To get around the dynamic range issues of current microphone pickups, Schwartz Engineering and Design has devised a laser-based pickup that detects voice-induced 'distortions' in a flowing stream of smoke and that then relies on proprietary digital signal processing to translate those distortions into audio.
To be honest, I was skeptical, as laser-based pickups have been tried before by Bell Labs, but the man behind the system is David Schwartz, who performed seminal work in audio compression in the early '80's that led to what is now MP3, so my interest was piqued.
Skepticism seemed all the more justified when I met David in our reception area holding what looked alarmingly like a really elaborate bong! The walk from reception to the meeting room did not go unnoticed by my colleagues.
Called the Particulate Flow Detection Microphone and shown in its latest rendition in the YouTube video below, the system does actually require a smoke source (regular smoke, that is.) That smoke is then pulled vertically through a tube by two fans at a rate of two to three inches per second. Near the top is a low-cost, off-the shelf laser set up such that the smoke passes between the source and the detector. At that same point, the vertical tube has an aperture through which Schwartz speaks to 'distort' the rising smoke.
As a tech editor, we see lots of demonstrations, this was one of the most bizarre. We also see lots of mishaps in even the most well-planned, pre-staged demonstrations. This was no exception. One of the fans got broken in transit so the rate of smoke flow was reduced accordingly. That was a problem.
Why? Smoke is not elastic, so it needs to be replenished in the beam path otherwise the sound pressure waves leave "trails" or after-effects in the intersection of laser and smoke. Also, reverberations in the detection chamber would affect the signal even more than they already do. The laser needs to "see" a clean, smooth slat--or at least as clean a slate as possible. The cleaner the smoke, the better. Two fans aid in that.
With one fan out of action, it became more difficult to detect Schwartz's speech, this translated to a barely audible replay on the accompanying laptop, but it definitely worked. I videoed the demonstration, but the audio is too low, but that's fine: Schwartz has posted better recordings on YouTube across each stage of development. Here's the latest:
In this version, he has figured out how to make an ultra-low noise-floor version that sets the smoke stream between the sound input aperture and the beam, instead of aiming the laser beam directly through the smoke column.
This way, when there is no sound, the beam crosses the detection chamber without any interference by smoke: it is dead quiet. As soon as any sound pressure affects the smoke, it intrudes into the beam's path, modulating the laser and thus yielding the audio signal.
The new mode does not replace the previous mode, but rather gives mic designers more options, which has the potential of leading to different applications for both modes. Combined modes using two or more lasers are disclosed in the patent.
Though demonstrated at the recent Audio Engineering Society event in New York, it clearly has a long way to go before becoming a practical system, but Schwartz is confident he can get into a more integrated and portable system. If not, I'm sure it can be adapted to 'other' purposes.
For more information, review the patent and an accompanying updated paper.