Ultrasound or ultrasonic waves are sound waves with frequencies above 20,000 Hz. These waves are undetectable by humans as they lie above the audible range.
Ultrasonic waves have high frequencies and short wavelengths.
We know that the velocity of sound waves in air is 330 m/s. Therefore for an audible sound with frequency 1000 Hz we can calculate the wavelength using the wave equation:
velocity = wavelength x frequency
wavelength = velocity/frequency
wavelength = 330/1000
wavelength = 0.33m
However, if the sound is in the ultrasonic range for example 100,000 Hz then the wavelength can be calculated as:
wavelength = 330/100000
wavelength = 0.0033 m = 0.33 cm
Hence the ultrasonic wave with is shorter wavelength can be transmitted as a beam and undergoes little diffraction; these properties of ultrasound are put to use in various applications.