# Generate sound that can only be heard in a particular direction?

Since sound waves travel in straight lines, is it possible to generate sound such that it can only be heard in a specific direction away from it's source? Normally when we generate a sound, for example when we say something, the sound we make can be heard all around us. The person left and the person right to us can hear us when we say something. I assume this is because when we say something we don't generate a disturbance on one particular particle that propagates the disturbance, but we do this on many particles in many different directions. So I was wondering if it is theoretically (or even practically) possible to make a sound (obviously by a machine and not our voice) that can be heard only for example in front of the source in a specific radius?

EDIT: I assume it is not possible because particles aren't perfectly lined up, so when they hit each other and propagate the disturbance the direction of the sound wave gets deflected. But then again, I read that sound waves travel in straight lines, so what does this mean exactly?

• Your best bet is probably standing sound waves in a tube or tunnel. The maxima would be audibly at some repetitive distances and inaudible in the minima.
– Gert
Dec 5, 2021 at 20:47
• the answer is yes, and it is commercial see holosonics.com Dec 5, 2021 at 20:53
• @annav amazing! do you have any article suggestions to read up on this technique? Do you know why my intuition was wrong that it is not possible because particles are not perfectly lined up? Dec 5, 2021 at 21:05
• just found it by google "directed sound". I knew of the technology of using reflected sound in open air theaters and clubs ( I live in Greece) so that the sound does not propagate outside and disturb neighbors. Sound is modeled with wave equations, the same as light, so it makes sense that a beam can be constructed. Dec 5, 2021 at 21:22
• I haven't looked further into holosonic's technology but to my knowledge there are two quite prominent ways to create a narrow (sound) beam. One is already touched upon by Niels Nielsen and can be found in various "flavours". For more information on it you can look for terms like "phased arrays" and "beamforming". The other way is to use modulated ultrasounds. Since ultrasounds can be very directive, you can combine them (pretty much like creating a beating effect) to create modulation in some audible frequencies. Again, I am not sure which one (or if it's a different one) holosonics use. Dec 6, 2021 at 23:43