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I'm sorry if this is the wrong place to ask. I understand that for typical microphones, the sound waves need to physically reach the microphone and cause vibrations in the diaphragm before an electrical signal can be created.

My question is, is there existing technology that makes it possible to detect soundwaves from a distance before the soundwaves physically reach the actual sensor? (i.e if the distance is x metres, we have a buffer time of x/(speed of sound) before the waves hit the sensor) Googling has not gave me much leads and I'll really appreciate someone pointing out some keywords and concepts I can further research on. Thank you!

Ymi
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    Depends how you define a sensor. For example a laser can be used to detect vibration of other objects so in essence detected before the sound arrives at the lasers position. But then you could argue that the object is the sensor. See the situation does not differ from having a microphone at the position of object. – joojaa Nov 08 '20 at 08:38
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    This depends on the context. Speed of light is faster than sound. So in the case of explosion, one could see the explosion before one hears it. – Mahendra Gunawardena Nov 08 '20 at 12:57
  • Yes. Put a second microphone (or any other suitable transducer) closer to the sound source. – Transistor Nov 08 '20 at 14:34
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    I've read that one can spy on audio by measuring vibrations from window glass using a laser. You are still getting signals, but since light is faster than sound you get the signal before the sound wave gets to you. – Eric S Nov 08 '20 at 23:39
  • What exactly do you need this for? Are you planning to record sound and need to know when to start recording? A microphone will only react to sound so you would need some type of electromagnetic signal (light, radiowave, microwave, etc.) that is transmitted from the source and can be detected by a different sensor and start the recording before the sounwave reaches the microphone. – Gwyn Dec 08 '20 at 19:04

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The speed of sound is not a constant. The speed depends on the material the sound is traveling though. The speed of sound in aluminum, for instance, is roughly 20 times faster than air. If the microphone is mounted in a solid mount connected to the sound source, then the sound wave can propagate to the transducer through the mount faster than through the air.

Another approach is to use a laser to reflect off a window near the sound source. The window vibrates as a function of the sound which modulates the reflected laser light. This, I believe is actually used in surveillance. Then you are dealing with the speed of light which is much faster than sound.

Eric S
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Here is one way, demonstrated in the harbor explosion videos from Beirut. If the sound wave is sufficiently intense and the atmospheric humidity levels just right, the approaching sound pulse causes transient condensation of water vapor, producing a thin shell of mist that traces the travel of the sound pulse. For supersonic shock waves, this effect is even stronger.

Another way (again, for huge sound waves, as from a big explosion) you shoot an array of rockets straight up in the air which leave smoke trails behind them. Then you set off the explosion and film the smoke trails from a variety of angles. Because a very strong shock wave in air significantly compresses the air right at the wave front, its index of refraction increases significantly and on the film, the smoke trails appear to have kinks in them due to the shock wave.

niels nielsen
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