This is another reason to mount the ultrasonic detector facing the possible point of entry. These need to be very small, smaller in fact than the tweeter in a hi-fi loudspeaker, because the moving parts must move very rapidly and so their mass must be kept to a minimum.Ultrasonic detectors usually have a range up to 30ft, but they have a sensitivity control which can reduce the range down to about 10 ft.

The term ultrasonic denotes sound frequencies that are Terminal Blocks above the range of human hearing, the upper limit of which is 16 kHz in healthy people in their twenties. The same unit contains a microphone with amplifying and processing circuits. The polar pattern is in the form of a narrow lobe and so the device affords little protection at the sides.Let's cover ultrasonic detectors. The ultrasonic detector can thus be connected into a normal loop, but it is preferable for space detectors to be connected to a separate zone from the perimeter sensors so that rapid identification of the alarm source can be made. Should movement of an object occur, the sound reflected from it will undergo a change of frequency owing to the Doppler Effect.. When the two frequencies are close, the third is often termed a beat note.

The frequency is generated by an electronic oscillator and is fed to one or more loudspeakers. When there is no movement, both direct and reflected sounds are of the same frequency. As the Doppler Effect is greatest for forward or backward motion, the sensor is more sensitive to these than to side-to-side movements. A power supply of 25 - 50 mA is required from the control box. This is an advantage of an ultrasonic detector alarm system as the units can then be made small and unobtrusive. The effect can sometimes be heard when two internal combustion engines are idling at slightly different speeds: the beat note is heard by the ultrasonic detector as a throbbing sound that varies in frequency as the speed of one or other of the engines changes.Ultra-high frequency sound is thus projected into the protected area of the ultrasonic detector.

The frequencies used in ultrasonic detectors vary between different models, ranging from 23 to 40 kHz. Thus the ultrasonic detector microphone now picks up two frequencies: the original received directly from the loudspeaker, and the shifted frequency reflected from the moving object.The processing circuits in the ultrasonic detector detect the presence of the beat note and actuate the output relay which is of normally closed type. Some of it is received directly by the microphone from the loudspeaker, while some is picked up after being reflected from walls and objects in the room.If two different frequencies are mixed, the result is that a third one appears that is the difference between the two. It should therefore be pointed towards the expected point of entry

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