Is it theoretically possible to project sound in a line, like flashlights? I was wondering if there is a way to project sound waves on a particular point, like a laser? So that you can you can privately send a voice message to a specific person in crowd.
The very first idea that comes to mind is to place a parabolic surface next to the sound source, like in flashlights. This solution looks alright at first, however, there's a big problem: diffraction. Sound waves are 1.7 cm to 17 m in wavelength, that means they'll immediately diffract. And I want to conclude that, even though we can perfectly line up sound waves in a straight direction, aiming sound to a particular point is not possible, because the sound diffracts hardly.
So I'm here to ask: Is this conclusion right? Isn't there any way to avoid diffraction?
Thanks in advance.
 A: You cannot prevent diffraction. It comes with waves. Even laser beams spread from diffraction. 
It is possible to generate a laser bean with a Bessel function cross section. In this case, the beam angle is 0. This has been used for applications such as laser drilling. It is possible to do the same thing with ultrasound. http://en.wikipedia.org/wiki/Bessel_beam
Ultrasound beams are used in medical applications. http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0CCgQFjAB&url=http%3A%2F%2Fwww.tankonyvtar.hu%2Fhu%2Ftartalom%2Ftamop425%2F0006_Orvosbiologiai_kepalkoto_rendszerek%2FOB_ultrah_4.pdf&ei=_d-vVIi4B8WZNufCgdAN&usg=AFQjCNEvpeAH11_RGgj4WqDt0YDn_9jUyA&sig2=lw99ih7FU3tRo-72HNhCMg&bvm=bv.83339334,d.eXY
And weapons. http://en.wikipedia.org/wiki/Long_Range_Acoustic_Device I was under the impression that weapons had a much narrower beam angle than 30 degrees. But perhaps not. 
A: There is a fair amount of research that has gone into generating audible sound from ultrasound. The audiospotlight (no affiliation), for example, can "beam" sound over a couple of meters with a relatively narrow width. Lasers have a much narrower beam and can cover greater distances, but under the right conditions you could probably target an individual (or small groups of individuals) in a crowd.
A: I'm not sure what you mean that they'll immediately diffract because of their wavelength? Parabolic sound amplifiers work and I've seen (heard?) them implemented at the Exploratorium in San Francisco: http://en.wikipedia.org/wiki/Acoustic_mirror .
