Your Auditory System

Your Auditory System

Your Auditory System

Your ears convert vibrations in the air into nerve impulses in three stages. Your outer ear funnels sound waves to your eardrum and the waves cause that delicate layer of skin to vibrate. Then tiny bone structures in your middle ear amplify the mechanical vibrations of the eardrum and pass the amplified motion to your inner ear. The cochlea is the main structure in the inner ear. It is a coil about 3 cm long, filled with fluid and lined with nerve endings. Vibrations from the bones in your middle ear cause the fluid in the cochlea to vibrate and that motion generates auditory nerve impulses.

About forty percent of the nerve impulses from an ear on one side of your head go the same side of the brain, and sixty percent are transferred to the opposite side of the brain.

With both ears operating you are able to determine the approximate direction of a source of sound. And, if you are familiar with a source of sound, you can also make a reasonable estimate of the distance to the source.

Human ears are sensitive to a wide range of vibrations, from a low hum at 20 Hertz (cycles per second) to a high squeak at 20 000 Hertz. Your ears can also detect a wide range of intensities, from a pin dropping on the floor, to a jet engine. Your ears can also detect more than one frequency at a time; when two notes are played on a piano you can detect both notes. This ability lets you interpret speech and enjoy music.

Your ability to interpret speech also depends on the social context of a conversation, since you do not always hear every word that is spoken. In an experiment to test the ability of subjects to perceive spoken language, portions of the words in a conversation were blanked out at random. As long as a conversation had a known context, subjects were still able to understand about 85% of the words spoken, even when 50% of the sounds were deleted.

For a right-handed person, the right ear (feeding most of its impulses to the left hemisphere) dominates in perceiving speech and language. The left ear (feeding most of its impulses to the right hemisphere) dominates in perceiving non-verbal sounds.

Your ability to detect high frequencies declines with age, at the rate of about 150 Hertz per year after age 40.