about two channels one running at 20kHz, the other at 20kHz +the audio. Shouldn't they beat at audio frequency ?
laserjocky (author)  steveastrouk6 years ago
So one channel would be a sort of reference, then the second channel would be frequency modulated by the input? The added frequency would cause audible beats I suppose. I can imagine that working, but I think I'm stuck with amplitude modulation at my level of experience. Thanks for your help!
lemonie6 years ago

You'd want Amplitude Modulation yes. However, I'm not convinced that this would work as you want it to - is there a precedent for this?
http://en.wikibooks.org/wiki/Sound_Synthesis_Theory/Modulation_Synthesis

L
laserjocky (author)  lemonie6 years ago
My physics professor seems to think it's possible but is short on detail. I did an oversimplified proof of concept with a pair of 555s and a MOSFET (low freq on the gate, high freq on drain), and I was able to hear both the barely audible carrier and the lower frequency. This may just be effects of my choice of components though, or lack thereof. EE is next term, haha. I'm going to try both methods.

To me it seems like I will need a signal multiplier to do this properly, if this is even physically possible. 20k Sine carrier between 0 and 1v, multiplied by the audible input, so the envelope exactly resembles the audible input.
6 years ago

Well I hope you get it to work, best wishes.

L