# 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?

• Yes, there is a full analogy with the laser. For that the sound beam must be wide and the crowd very rare. Jan 9, 2015 at 20:51
• There was a Kickstarter project from 2012 that claims to have created a sound "laser". I have not tried it, but it may be worth a look. kickstarter.com/projects/richardhaberkern/… May 2, 2017 at 0:09

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

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.

• "This has been used for applications such as laser drilling." Do you have any references here? A perfect Bessel beam cannot exist, because it would be of infinite extent. However, I would be greatly interested in any analysis that looks into the effect of inevitable deviations from the true Bessel beam on its success in laser drilling. I was under the impression that laser drilling, although having excellent repeatability and control, couldn't really drill much deeper than conventional mechanical drills, i.e. a hole aspect ratio (depth/diameter) is limited to be less than of the order ... Jan 14, 2015 at 10:37
• .... of between 30 to 50. Do you know whether my impression is correct, or could the aspect ratio be considerably greater than this and that I have been talking to the wrong people? Jan 14, 2015 at 10:38
• +1 BTW: especially for reminding me of the merits of the Bessel beam for drilling - this is relevant to a design problem I have at the moment. Jan 14, 2015 at 10:39
• You are probably going to use a CO2 laser and ZnSe a lens for drilling? A company that manufactures lenses would be a good bet for information. I am a bit out of date. It has been 30 years since I worked for one. I would not expect beam diameter to be the limiting factor. At 10.6 um, indices of refraction are very high. A singlet lens can be diffraction limited. The limit would more likely be producing an accurate Bessel profile. A conical axicon lens isn't perfect. Depending on your budget, perhaps an aspherical lens could be designed? Jan 14, 2015 at 14:19
• @WetSavannaAnimalakaRodVance osa-opn.org/home/articles/volume_24/june_2013/featurres/… Jan 21, 2015 at 14:10

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.

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 .

• I believe that they are mostly used to receive sound. Of course they can be used like flashlight parabolic mirror, but the problem is that they (or any device else which works in this way) can't do it sharply; even on paper. Jan 9, 2015 at 14:50