Ultrasonic Control


Ultrasonics is the study of all sound like waves whose frequency is above the range of normal hearing. Audible sound is in a range of about 30 to 20,000 hertz (1 Hz = 1 cycle per second). The basic modern source of ultra sound today is specially cut crystals of materials like quartz or ceramics such as barium titanate and lead zirconate. Alternating electrical voltage it applied to opposite faces of a plate made of the material. This produces an alternating electrical expansion and contraction of the plate at the impressed frequency. This phenomenon is known as piezoelectricity when it occurs in crystals. The frequency of alternation is given by the formula, f = c/2l, where c is the speed of sound in the object and l is the thickness of the plate. If the size of the alternating expansions become very large, the plate is said to exhibit resonance.


Ultrasonics at one time was used in TV remote controls. Problems occurred when other sources of ultrasonic sound affected the TV. Plus, the high frequencies used in the remote controls made bothered dogs due to their ability to hear high frequencies. Despite this, the ultrasonic remote control was the industry standard for two decades.


Telephone answering machines also use ultrasonics.


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