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Author Topic: Sound Spectrum of a Cavity  (Read 2531 times)

Offline lightarrow

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Sound Spectrum of a Cavity
« on: 19/08/2006 03:11:20 »
Is it possible to make a sound analogous of a blackbody?

That is, a cavity which absorbs perfectly all sound frequencies (at least in a broad range) converts them into some form of energy (for example heat, but not necessarily), put itself in energetic equilibrium and then re-emits sound frequencies according to its energy?

If that would be possible, at least theoretically, which would be the sound spectrum of the cavity? Would it be independent of the material and dimension of the cavity, as in the luminous cavity?


 

Offline eric l

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Re: Sound Spectrum of a Cavity
« Reply #1 on: 19/08/2006 09:33:52 »
quote:
Originally posted by lightarrow

Is it possible to make a sound analogous of a blackbody?


I don't think so.  Light is mainly the result of electrons falling from a high energy level to a lower one; absorbing light is using the energy of the light to make electrons jump from a low energy to a higher one.
Sound is a question of deformation of matter, wether that is air (or another gas) or a conventional liquid (like water) or a conventinal solid.  
You can absorb the energy of sound, but I don't think you can store it in such a way that it can be transformed back into sound again.
Although !  You can make a violin string (or piano string or guitar string) sound in resonance with a sound from an exterior source; when you stop this outside source, the string will go on vibrating till it fades out.
Am I contradicting myself now ?
 

Offline Soul Surfer

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Re: Sound Spectrum of a Cavity
« Reply #2 on: 19/08/2006 11:41:44 »
Yes but most times it would not work the way you are thinking because of the conditions.  

It is quite possible to build both sound and electromagnetic anechoic chambers in which all sound or em radistion is absobed  A hole in the side of such a chamber would be your black body source.  However when you talk about a black body source of radiation  the important feature of the source is that it is at a significantly different temperature (usually much hotter)  than its surroundings so its radiation can be detected.


Sound needs a medium to be transmitted so lets assume that the medium we are working with is air.  There is a fundamental sound noise level associated with the molecules of gas hitting an object this can be seen in small objects under the microscope by their brownian motion  This produces a mechanical noise in a microphone for large microphones this is usually insignificant compared with the electrical noise in the amplifier but for very small sensitive microphones it is probably detectable with care.

This noise level is clearly dependant on the temperature of the gas so if we had a very hot anechoic chamber with a hole in it cold air around it it might just be possible to a slight thermal sound from the hole.

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Re: Sound Spectrum of a Cavity
« Reply #2 on: 19/08/2006 11:41:44 »

 

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