Ultrasonic propagation second sound

An acoustic wave (sound) is a propagation of pressure oscillation in medium such as air or liquid water with the sound velocity [1]. Ultrasound is inaudible sound and its frequency of pressure oscillation is above 20 kHz (20,000 oscillations per second) [2]. For convenience, an acoustic wave above 10 kHz in frequency is sometimes called an ultrasonic wave. 

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