In human beings, frequency range of hearing from 20 Hz to 20,000 Hz is called audible range. Human ear is sensitive to frequencies between 20 Hz to 20,000 Hz.

Human ears cannot hear sounds of frequencies less than 20 Hz and above 20,000 Hz. 

(a) Sound of frequency less than 20 Hz is called infrasound.

(b) Sound of frequency higher than 20 kHz is called ultrasound.

Sound is a form of energy. Sound is produced due to vibration of different objects thus, produces a sensation of hearing in our ears. 

Propagation of sound through air. Sound travels through air in the form of longitudinal waves. Consider a vibrating tuning fork producing sound waves as shown in Fig. 12.8. Its both prongs first move inward and then outwards and so on. We focus our attention to prong B only. As this prong moves from B to B₁ it compresses the layer of air in front of it. As this compressed layer moves forward, it compresses the next layer and so on. So a wave of compression moves forward. When the prong moves backward from B₁ to

Fig. 12.8. Tuning fork producing sound waves in air.

B₂, the pressure of air in the adjoining layer decreases. The next layer, being at higher pressure tends to move it towards right and so on. So a wave of rarefaction moves forward. The vibrating tuning fork continues to send a series of compressions and rarefactions.

These waves finally reach the ear and force the tympanic membrane to vibrate with their frequency and thus cause the sensation of hearing.

Compression is the region of high pressure and rarefaction is the region of low pressure. Higher the pressure in a region, higher is the number of particles per unit volume and hence higher is the density of the medium. So, a sound wave propagates through a medium as the variation in its pressure or density.

The audible range of frequency extends from 20 Hz to 20,000 Hz, for an average human ear.