All of the five sense receptors in human beings are extremely complex pieces of bioengineering. It is a marvel of nature and has certainly not found any counterparts in mechanical engineering. The human ear is an extremely sensitive piece of equipment for the discernment of sound. Before we get into the details of how the human ear is made and how it hears sound, let us understand the nature of sound itself.

Sound waves can be compared to the ripples in a pond when it is disturbed by a stone falling on its surface. As ripples move out from the source of disturbance, similarly, ripples of differences in air pressure move out from the source of disturbance. Sound waves move out as differences in air pressure (compressed air and rarefied air). These fluctuations when hit an eardrum, the pressure differences manifest as sound.

Sound travels as waves of pressure through air at a speed of 740 miles (approx. 1184 km) per hour. There are two features of sound that we can measure – pitch and intensity. Pitch, also known as frequency, is a measure of how rapidly the waves change from above to below the ambient pressure. In other words, it is a measure of how many crests pass a point in a second. The greater the number, the higher is the pitch.

Intensity, or loudness, amplitude is a measure of how far the waves are above or below the ambient pressure. The greater the amplitude, the louder is the sound. Pressure levels of sound are measured as decibels (dB). These are logarithmic measurements, i.e., an increase of 10 dBhL corresponds to the loudness being doubled. It means that a sound of 80 dBhL is twice as loud as a sound of 70 dBhL.

The human ear is made up of three parts; the outer ear (pinna, ear canal and eardrum), the middle ear (the three bones called ossicular chain) and the inner ear (semicircular canal cochlea). The outer ear collects the sound waves and transmits them towards the eardrum, or tympanum. The tympanum vibrates and these vibrations are carried inside and amplified further by the ossicular chain. Further amplification takes place in the inner ear and the sensations are taken to the brain for analysis.

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