a very stupid question

[siguy]

Since sound is just pressure waves and pressure is (gas law i think) density of air and density alters refraction, can't one just make a mic that uses lasers to measure density of air at a specific point at a very high rate, oh say ~100000 times per second? it could be a big open thing so as not to have cavity resonance effects, maybe even a beam (or series of beams i think) projecting from floor to ceiling, with the "pickup point" somewhere in the middle.

[geoff]

"siguy" > wrote in message
...
Since sound is just pressure waves and pressure is (gas law i think) density
of air and density alters refraction, can't one just make a mic that uses
lasers to measure density of air at a specific point at a very high rate, oh
say ~100000 times per second? it could be a big open thing so as not to have
cavity resonance effects, maybe even a beam (or series of beams i think)
projecting from floor to ceiling, with the "pickup point" somewhere in the
middle.

That's two stuipid questions. Or one, twice, to be exact.

geoff

[Scott Dorsey]

In article >,
siguy > wrote:
>Since sound is just pressure waves and pressure is (gas law i think) densit=
>y of air and density alters refraction, can't one just make a mic that uses=
> lasers to measure density of air at a specific point at a very high rate, =
>oh say ~100000 times per second? it could be a big open thing so as not to =
>have cavity resonance effects, maybe even a beam (or series of beams i thin=
>k) projecting from floor to ceiling, with the "pickup point" somewhere in t=
>he middle.

Yes, it's been patented.

It's also possible to use lasers to measure air velocity and effectively
create a figure-8 microphone.

The good part: no moving masses therefore no mechanical damping issues.

The bad part: linearity and noise issues, since the range of pressures
being dealt with is such a great one.

In fact, making a good pressure-response (omnidirectional) microphone is
pretty easy. Even Panasonic can do it for five bucks. It's making it
directional in a consistent and even way that gets difficult. So using
a laser to make a pressure-response microphone is not solving an interesting
or important problem.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."