Introduction

Hearing is easy to take for granted. But appreciation for normal hearing usually grows when hearing loss occurs. The importance of hearing, however, goes beyond the mere perception of sound. It also affects a person's interactions, thoughts, and skills. Several common expressions, with their emphasis on the ear as symbolic of hearing, give hearing its due. They describe attention ("lend an ear"), the ability to play a song after only having heard it ("playing by ear"), the act of willfully ignoring ("turn a deaf ear"), forgetfulness ("in one ear and out the other"), and suspicion and deceit ("the walls have ears").

See the figure A Look Inside the Ear.

The ear is a complex organ capable of perceiving the minute changes in air pressure that make up sound. The ear has three parts: the outer ear, the middle ear, and the inner ear. The outer ear consists of the external ear (pinna) and the ear canal. The external ear focuses sound waves into the ear canal, which boosts the volume of frequencies critical to human hearing. The ear canal leads to the eardrum, which separates the external ear from the middle ear. The eardrum vibrates in response to sound waves and transmits the vibrations to three tiny bones in the middle ear called ossicles. The ossicles amplify the vibrations and transmit them to the cochlea. Within the cochlea—a fluid-filled structure in the inner ear—small hairlike projections (cilia) conduct electrical current when moved by sound vibrations. The signal is then transmitted by the auditory nerve to the brain.

The eustachian tube connects the back of the pharynx (throat and nose) with the middle ear. It functions like a safety valve, allowing fluid that accumulates in the middle ear (for example, from ear infections) to drain into the throat. The eustachian tube allows air pressure in the middle ear to balance, or equalize, with the air pressure in the throat. When the nose is stuffy because of an allergy, a viral illness, sinusitis, a large adenoid (common in children), or a tumor, the eustachian tube may not open normally. Without this equalization of pressure, hearing loss may result.

Numerous disorders can affect hearing and the ears. Older people commonly experience some degree of hearing loss. Also common among older people are tinnitus and accumulation of earwax.