Recent articles like this one in USA Today are crowing about HyperSonic Sound (HSS), which “creates a laserlike beam of sound.” Invented by Woody Norris, with a patent owned by American Technology Corp. (ATC), this new technology claims to allow focusing sound and projecting it long distances without anyone outside the beam hearing it.
While my gut feeling is that this is legitimate science, I can’t quite sink my teeth into the description of how it works, even tho I am no slouch in acoustic theory and physics:
The sound “slows down”? Other than a doppler effect or a change in transmission medium, why would that happen?
It makes me think of a heterodyne or interference effect, where the combining of two high-frequency signals with similar wavelengths produces a third frequency equal to their difference. This principle is used in superhetrodyne AM radios (as RF) and by piano tuners (as sound). I wonder if an ultrasonic frequency is being modulated with an audio one? And if so, how would striking an object cause demodulation?
Can anyone explain the HSS concept in a little more detail? Google searches turned up more news articles, but so far I haven’t found any technical papers.
The idea is analogous to modulation and demodulation in radio. In radio transmissions, a baseband audio-frequency signal is sent by (simplified explanation) modulating it to a high frequency, essentially adding the signal frequencies to a carrier frequency in a mixer. At the receiver, this is demodulated by subtracting the carrier frequency to return the signal to the audio frequency range. The modulator and demodulator work by using nonlinear devices to mix the frequencies of the signal and a carrier; among the terms in such a mixture are the sum and difference frequencies. Amplifying the appropriate frequency ranges gives you either a transmitter or a receiver.
The idea here is to do something similar with ultrasound waves. If you beam two ultrasonic waves of slightly different frequencies–think of one as the carrier frequency and the other as the modulated signal–then when they hit an object the nonlinear response of the object will usually cause some response at the difference frequency. If this is in the audible range, the vibration of the object at this frequency (the “beat” frequency) will be audible even though the original carrier waves were not.
Sorry, that was probably a little simplified for you; I missed the last part of your post. Most objects have nonlinear responses to applied forces, and just about any nonlinear response is sufficient to cause at least some frequency mixing into the sum and difference frequencies.
One problem is that the mixing is probably pretty inefficient in general, meaning that you need a higher power density in the ultrasonic waves than you would for audio-frequency speakers. But you can probably make ultrasonic transducers a lot more efficient than audio-frequency speakers, and allowing beaming means you don’t have to broadcast the sound (as well as being its own feature).
I guess we are on the same track. I certainly understand beat frequencies and modulation. The part that I can’t quite grasp is the “non-linear” part, and why hitting a solid object would do anything other than just reflect sound waves (“The angle of incidence…”) And, Ophamaloskeptic, have you ever observed or experienced this phenomena (HSS) in a laboratory (or other) setting?
The Norris site has some tech docs in PDF form; I am downloading them to see if that helps.
First, no, I haven’t seen this effect in action (though I’ve heard of it before and have been tempted to try making one; I think the basic idea is pretty straightforward, though making a high-fidelity speaker out of it is probably somewhat more difficult). It would be interesting to play with.
A very heavy, solid, smooth object will indeed tend to reflect sound waves at equal angles. But most objects don’t meet this ideal. Walls are able to vibrate, for example, meaning that they have a nontrivial frequency response. If you’ve ever lived in an apartment complex or dorm, you’ve probably experienced vibrations of things in your room when some neighbor turned up his stereo; this is a resonant mode, which acts differently than simply reflecting the sound.
Most objects are also not linear in their responses, which is what allows for frequency mixing. For example, the whiteboard I’m looking at right now is anchored at its top but not at its bottom, so it swings relatively freely away from the wall at the bottom. Of course it doesn’t swing into the wall, though, so this acts something like a rectifier, reflecting the positive-pressure part of a wave but damping the negative-pressure part. This model only works at very low frequencies, of course, since it’s pretty large and has a low frequency cutoff, but lots of surfaces have similar sorts of nonlinearities.