How we hear

The sounds we hear are the result of vibrations of air, fluid, and solid materials in our environment. The vibrations produce sound waves, which vibrate at a certain speed (frequency) and have a certain height (amplitude). The vibration speed of a sound wave determines how high or low a sound is (pitch); the height of the sound wave determines how loud the sound is (volume).

Hearing is the result of these sound waves traveling through the ear and being converted into nerve impulses that are sent to the brain, which “hears” them.

• Sound waves enter the ear through the ear canal (external ear) and strike the eardrum (tympanic membrane), which separates the ear canal and the middle ear.
• The eardrum vibrates, and the vibrations move to the bones of the middle ear. In response, the bones of the middle ear vibrate, magnifying the sound and sending it to the inner ear.
• The fluid-filled, curved space of the inner ear, sometimes called the labyrinth, contains the main sensory organ of hearing, the cochlea. Sound vibrations cause the fluid in the inner ear to move, which bends tiny hair cells (cilia) in the cochlea. The movement of the hair cells creates nerve impulses, which travel along the cochlear (auditory, or eighth cranial) nerve to the brain and are interpreted as sound.