Unless, I didn’t read closely enough, Cecil didn’t seem to answer the question.
Would it be possible to build a device that could focus sound waves such that a person could speak to someone a couple hundred feet away while not being heard by someone nearby?
There are two elements involved in lasers’ – err – beam-ness.
One is coherence. This is easily produced. In fact, any audio speaker does it.
The other is wavelength. This, by definition, cannot be controlled. Visible light has short wavelengths; audible sound has long. The shorter the wavelength, the beam-ier the radiation can be.
So if “audio equivalent of a laser” means “stimulated emission”, the answer is that we have no way to do that, and no theoretical way comes to mind. If it means “coherent”, the answer is, trivially, yes. If it means “tight-beam behavior”, the answer is no, it isn’t possible, even in theroy.
John W. Kennedy
“Compact is becoming contract; man only earns and pays.”
– Charles Williams
I’m not aware of anything that could make sound waves behave like a laser. Other than the reasons we’ve covered, EM radiation is transverse (follows a sine-wave pattern like most of us are familiar with). Something actually travels from the source to the destination. Sound waves are longitudinal (compression and rarification) – the sound “wave” is air molecules bumping into one another front to back. Sound doesn’t travel, but its effects do. A sonic laser would be trying to cut something with nothing.
There are two projects going on that I know of, that involve using sonics as weapons (Three projects, if you count Cher’s recent album). One uses ultra-low frequency sound to shake crowds of people up inside, causing nausea and dizziness. The other uses a pair of high-frequency emitters to focus dissonant sound on a target, presumably with a painful effect. Neither of these are proposed as lethal weapons, and are intended for crowd control and/or softening up fortified defenders.
My info comes from The Discovery Channel, so it must be accurate, right?
As to the lethality of Cher’s Believe, as well as some other recent offerings by the various pseudo-divas: I know they make me wanna go out and put some blood in the streets…
In this month’s Scientific American (November 1999), there’s an article about Time-Reversed Acoustics. From the article, something like an acoustic hologram may be made. That is, focusing sound to only one person’s ear. The example they gave was projecting “Hello” to one person, and “bonjour” to another simultaneously.
So, while it’s unlikely that an audio “beam” can be devised, focusing sound on a small target looks to be doable.
Focusing sound is trivial to do, and has a centuries old tradition. ust think of all the 18th century ‘whispering galleries’ (generally an elliptical room) where a person whispering at one focus was easily audible to a person standing at the other focus (no matter which way the speaker faced) but was all but inaudible elsewhere in the room.
And don’t even get me started on antinodes and soliton phenomena
[Hmm… solitons with a multifocal source, with the sources of the different frequency components separated, might well produce a surprisingly tight conical beam… but based on my back of the envelope calculations, they could also produce ‘side bands’ – spontaneous soliton origination sites that would originate at sites away from the actual sound origin, and would diverge from the main directional axis. Perhaps the sum of all these sidebands would sum to an approximation of the standard conical dispersion…
I hate to tell you this but the technology for focusing audible sounds and even voices has been around for over 30 years. Look at the CIA project MKULTRA. You use microwaves, digital analogs and possibly elliptcal focusers. I can give links to anyone who wants to look at the info.
