The sound waves having frequency higher than 20,000 Hz is called ultrasonic waves or ultrasound. Human beings cannot hear ultrasound. Dogs, bats and dolphins can hear ultrasound. For example bats and dolphins can hear sound waves having frequencies of about 150,000 Hz. Bats and dolphins detect the presence of any obstacle by hearing the echo of the sound produced by them.
Ultrasound finds many technological applications.
Characteristics of Ultra Sound :
Ultrasound (or ultrasonic waves) are found very useful due to the following reasons :
Ultrasound (or ultrasonic) waves are high frequency sound waves. So these waves have short wavelength. These short wavelength sound waves can be reflected back from the smaller objects. Thus, ultrasound can detect smaller objects (< 1 cm size). The sound waves in the audible range cannot detect or 'see' objects smaller than having size ranging from a few tens of centimeters to a few metres.
Ultrasound beam is more directional and can be aimed towards any target just like a torch. These waves remain undeviated over long distances.