Sound is produced whenever a body vibrates in a medium. These vibrations travel through the medium with a certain speed depending on the density and elasticity of the medium. When vibrations reach our ear, sound is heard. Our ears are sensitive only to a limited range of frequencies 20 Hz to 20,000 Hz. This range of frequency is, therefore, called the range of audibility. However, the audibility range of a person decreases as he gets older since the ears lose the hearing sensitivity to both the low and high frequencies. The sound of frequency above 20,000 Hz is called the ultrasonic, while the sound of frequency below 20 Hz is called the infrasonic. Ultrasonic and infrasonic both are inaudible to human beings.
The wave velocity V, frequency f and wavelength λ are related as V = f λ, whereas the time period of wave T and its frequency f are related as f = 1/T. Note that the frequency or time period of wave depends on the vibrating source producing the wave, while the velocity of wave and hence its wavelength depends on the properties of the medium in which the wave travels.
The sound waves necessarily require a medium for their propagation and they are also called the elastic or mechanical waves. These waves travel in the medium through the vibrations of the medium particles about their mean positions. If the vibrations of medium particles are along the direction of propagation of the wave, thus forming compressions and rarefactions in the medium, the wave is called a longitudinal wave example sound waves in air, solid and inside a liquid.
The longitudinal waves can travel in solid, liquid as well as gas. On the other hand, if the medium particles vibrate normal to the direction of propagation, forming crests and troughs, the wave is called a transverse wave e.g. sound waves in a solid and on the surface of a liquid. Transverse waves are formed only in those media which possess rigidity. They can travel only in solids and on the surface of liquid.
The speed of a wave in a medium depends on the elasticity and density of the medium. Since the density decreases with increase in temperature and with the increase in humidity, so the speed of sound increases with the increase in temperature and with the presence of humidity. However the speed of sound is not affected by the change in pressure.