Light Mic

Microphones are among the most mature devices in the electronic musician's toolbox, and they all work according to the same principle: sound waves impinge
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FIG. 1: In a particulate flow detection microphone, a stream of air infused with particles passes through a chamber within the outer casing near the top, where the particles are perturbed by sound waves entering from the back. A laser beam passes through the stream, which modulates its intensity, and a photosensor converts that modulated light into an electrical signal analogous to the -original sound wave.

Microphones are among the most mature devices in the electronic musician's toolbox, and they all work according to the same principle: sound waves impinge on a diaphragm of some sort, causing it to vibrate in a pattern that is analogous to the acoustic waveform. The diaphragm's vibration is converted to an electrical signal whose waveform is analogous to the vibration (and thus to the acoustic waveform) by means of electromagnetic induction.

This process has just one problem: all diaphragms have a finite mass, which prevents them from changing their vibration instantaneously in response to an instantaneous change in the sound wave. Thanks to the immutable law of inertia, it takes a certain amount of time for the diaphragm to change its vibration to match the new sound wave, leading to distortion in the electrical signal.

California-based inventor David Schwartz has come up with an ingenious solution to this problem. Sound waves perturb particles, such as water vapor, suspended in a stream of air. The perturbations, which manifest themselves as changes in the density of the particles, are analogous to the acoustic waveform, just like a conventional diaphragm. Unlike a diaphragm, however, the particles in the airstream have far less mass and can thus respond to changes in the sound wave with virtually no delay or distortion.

To convert the perturbations in the particle stream to an electrical signal, a laser beam passes through the stream to a photosensor on the opposite side. The changes in density correlate to changes in the transparency of the stream, modulating the intensity of the laser light at the sensor, which converts the modulated light into an electrical signal that is analogous to the original sound wave.

In his patent application, Schwartz describes one possible design for what he calls a “particulate flow detection microphone.” As illustrated in Fig. 1, the stream enters the microphone at the bottom and travels through a tube into a sensing chamber near the top. Sound waves enter the chamber from one side, while the laser and photosensor are aligned on an axis perpendicular to both the stream and the direction of the sound waves. The stream then exits the chamber, replaced by unperturbed particles.

That last point is important — for the microphone to work properly, the airstream must be in constant motion so that the sound waves can affect “fresh” particles that have not already been perturbed. In a way, the airstream resembles a piano roll, representing the sound wave in a continuous strip rather than as a single object vibrating back and forth.

Schwartz has experimented with nonturbulent flow (technically called laminar flow), as well as with turbulent flow, and has found that both produce interesting results. In particular, a turbulent flow could be used to generate a “whiter,” more random noise floor, or one in which the noise is mainly in high frequencies, where it has a less perceptible effect on the audio.

The beauty of this idea is that it eliminates all nonlinearities in the conversion from acoustic to electrical energy. As a result, distortion drops to zero, and the electrical signal is a far more accurate representation of the sound wave than any conventional microphone can manage. I'm excited by the potential of this invention, and I hope to see it become available as a commercial product someday.