^{Photo (C)2009 Schwartz}

You might be wondering, what does that woman with the smoke and the laser beam in the picture above have to do with anything? It's a concept image; the one we used for promotion of my new thing at the Audio Engineering Society convention and exhibition in New York City last month. The response from attendees was heartening, but we're not done yet. There's been tons of press coverage, too. Here's a link to a recent story: http://www.spectrum.ieee.org/video/semiconductors/optoelectronics/laser-mic

So, I've been spending way too much time lately talking into a small smoky birdhouse. OK, it only looks like a birdhouse. This little box is the business end of a new kind of microphone. Inside it are a red laser and a silicon chip photo detector. Between the chip and the laser there's a nozzle spewing a ribbon of smoke.  When I talk, small ripples in the smoke scatter the laser's beam. This scattering causes variations in voltage coming out of the photo detector. The varying voltage is linearly proportional to the sound waves I make. The electrical signal from the detector is connected to the microphone input of my laptop computer. My voice ends up as a digital recording.

The challenge is to make this weird setup sound as good or better than a conventional microphone. In theory, it should be better, much better. At this point, 90 days into the project, I'm just glad it sounds like me talking. Back in early September, my recordings sounded a lot like Scooby Doo with a head cold, underwater.

Our (me and my son, Daniel) fifth prototype is very different from our first. Notable changes include more laser power, a glass lens that forms the laser beam into a horizontal line, better smoke (colder, denser, less energetic), a NACA duct-shaped nozzle, an acoustically 'dead' housing, separate exhaust and supply fans, and a band gap filter for the photo detector. I wish I could rank these improvements in order of importance but I can't.

The next immediate steps will be to get a nozzle made by a machine shop, switch to water vapor as the medium and to use a photo detector with a much greater dynamic range.

Down the road a bit, we may try to take advantage of the Schlieren effect to amplify the signal optically, which would allow us to reduce the smoke or vapor stream flow, which in turn would reduce the noise floor. One attraction of that approach would be the opportunity to incorporate a mirror inside the device. Then we'd have it all: laser, smoke and mirror.

For more about this subject, visit:

http://www.schwartz-engineering-design.com/