Seeing the sound to locate its source

Seeing the sound to locate its source. Several microphones arranged in a given pattern can be used to locate a sound source by analyzing the phase mismatches of the signal at different receivers. That long-established technique is called beamforming. The top panel of the figure shows the output from a line of 19 microphones in response to a simulated incident plane wave. The dark lane at 0° identifies the direction of the source as being broadside; that lane would shift up or down for a source in a different angular direction. The other dark lanes appearing at high frequencies are unavoidable artifacts—spatial aliasing—due to the discrete separations of a finite number of microphones. Those artifacts limit the technique to the lowest frequencies. If infinitely many microphones formed a continuous line, the artifacts would go away and a larger frequency range could be used. Researchers from the Danish Fundamental Metrology Institute and the Technical University of Denmark recognized that sound creates variations in pressure—and thus in density—and that those variations affect the phase velocity of light. So they replaced the line of discrete microphones with a continuous laser beam, generated in a laser Doppler vibrometer (LDV) that typically samples a vibrating surface. Instead, they had the beam reflect off a rock-solid mirror and back into the LDV, thereby sampling the sound fluctuations along its path. The bottom panel shows an experimental realization of the researchers’ theoretical analysis, with 0° now representing the head-on direction. With the LDV and reflector mounted on a turntable, the beamformer can be steered to find an acoustic source without any trace of spatial aliasing. The technique may prove useful where microphones cannot be deployed, such as in high-temperature environments. (A. Torras-Rosell, S. Barrera-Figueroa, F. Jacobsen, J. Acoust. Soc. Amer. 132, 144, 2012.)