This is ultrasound, vibrations too fast for us to hear. Vibrations Too Fast To Hear. Human hearing can't go beyond about 18,000 vibrations per second, or 18 kHz. There are mammals that can hear well above this. Bats and whales use echolocation that can reach frequencies above 100 kHz.
Higher frequencies have a shorter wavelength.
A smaller size allows them to reflect from objects more readily and to give greater information about those objects. Extremely high frequencies are difficult to generate and to measure. Generally, the higher frequencies are used for medical imaging, such as investigating a fetus in the mother's womb. Lower frequencies could affect the tissues being studied or introduce unwanted noise into the imaging process. Ultrasound diagnosis is a process which translates electric pulses to high frequency sound vibrations and then listens for the echoes.