Sound is a form of energy which produces a sensation of hearing in our ears. It is produced due to vibration of different objects. Sound waves are longitudinal waves and propagate in a medium in the form of compressions and rarefactions. These waves cannot travel in vacuum. A sound wave can be described by its three important properties i.e., frequency, amplitude and speed. It can be reflected at the surface of a solid or a liquid and follows the laws of reflection, like light. The repetition of sound caused by the reflection of sound waves by a tall building or a mountain is known as echo. The time interval between the original sound and reflected one should be at least 0.1 s in order to hear a distinct sound. However, the persistence of sound due to the repeated reflections of sound is known as reverberation.
Human beings and most of the animals are able to hear with the help of a sense organ, called the ear. It consists of three basic parts - outer ear, middle ear and inner ear.
Human beings can hear the sounds of frequencies between 20Hz to 20,000Hz. It is known as the audible range. The sounds of frequencies less than 20 Hz are called infrasonic sound or infrasound whereas the frequencies higher than 20 kHz are called ultrasonic sound or ultrasound. Ultrasound has many medical and industrial applications.