How Our Hearing Functions

The shape of the fleshy part of the outer ear, called the auricle, or pinna, is designed to collect sound waves and direct them  into the ear canal, where they soon reach the eardrum to vibrate, and the eardrum, in turn,  causes the three bones  in the middle ear to vibrate. Next the vibrations are transmitted into the inner ear, fluid-filled sac encased in bone. Here the vibrations move through the fluid in the cochlea, the snail-shaped hearing part of the inner ear that contains the hair cells.  the fluid in the cochlea triggers the top portion of the hair cells to produce  readable nerve impulses.  These impulses are then transmitted to the brain, where they are decoded and interpreted as sound. 

The limbic system helps the brain decide which sounds to pay attention to and which to dismiss. For instance,  a mother may not consciously hear the normal sound of a child at play, but she will respond instantly to a cry of alarm. Hearing with two ears enables us to hear in stereo, which is very useful.  It allows us to identify where the sounds are coming from.  Yet, when a sound consists of speech, the brain can only understand one message at a time.  "This is why," says the book The Senses," when listening to someone whole talking on the telephone, people cannot readily take in what the person next to them is saying." 

Next time: How Noise Damages Our Hearing

From the Awake! magazine, 2002