Topic: Research: Microphone with no moving parts

[digifish_music]

This is an interesting paper, recently published, that outlines research being done into a microphone that measures the change in air pressure at an interface between a lens and air, as changes in refractive index of the air as the pressure changes...

The Acoustical Society of Japan: http://www.jstage.jst.go.jp/article/ast/29/4/283/_pdf

Abstract: A new microphone, which uses the optical total reflection on the boundary surface
between glass and air, has been proposed. The intensity of reflected light changes by the sound
pressure, as the fluctuation of air density due to sound changes the refractive index of air and the
change in the refractive index changes the angle of total reflection. The sound pressure can be
measured as the change in the intensity of reflected light. In this paper we investigate theoretically the
sound pressure sensitivity of the proposed microphone in certain types of the shape of curved boundary
surface. As the sensitivity can be improved considerably by employing a plane wave light source, such
as laser light, and the curved surface with a large radius of curvature, the microphone is expected to be
practically used. The microphone detects the sound pressure without mechanical vibration and can be
made small, so its upper limit in the frequency range is very high in principle.

...so the potential is ruler-flat, 1Hz-100kHz frequency response in something the size of a pen?

Previous paper by the same authors...  http://www.jstage.jst.go.jp/article/ast/25/1/45/_pdf

digifish

[John Willett]

Interesting - but please note that Sennheiser already have an optical microphone on the market - this one though, is based on reflecting light off a moving diaphragm.

But, because there is no metal involved, it can be used in CAT scanners and in places where any electronics can cause an explosion (gas and petroleum plants, for example).

[digifish_music]

Interesting - but please note that Sennheiser already have an optical microphone on the market - this one though, is based on reflecting light off a moving diaphragm.

But, because there is no metal involved, it can be used in CAT scanners and in places where any electronics can cause an explosion (gas and petroleum plants, for example).

Yes, well I was interested in this mic precisely because it has no moving parts. A solid state mic

The idea of reflecting a laster off a diaphragm is similar to the technique used in Atomic Force Microscopes etc.

http://en.wikipedia.org/wiki/Image:Atomic_force_microscope_block_diagram.png

I have been expecting these technologies to make their way into microphones at some point in the near future. You could build a 24 bit digital mic no problem if you go straight from the photo-diode to digital signal. Imagine a CCD like detector in a strip where each cell represents a bit-value...or you probably could get away with just a couple of photo detectors and interpolate.

EDIT: quite a few companies out there with products it seems...  e.g. http://www.optoacoustics.com/content.php?id=21

digifish

[ghellquist]

I have been expecting these technologies to make their way into microphones at some point in the near future. You could build a 24 bit digital mic no problem if you go straight from the photo-diode to digital signal. Imagine a CCD like detector in a strip where each cell represents a bit-value...or you probably could get away with just a couple of photo detectors and interpolate.

Well, frankly, no. You will be hard pressed to go to that many bits. The reasons are several, but basically what we are talking about is an enormous signal to noise ratio that is very difficult to handle.

Gunnar