 Ultrasonic motion detectors generate inaudible sound
waves/patterns and recognize a disturbance in those patterns caused by an intruder. These
are compact devices which make the installation simple. The ultrasonic transmitter
transmits high frequency, inaudible sound waves into the protected area and the receiver
picks up the energy reflected back to the unit by the walls, ceiling, floors and other
fixed objects in the enclosed area. The received signal is processed and analyzed by the
signal processor. An intruder moving into the protected area causes some waves to be
reflected resulting in change of pattern and hence change in strength of signal received
by Receiver. If there is no movement within the area of coverage, the signal strength
remains unchanged. These changes causes the unit to detect the movement and sound the
alarm.
While both ultrasonic and microwave use the same principle to detect motion, a microwave
uses a much higher frequency and as a result it can detect motion through most walls &
partitions. Motion Sensors generate signals in the range between 19 and 40 kilo hertz
which is above the frequencies that a normal ear can hear. Ultrasonic energy is contained
completely in the area it is operating. It will not penetrate walls or windows but is
absorbed by carpet, curtains and false ceiling. Obstructions within a room will reflect
the ultrasonic energy and distort its shape pattern.
Typical detection range of ultrasonic units is 20 feet wide by 30 feet long in a room with
a ceiling up to 12 feet high. Ultrasonic detectors can be prone to false alarms due to
excessive air currents and other extraneous ultrasonic noises. Proper application and
installation of this equipment is very important. The ultrasonic units can be mounted on
any plane, vertical or horizontal or in any location - ceilings, walls, corridors -
projecting the ultrasonic pattern at any desired angle. These units provide volumetric
space protection.
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