Understanding hearing EXPECTED LEARNING To learn how our ears allow us to hear and balance.

The Auditory System: a quick overview The auditory system is divided into three major parts: The outer ear which is the part you see. The middle ear which includes the eardrum and is involved in the hearing process. Lastly the inner ear which includes the cochlea and semicircular canals (vestibules). It is involved in both hearing and balance.

Outer (External) Ear The primary job of the outer ear is to collect sound waves and direct them to the middle ear. Due to sound travelling relatively slowly in air, 332 meters per second, there is a slight delay between the time that a sound will reach one ear and then the other. Why is this important? What can it tell us?

Middle Ear Sound travels down the ear canal to the tympanic ear, also known as the ear drum, causing it to vibrate. This vibration causes vibration of the three ossicles of the middle ear, which in turn causes the oval window at the start of the inner ear to vibrate. Several skeletal muscles are attached to the ossicles to dampen excessively loud noises to help protect your ears.

Inner Ear The inner ear is the most complex part of the ear. The cochlear is a fluid filled tube which is curled up like a snails shell. It is the movement of the fluid in the cochlear causing the vibration of the basilar membrane that produces the signals which are sent to the brain allowing us to hear.

The Human Ear Watch Auditory Transduction video! You will need to have enabled macros and to view this as a slideshow for this slide to work properly! The Human Ear Watch Auditory Transduction video!

The Auditory System: a quick review The auditory system controls your sense of hearing. Hearing begins with sound waves—vibrations in air, water, or solid material. A sound can be described by its pitch (frequency - hertz) and it’s loudness (amplitude - decibels)

Decibel Level of Some Common Sounds Decibels Source Exposure Danger 180 Space shuttle launch Hearing loss certain within 150 feet of launch pad 140 Threshold of severe pain - 130 Rock band Painfully loud at 15 feet 120 Sandblaster, thunderclap Immediate danger 100 Heavy auto traffic, lawn mower 2 hours 60 Normal conversation No danger 40 Quiet office 30 Quiet library 20 Soft whisper 10 Normal breathing Minimal detectable sound

Hearing “IMAGINE that you stretch a pillowcase tightly across the opening of a bucket, and different people throw ping-pong balls at it from different distances. They can each throw as many balls as they like, and as often as they like. Your job is to figure out, just by looking at how the pillowcase moves up and down, how many people there are, who they are and whether they are walking towards you, away from you or standing still. This is essentially the problem your auditory system has to contend with when it uses the eardrum as the gateway to hearing.” Source: http://www.newscientist.com/article/mg19726441.500-music-special-the-illusion-of-music.html

Theories of hearing Two main categories of theories: Place theories These theories suggest that the analysis of sound occurs in the basilar membrane of the ear, with different frequencies and intensities affecting different parts (places) of the membrane. Each sound wave causes a travelling wave on the membrane which causes displacement of hair cells on the membrane. The displacement of hair cells triggers specific information about the pitch of the sound. The bigger the stimulus the bigger the wave

Theories of hearing Frequency theories These theories suggest that the analysis of a sounds pitch and frequency occurs in the brain, in the auditory area of the cortex. They claim that the entire basilar membrane is stimulated by a sound and its overall rate of responding somehow provides information to the brain for processing. Neither type of theory is correct in isolation- hearing actually occurs in a way that involves both of these theories, so modern researchers have developed theories that are a combination of the above.

Sound localisation Some examples: a mother knowing where to turn when she hears her baby crying a person walking home at night knowing where their keys hit the ground even though they didn’t see them drop a teacher knowing which student to ask to be quiet when they talk in class It is influenced by two main factors: Interaural time differences: a sound produced to the left of your head will arrive at the left ear first Interaural intensity differences: a sound produced to the left of your head will have different intensities when it reaches each of your ears, it will be perceived as slightly more intense by the left than right

Sound localisation Other influencing factors: head and body movements, ridges and folds of the external ear, visual cues and previous experiences If the sound isn’t right in front of you, you need to use head and body movements to find the source of the sound. You rotate your head and/or move your body to try and determine it.