The human ear is divided into three parts: the outer ear, the middle ear and the inner ear. The outer ear collects sound energy and channels it to the middle ear via the ear drum. The sound energy is converted into mechanical vibrations of small bones in the middle ear. These vibrations are transmitted to cochlea in the inner ear.

A membrane in the cochlea picks up these vibrations and converts them into neural impulses. These impulses are then transmitted to the brain along the auditory nerve.

When blockages exist anywhere in the passage from the outer ear to the middle ear, they result in conductive hearing loss. This results in the loss of ability to hear faint sounds, and particularly those of high frequencies.

When the cochlea is affected by ageing or disease, it results in sensory neural hearing loss. This affects the quality of the sound detected and results in loss of ability to understand speech and discriminate various sounds, especially in the presence of noise. The ability to locate sounds also decreases with age, particularly for sounds with relatively low volume and short duration, such as the tones generated by watches.