Sound is a form of energy. Each and every living and nonliving in this world produces sound.

On the basis of frequency, sound waves are classified into three types:

(i) Audible sound : The human ear can hear the sound waves of frequencies ranging from 20 to 20,000 Hz. This range is known as audible sound.

(ii) Infrasonic sound : The sound waves of frequency less than the audible range (<20 Hz) and cannot be heard by human ear. This sound is known as ‘Infrasonics’ (or) infrasonic sound.

(iii) Ultrasonic sound : The sound waves of frequency of more than 20KHz (or) 20,000 Hz and cannot be heard by human ear. This sound is known as ultra ‘Ultrasonics’ (or) ultrasonic sound.

Ultrasonic sound waves are also called high frequency sound waves. Human ear cannot hear this sound. But bats are capable of hearing this sound. Bats are able to avoid obstacles in the dark by producing ultrasonic sound waves and by detecting the echoes from the obstacles.

Properties of Ultrasonic waves

1. The frequency of ultrasonic waves is more than 20,000 Hz.
2. They travel through long distance with minimum energy loss.
3. They are highly energetic waves.
4. Ultrasonic waves are longitudinal in nature.
5. Energy can be increased by increasing frequency of ultrasonics.
6. They produce heating effect when passes through a medium.
7. It undergoes reflection and refraction like sound waves.