The Sense of Hearing

Our sense of hearing allows us to perceive the loudness, pitch and timbre of a sound, as well as to localize where a sound is coming from.

The outer ear helps to ‘gather’ sound which passes down the ear canal until it reaches the eardrum. The eardrum and middle ear systems convert the pressure wave into a pattern of vibration. This vibration is transferred to the inner ear, or cochlea, where it causes a pressure wave in the cochlear fluid. This pressure wave sets a membrane in the cochlea in motion. Hair cells located on the membrane detect the motion, and in turn, cause activity in the fibers of the auditory (hearing) nerve. The brain interprets this nerve activity as sound.

Normal hearing covers a frequency range of sounds between 20 Hz and 20,000 Hz (20 kHz). However, this range varies significantly with age. Usually, the sensitivity for high frequencies diminishes with advancing age. The majority of speech sounds are within a frequency range of between 100 and 8000 Hz. The human ear is most sensitive to frequencies around 1000–3500 Hz. Sound frequencies above the hearing range are known as ultrasound.

The brain also exploits the fact that we have two ears. Having two independent listening points on either side of our head allows nerve pathways in the brain to compare and contrast the signal from both sides. This can help us to hear more easily in noisy situations, and helps us to determine which direction a sound is coming from.