Brian E. Anderson
Time Reversal Acoustic Private Communications
Time reversal is a signal processing technique that allows audible sound to be focused to a target location from remotely placed sound sources. A calibration signal must be obtained to inform the delivery system how sound gets from the source location(s) to the intended target location. This calibration signal can be recorded in advance before it is needed, a numerical simulation of the room can provide the calibration signal, or it may be obtained covertly using several ideas developed by Brian Anderson. Below are some proof of concept experiments conducted in a typical conference room.
Two Speech Foci
Two speech samples are simultaneously focused to two different seat locations. Microphones were used to obtain the calibration signal (impulse response) and allow the viewer to see where the focused speech is optimal. The microphones could be removed once the calibration signal is obtained.
Speech is focused to one seat location and white noise has been added to mask the speech like sounds heard elsewhere in the room.
Focused Pulses from Outside
Speakers are placed outside the room and sound is focused within the room. Here sound pulses are focused instead of speech. The sharper the pulse the better the speech delivery can be.
Laser Calibrated Focus
A laser Doppler vibrometer is used to obtain the calibration signal and then speech is focused to that seat location. The microphone in the video was only used to quantify the performance and as a visual indication of where the speech is focused to. Infrared laser vibrometers are commercially available so that a visible light laser does not have to be used.