The travelling of sound is called propagation of sound. Sound is propagated by the to and fro motion of particles of the medium. When an object vibrates, the particles around the medium vibrate. Sound can be modelled in terms of pressure or terms of displacement of molecules. The human ear is sensitive to frequencies between 20 Hz and 20 kHz. The particle in contact with the vibrating object is first displaced from its equilibrium position. It deals with concepts such as the production of sound, propagation of sound, the reflection of sound, range of hearing, applications of ultrasound, the structure of the human ear. A few numerical are also there.