(apologies if this is in the wrong forum)
I’m looking for a “sound concentrator” - that is, some device that lets a CD player aim its sound in a certain direction. I’d like to play some music that will hopefully be aimed in only one direction (for example, to only be mostly heard to the right, but not to the left). Headphones will not help for my purposes.
Does this even exist?
Thanks,
-Wallet-
You want a Parabolic speaker.
You can focus the sound with something shaped like a dish. Turn the volume low, point the player toward the dish and anyone in line of the dish can hear it clearly. This is an example, people can stand in front of them and whisper to each other. The downside is that it may focus the sound too much so you’d have to be in exactly the right spot to hear it. Same principle as the parabolic speaker.
the problem with concentrating sound waves by these methods (along with the very clever HSS Speakers), is that they suck for music, since they won’t go much below 600 hertz. So you’re missing all the bass, and an important segment of the midrange.
The good news is that once you’ve defeated the laws of physics, this problem will prove eminently solvable. 
You could also make a lens out of a bag of gas having higher density than air. Or, you can give your lens concave surfaces and fill it with a gas of lower than air density.
Another possibility is to arrange many speakers on the surface of a sphere centered on your target.
The whole bass thing is a problem for systems you set up that have dimensions smaller than the wavelength of the sound you want to hear. The bigger the dimensions, the lower your focussed frequencies can go.
As alluded to above - the critical problem is the wavelgth of the sound. Sounds travels at 340m/s. Any focussing system pretty much ceases to be effective when the waveleght is of the same order in size as the device. A one metre focussing thingy will only be effective for frequencies above about 300 Hz.
The same issue affects how a directional emitter will perform. A horn for instance can be quite directional for short waveleghts, but once the waveleght is of a similar size to the diameter of the horn it begins to emit sound pretty much omnidirectionally.
A surprising amount of sound we hear indoors is not directly radiated to our ears anyway. Unless you are close to the source the majority of the energy has been reflected of a least one surface, usually more. The clue here is that well placedsound adsorbing materials (which is also hard to do for low frequencies) may be just as, if not more, important than simply directing the initial sound energy.
Sound is really hard. A good examle of how hard is in movie making. The lighting and cameras have the great attribute that light travels in a stright line, and apart form being careful about shooting into a mirror, there is little problem. The sound recordist has to worry about every single bit of ambient noise. A distant truck, plane, or member of the crew coughing is enough to ruin things in a manner that no amount of processing can fix.
From Meyer Sound. Probably costs $10,000.
There are directional sound devices on the market. A few years back I heard one at a trade show. (Maybe from a company listed here.) It was really eerie. Stand in one spot: the music is clear. Move a few few away: the music drops off to nothing. The ones I’ve seen look like flat panels but are an array of tiny transducers.
I don’t think they are being marketed to consumers at this time.
IIRC correctly the main application in the development of these things was crowd control, as they allowed the relevant authorities to shout “Oi, you!” at specific targets. Safety concerns might keep them off the domestic market, as it’s similar technology that non-invasively breaks up kidney stones.
These particular directional audio systems work by modulating encoded audio onto two ultrasonic carrier wave sources, which when brought together at a point interfere with each other in such a way as to give a product that reconstitutes the original audio, and/or partially jellifies the hapless recipient.
Radio headphones are good, they won’t crumble your bones so much.
I think the ones you may be thinking of are here:
They’re mostly for pro-grade installations, but they do sell to consumers as well.
The technical information is interesting:
They’re much more directional than a parabolic dome: http://holosonics.com/tech_directivity.html
The Exploratorium in San Francisco (and, think, in some other science museums) has an elliptical sound concentrator. Rather than directing the sound along one direction, it concentrates it at a particular focus, although you really need a good portion of the ellipsoid to hear the effect.
