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