Commercial laser microphones intended to collect speech from a distance are advertised at costs ranging from US$ 5000 to US$ 20,000 on the world wide web. Suppliers claim these devices will operate at distances of "up to" 100 yards/meters.
The literature conjures visions of office
windows vibrating in sympathy to speech and invisible light reflecting at
varying angles. The commercial models seem to favor 830 nM. infrared lasers with
optical beam spreaders, prisms, splitters, polarizers and photo-diodes
configured as a Michaelson Interferometer. The "light", being
invisible, does not make them an effective security awareness tool.
The demonstration system shown here is a very effective security awareness tool that can be produced at a fraction of this cost from quite common parts. The top photo shows a 640 nM. red laser from a classy pocket pointer ($10) clamped to the side of a 10 inch chromed brass toilet filler tube ($2) at Home Depot supported in a hobby vise ($10). The middle photo shows the Radio Shack photo transistor ($2), wrapped in layers of heat-shrink tubing ($0.50), and stuffed into the left end of the toilet filler tube. The yellow wire connects to an an audio amplifier with headphones. A dollar store Walkman would do. Optical alignment is achieved, with the phototransistor removed, by adjusting the laser position on the tube so the distant spot is centered when viewed through the tube.
The "secret" to the effectiveness of this demonstration system is the use of "mini-dot" targets prepared from reflective adhesive automotive safety tape. When affixed to surfaces stimulated by sound, this tape modulates the intensity of the red laser light reflected to the phototransistor. Cardboard boxes, calendars, bubble-wrap, and thumb-tacks are examples of surfaces which can be prepared in this manner.
© OFARTS Canada 2002-2007 Old Foreign Affairs Retired Technicians, Canada