# Is there a way to use constructive interference to create a specific sound in a specific location?

With 8 speakers, one in each corner, can a combination of waves be emitted that are silent from the sources but form a coherent emission somewhere between?

I mean that even if you walked around the room the sound of something, say a ball bouncing, would stay in the position that it sounds like its bouncing in relative to the walls and not because your position is being tracked and each speaker adjusts its volume accordingly, silencing as you approached and vice versa, but because the pressure changes in each speaker are beyond the 20 to 20k Hz range we hear until a combination of interference creates a wave that is formed at a controllable location and actually emits from there. An opposite to dead spots.

We have Phased arrays to steer sound without moving the array if we're not using the parametric array effect to 'beam' the sound but maybe some combination of these. Acoustic phase conjugations which can create a localized vibration or sound wall that could perhaps be enough of a non-linear medium to scatter the ultrasonic propagation? There is Intermodulation, primarily considered a distortion but all combined this could be possible?

• What does "silent from the sources" mean? Nov 22 '16 at 22:09
• @immibis Probably that you can't hear a thing unless you're in the special location where the beams are focused.
– Kaz
Nov 22 '16 at 22:12
• @Kaz Nearly. I want the sounds coming from special locations no matter where you are.
– Ryan
Nov 22 '16 at 23:47
• Search for "phased arrays" (actually "phased array antennas"). That's what you are looking for Nov 22 '16 at 23:56
• A relevant question on the Physics Stack Exchange: Is it possible to create an audible sound source in mid air by intersecting ultrasonic sound beams?
– user39382
Nov 23 '16 at 0:15

Not in any practical sense, no.

Constructive/destructive interference can only change the amplitude at a given frequency; it cannot produce a frequency shift. Combining multiple ultrasonic signals will only give you another ultrasonic signal; it will never result in an audible frequency.

Audio "dead spots" are the result of multiple audio paths that happen to perfectly cancel out at that spot. They are, in a sense, "unstable", because the balance is only hit at that one exact location (and, in most cases, even then only at specific frequencies). At most other locations, the sound can be heard normally.

There's no way to get the opposite effect you're describing, of a set of signals that cancel out everywhere except one location. This is because there's no way to make signals cancel out everywhere, besides making them perfectly equal and opposite, and originating from the same point. (At which point, there's effectively no signal being transmitted at all.)

What is possible is to produce a single point of constructive interference, where a sound is perceived louder than it is elsewhere -- this is simply the opposite of the "dead spot" effect I described earlier. However, this doesn't translate to the effect you're describing: the sound is not re-emitted from that location; it can only be heard by a listener in that location, and they will hear it coming from all of its "real" sources.

• Constructive/destructive interference can only change the amplitude at a given frequency; it cannot produce a frequency shift — well, I’m not sure about that. What about beat frequencies? Nov 22 '16 at 23:27
• This answer is incorrect, you can create audio sounds using ultrasonics. There are many products available, this being one: ultrasonic-audio.com I've seen these used in many museums to produce stereo sound around exhibits. Nov 22 '16 at 23:28

Yes for focusing 1 point, No for silent eleswhere

• beam forming coherence standing waves can be created to simulate the sound in the middle of your head, like whispering into a large parabolic dish or concave wall centre.

• this does not make everywhere else sound silent , it just sounds like its coming from the speaker.

• Although dead spots can be created by standing waves.
-Look up Ricean Fading, the same thing happens to WiFi on fringe signals.

This phase property is what defines the quality for those who have a passion for high fidelity audio with the ability to see the orchestra play positions on stage. Unfortunately mixing is such these days that this property is obscured by the needs to be compressed and sound good on radio resulting in some phase distortion.

Also speaker linear phase response or maximally flat frequency response is an expen\$ive linear property.

• This is for stereo only though? With surround sound there is flat plane you have to center yourself within?
– Ryan
Nov 22 '16 at 22:37
• It applies to stereo, quadraphonic or octagon sound like in Epcot centre theatres Nov 22 '16 at 22:39

You should check out this vid on sound wave patterns. The interference here would be similar to what you're trying to achieve. https://www.youtube.com/watch?v=v4ehaGeo6vs

However you probably want very directional speakers, the effect of which could be ruined by diffraction depending on the size of your room.

I don't know if I should answer my own question but no one has yet put all these together...

We have Phased arrays to steer sound without moving the array if we're not using the parametric array effect to 'beam' the sound but maybe some careful combination of these. Acoustic phase conjugations which can create a localized vibration or sound wall that could perhaps make the air enough of a non-linear medium to scatter two converging ultrasonic wave's propagation at a point in mid air via Intermodulation distortion.

All combined even with lots of maybes, this seems quite possible to me.