Sound is a mechanical wave that propagates through a medium (for example air, water, or biological tissues). It arises from the oscillation of particles in the medium and propagates as a longitudinal wave formed by alternating regions of compression and rarefaction.
The prefix ultra- means "beyond" or "above." In this case it denotes frequencies higher than the upper limit of human hearing. Humans can perceive sound approximately in the range of 20 Hz to 20 kHz. Waves with higher frequency are called ultrasound.
In medical imaging, much higher frequencies are used, typically about 2–25 MHz. These frequencies are many times higher than audible sound and therefore cannot be detected by the human ear.
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