The phonautograph was the first, but hardly the last audio device to use a diaphragm to capture sound pressure waves. You can see drawings of de Martinville's invention and listen to it here:
Even today, a diaphragm or plate is integral to every kind of microphone. Because diaphragms interfere with sound, they are a necessary evil. At least they were until now.
In 2005, I designed a diaphragm-free audio transducer based on a laser, smoke and photocell. The idea occurred to me shortly after having dinner in a fancy restaurant with my wife. On the table between us was a little oil lamp. A thin column of white smoke rose from it, passing between us. Every time either of us spoke, the smoke wavered. Of course, most of the movement was due to exhaled breath. But I figured that somewhere in that stream, sound waves had made microscopic disturbances.
As a result of much research, bench-top experimenting, and legal business, U.S. Patent 7580533 issued a few weeks ago. It can be downloaded for free from www.uspto.gov. Here's the link:
Note that the patent contains further links to prior art, which show many attempts to directly detect sound in air.
Both the first not-completely-ugly proof of concept and the first prototype of the smoke and laser mic are demonstrated in videos on YouTube. Here's a link to one of them:
I know, it sounds like crap. The next prototype will be better.
This is the first somewhat public writing about it. I'm hoping some of the people who read about the mic here will be inspired to study the patent, look at the videos, and build their own version to study the concept. There is much I don't know about how to optimize this device. Any insights will be appreciated.
As most of you know, modern microphones are very good. About the only improvements anyone spends time on are to make them smaller and cheaper. However, no matter how good they are, they contain a distortion producing mechanism: the diaphragm or plate. Because that element has mass, it has intertia, which inhibits instant response. The diaphragm also must be supported, which restricts its motion. It resonates, adding its own signal to the sound. Ideally, sound would be detected directly from the air.
Over the past 30 years, more than a dozen engineers have tried to directly detect sound pressure waves in air. The most notable attempts are shown in patents issued to Bell Labs and Brookhaven National Labs. They are referenced in my patent, above. Neither of those designs were ever publicly demonstrated or commercialized.
The key to "seeing" sound pressure waves in air is to introduce small particles or droplets into the air. That smoke or fog moves with the air. A laser passing through the mix will be disturbed by the variations caused by sound. Those variations can be detected by a fast photocell and converted to an electrical signal.
Because a small stream of smoke or fog has little mass and can be constantly replenished, it is easily modulated by sound and does not have any appreciable reaction force. In theory, the dynamic range of this kind of mic could exceed 130 dB and its response to rapidly changing sounds should be nearly instantaneous. Of course, it may be another year before we get close to those goals.