Piggy-backed sound in a vacuum?

[padren]

I have no idea if this is ludicrous or not - I don't know anything about the underlying science.

 

I am curious, about the ability to passively detect sound through a vacuum like space through the aid of specific devices.

 

In short, if you had a metal module in space, with mechanical elements clanking away within it, and it is radiating infra-red heat, could the disturbances and minute vibrations that the sound waves were causing in the metal structure of the module be detected remotely with correct sensory equipment to detect the fluxuations via the light radiation and reproduce the sounds through speakers?

[DV8 2XL]

Sound will not propagate through a vacuum. Sound is defined as mechanical vibrations propagating through a medium. No medium, no sound.

[padren]

Sound will not propagate through a vacuum. Sound is defined as mechanical vibrations propagating through a medium. No medium, no sound.

 

I am aware of that element.

 

Infrared heat/light does though, and since sound induces vibrations in the mediums that radiate infrared light, could the disturbance in the stream of photons that are caused by the vibrations sound induces in the medium be detected and used to reconstruct the causative sound waves?

 

You can "see" a tuning fork vibrate even though its in a vacuum chamber - could the correct detection equipment recontruct the tone from observing the fluxuations in the light bouncing off that tuning fork?

[DV8 2XL]

The process your referring to is called modulation: the superimposition of one signal upon an other. This is how sound waves may be carried great distance riding on electromagnetic waves and reconverted back in to sound at some distant place. Its technical name is - radio.

[padren]

The process your referring to is called modulation: the superimposition of one signal upon an other. This is how sound waves may be carried great distance riding on electromagnetic waves and reconverted back in to sound at some distant place. Its technical name is - radio.

 

Yes, but you need a radio transmitter for that - you need special gear at the source.

 

I am referring to a passive system, involving naturally emmitting light radiation (infrared light due to heat loss) from the source object. I am curious if the sound vibrations from within the source object could induce this condition naturally in the infrared light waves it is emmitting.

Would there be a micro-effect of the vibrations causing alternating red-blue shift in the light waves, that could be deconstructed to reconstitute the original sounds that induced them?

[swansont]

Blackbody emissions (which are not just IR) are a continuum given by Planck radiation formula. explained here What you would need to do, as you say, is discern the Doppler shifts in that spectrum, against frequencies that are already present. I suspect the answer is no, you can't, but haven't run the numbers.

 

However, for a single frequency reflected off the object, the answer would be yes. One can shine a laser onto a window of a room, and detect a conversation within, using a laser vibrometer

[elas]

Surely if the sound wave is greater than the carrier wave one looks for doppler shift, but if the sound wave is shorter than the carrier wave (as it might be with a heat wave) one looks for a superheterodyne wave, which has, I believe, a much shorter range than the doppler effect.

[swansont]

Surely if the sound wave is greater than the carrier wave one looks for doppler shift, but if the sound wave is shorter than the carrier wave (as it might be with a heat wave) one looks for a superheterodyne wave, which has, I believe, a much shorter range than the doppler effect.

 

Heterodyning still works on mixing pure tones, which you don't have with a continuum of a blackbody.

[padren]

Thanks for the explanation and finding the laser vibrometer, my next question was if you could do it with an active beam, and it does appear an active beam in the infrared spectrum (where in it I am not sure) can suffice.

 

Is the need for an active beam that there are too many ambient fluxuations, or that the frequency is not sufficient in low blackbody radiation?

 

Would ambient visible light and UV radiation reflecting off an object be effective for a passive system?

[swansont]

The shift is small, because the sound wave add less than 20 KHz to the beam. So unless you know what you're looking for, I'm guessing it's hard to detect that shift from a continuum that may not have a known distribution i.e. you don't know the temperature already, and there are fluctuations on that. The emission at a given frequency and the frequency 20 kHz away are roughly equal in magnitude. So, lots of noise. But with a laser, you have a reference, and are measuring difference on top of that. As such, you get to reject a lot of the noise present in the system. The narrow, stable frequency source is what you need.

[YT2095]

I remember a project in Everyday Electronics (circa 1986) called talk by flashlight, where basicly you had a head phone each and a mic and the case of an old flashlight with IR leds and receivers, basicly your voice gets converted to electricity that then powers an LED and transmits that to your friend that amplifies this modulated light and hears you etc....

 

the interesting part was using them in the daytime, point it at a car window or whatever and you could actualy hear the engine running!

 

so I guess if instead of reflected sunlight you used a Pyro sensor like those in the PIR house alarms or the in-ear thermometers, and some way of narrowing it`s directionality then yes, you would be able to "hear" via far infrared

[swansont]

I remember a project in Everyday Electronics (circa 1986) called talk by flashlight' date=' where basicly you had a head phone each and a mic and the case of an old flashlight with IR leds and receivers, basicly your voice gets converted to electricity that then powers an LED and transmits that to your friend that amplifies this modulated light and hears you etc....

 

the interesting part was using them in the daytime, point it at a car window or whatever and you could actualy hear the engine running!

 

so I guess if instead of reflected sunlight you used a Pyro sensor like those in the PIR house alarms or the in-ear thermometers, and some way of narrowing it`s directionality then yes, you would be able to "hear" via far infrared [/quote']

 

But you were probably modulating the intensity and not the frequency in the LED. I've done the FM-to-AM with a laser diode fed from a tape player, sent it across the lab onto a photodiode and then into a boom box, playing some tune, as a demo. Fiber-optic communications without the fiber.

[YT2095]

oh yeah, 100% is was pure AM (Amplitude Modulation), but FM wouldn`t be too terribly difficult either, infact it would allow for greater selectivity depending on your intermediate frequency (IF). it wouldn`t be my 1`st choice though, I`de stick with the AM and go for a sideband personaly, esp for a passive system.