Electrical Energy Into Mechanical Energy

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4.1. Principle of Operation
Ultrasonic sensors are devices deployed to convert electrical energy into mechanical energy. Ultrasonic waves are longitudinal waves which move as a series of compressions and rarefactions across the direction of wave propagation through the medium [37]. In addition to distance measurement, they are also utilised in ultrasonic material testing to detect; air bubbles, cracks, and other defects in products, detection of object and position, ultrasonic mouse, etc [37].
Ultrasonic sound waves are mechanical vibrations that display all of the same features as audible sound waves, only they operate at higher frequencies. Audible sound wave frequencies range between 20 Hz to 20 kHz. In contrast ultrasonic waves range from 20 kHz to 800 MHz, Generally, ultrasonic waves that are used for the testing of materials range from around 0.5 MHz to 20 MHz [38]. Ultrasonic waves are transferred via vibrating particles; therefore, ultrasonic waves require a transmission medium whether it is solid, liquid, or gas along which to propagate [3].
The velocity of sound is not constant, but, it alters in dissimilar media and also in the same medium at different temperature. For instance, it is estimated to be roughly around 331.9272 meters per second at 0°C in air medium; however, it increased to around 344.4240 meters per second at 20°C in the same air medium, or an increase of around 0.6248 meters per second for every centigrade degree rise in temperature [39]. Sound
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