Hearing (a.k.a. Hearing (a.k.a. AuditionAudition))

Our auditory sense

If a tree falls in the forest If a tree falls in the forest and no one is there to hear and no one is there to hear

it, is there any sound?it, is there any sound? The answer to this philosophical question is quite The answer to this philosophical question is quite

simple when it comes to science…simple when it comes to science… Yes, because sound is defined physically in terms Yes, because sound is defined physically in terms

of changes in air pressure. These changes in air of changes in air pressure. These changes in air pressure could be recorded in the absence of an pressure could be recorded in the absence of an observer.observer.

If, however, the question is “Is there any If, however, the question is “Is there any noisenoise?,” ?,” then the answer is quite different…then the answer is quite different…

No, because noise is a psychological correlate of No, because noise is a psychological correlate of sound generated by brain activity. Without an sound generated by brain activity. Without an observer, there is no noise.observer, there is no noise.

Hearing: The Auditory SystemHearing: The Auditory System Stimulus = Stimulus = sound wavessound waves

(vibrations of molecules (vibrations of molecules traveling in air)traveling in air)• Amplitude (loudness)Amplitude (loudness)• Wavelength (pitch)Wavelength (pitch)

Wavelength/Frequency Wavelength/Frequency The number of complete wavelengths that pass through The number of complete wavelengths that pass through

a point at a given time determines the pitch (range of a point at a given time determines the pitch (range of high and low sounds) of a sound. high and low sounds) of a sound.

• AmplitudeAmplitude is a measure of the physical strength is a measure of the physical strength of the sound wave (shown in its peak-to-valley of the sound wave (shown in its peak-to-valley height). height). *It is a description of sound pressure and it is *It is a description of sound pressure and it is measured in measured in decibelsdecibels (db). (db). **The amplitude determines how loud the sound is. The amplitude determines how loud the sound is. The higher the crest of the wave is, the louder the The higher the crest of the wave is, the louder the sound is perceived. sound is perceived. *When you turn the volume down on your stereo, *When you turn the volume down on your stereo, you are decreasing the amplitude of the sound you are decreasing the amplitude of the sound waveswaves

The Structure of the EarThe Structure of the EarThe External Ear, Middle Ear, The External Ear, Middle Ear,

and Inner Earand Inner Ear

The Outer/External EarThe Outer/External Ear

The outer ear is the part of the ear that people The outer ear is the part of the ear that people can see. It's what people pierce to wear earrings.can see. It's what people pierce to wear earrings.• The main job of the outer ear is to The main job of the outer ear is to

________________.________________.

The Middle Ear: Good VibrationsThe Middle Ear: Good Vibrations After sound waves After sound waves

enter the outer ear, enter the outer ear, they travel through the they travel through the ear canal and make ear canal and make their way to the middle their way to the middle ear. ear.

The middle ear's main The middle ear's main job is to take those job is to take those sound waves and turn sound waves and turn them into vibrations them into vibrations that are delivered to that are delivered to the inner ear. the inner ear.

To do this, it needs the To do this, it needs the eardrum, called the eardrum, called the tympanic tympanic membranemembrane, which is a , which is a thin piece of skin thin piece of skin stretched tight like a stretched tight like a drum.drum.

The Middle Ear: Good Vibrations (cont.)The Middle Ear: Good Vibrations (cont.) The eardrum separates The eardrum separates

the outer ear from the the outer ear from the middle ear and the middle ear and the ossicles. ossicles.

OssiclesOssicles: the three : the three tiniest, most delicate tiniest, most delicate bones in your body. bones in your body. They include:They include:• The The malleusmalleus, which is , which is

attached to the eardrum attached to the eardrum and means "hammer" in and means "hammer" in LatinLatin

• The The incusincus, which is , which is attached to the malleus attached to the malleus and means "anvil" in and means "anvil" in LatinLatin

• The The stapesstapes, the smallest , the smallest bone in the body, which bone in the body, which is attached to the incus is attached to the incus and means "stirrup" in and means "stirrup" in LatinLatin

When sound waves reach the When sound waves reach the eardrum, they cause the eardrum, they cause the eardrum to vibrate. eardrum to vibrate.

When the eardrum vibrates, When the eardrum vibrates, it moves the tiny ossicles — it moves the tiny ossicles — from the hammer to the anvil from the hammer to the anvil and then to the stirrup.(help and then to the stirrup.(help sound move along on its sound move along on its journey into the inner ear)journey into the inner ear)

The Middle Ear: Good Vibrations (cont.)The Middle Ear: Good Vibrations (cont.)

The Inner Ear: Nerve Signals Start HereThe Inner Ear: Nerve Signals Start Here When the stirrup vibrates, it hits When the stirrup vibrates, it hits

against the against the oval window, oval window, the the membrane surrounding a snail-membrane surrounding a snail-shaped structure called the shaped structure called the cochleacochlea

The cochlea is a small, curled tube The cochlea is a small, curled tube in the inner ear that is filled with in the inner ear that is filled with liquid, which is set into motion, like liquid, which is set into motion, like a wave, when the ossicles vibrate. a wave, when the ossicles vibrate.

The formerly airborne sound wave The formerly airborne sound wave becomes “seaborne” as the becomes “seaborne” as the vibrations set the fluid into wave vibrations set the fluid into wave motion.motion.

This fluid wave spreads through This fluid wave spreads through the cochlea, causing a the cochlea, causing a sympathetic vibration in the sympathetic vibration in the basilar membranebasilar membrane, a thin strip of , a thin strip of tissue running through the tissue running through the cochlea.cochlea.

This is where transduction, the This is where transduction, the conversion of vibrations into neural conversion of vibrations into neural messages, will happen…messages, will happen…

Transduction in the CochleaTransduction in the Cochlea Much like vision, the Much like vision, the

psychological sensation of psychological sensation of sound requires that waves be sound requires that waves be transduced into neural transduced into neural impulses and sent to the impulses and sent to the brain.brain.

The swaying of tiny hair cells The swaying of tiny hair cells (the auditory receptors) on (the auditory receptors) on the vibrating basilar the vibrating basilar membrane (much like the membrane (much like the swaying of buildings during an swaying of buildings during an earthquake) stimulates earthquake) stimulates sensory nerve endings sensory nerve endings connected to the hair cells. connected to the hair cells.

The axons of these nerve cells The axons of these nerve cells converge to form the converge to form the auditory nerve, auditory nerve, which sends which sends neural messages (via the neural messages (via the thalamus) to the temporal thalamus) to the temporal lobe’s lobe’s auditory cortex auditory cortex in the in the temporal lobes.temporal lobes.

Ear Sound Waves 1Ear Sound Waves 1

Instructor’s Notes

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Right-click on animation for playback controls.

Ear Sound Waves 2Ear Sound Waves 2

Instructor’s Notes

Copyright © Houghton Mifflin Company. All rights reserved.

PowerPoint® 2000 or better with Flash® plug-in required to view animations.

Right-click on animation for playback controls.

Eustachian TubeEustachian Tube

See http://www.sumanasinc.com/webcontent/animations/content/soundtransduction.html for an animated demo of the process of hearing.

How do we perceive How do we perceive differences in pitch?differences in pitch?

There are two theories:Place Theory and Frequency Theory

Helmholtz’s Place TheoryHelmholtz’s Place Theory Hermann von Helmholtz Hermann von Helmholtz

(1863) proposed that (1863) proposed that perception of pitch perception of pitch corresponds to the corresponds to the vibration of different vibration of different portionsportions, or places, along , or places, along the basilar membrane.the basilar membrane.

Thus, different places have Thus, different places have different pitches, like keys different pitches, like keys on a piano.on a piano.• So some hairs vibrate when So some hairs vibrate when

they hear high pitches and they hear high pitches and others vibrate when they hear others vibrate when they hear low pitches.low pitches.

Helmholtz’s Place TheoryHelmholtz’s Place Theory Problem with the theory? Problem with the theory? It can’t explain how we hear It can’t explain how we hear

low-pitched sounds, because low-pitched sounds, because the neural signals for low-the neural signals for low-pitched sounds are not so pitched sounds are not so neatly localized on the neatly localized on the basilar membrane.basilar membrane.

Frequency TheoryFrequency Theory Frequency theoryFrequency theory: which holds that : which holds that

perception of pitch corresponds to perception of pitch corresponds to the rate at which the entire basilar the rate at which the entire basilar membrane vibratesmembrane vibrates• Causing the auditory nerve to fire at Causing the auditory nerve to fire at

different rates for different frequencies. different rates for different frequencies. Thus, according to this theory, the Thus, according to this theory, the

brain detects the frequency of a tone brain detects the frequency of a tone by the by the rate at which the auditory rate at which the auditory nerve firesnerve fires..

All the hairs vibrate but at

different speeds.

Frequency TheoryFrequency Theory

We sense pitch by the basilar We sense pitch by the basilar membrane vibrating at the same membrane vibrating at the same rate as the sound (if a sound wave rate as the sound (if a sound wave has a frequency of 100 waves per has a frequency of 100 waves per second, then 100 impulses per second, then 100 impulses per second travel up the auditory second travel up the auditory nerve).nerve).

Problem with the theory? Problem with the theory? • An individual neuron cannot fire faster An individual neuron cannot fire faster

than 1000 times per second. How, than 1000 times per second. How, then, can we sense sounds with then, can we sense sounds with frequencies above 1000 waves per frequencies above 1000 waves per second (roughly the upper third of a second (roughly the upper third of a piano keyboard)?piano keyboard)?

All the hairs vibrate but at

different speeds.

Frequency TheoryFrequency Theory

This problem can be explained This problem can be explained using the volley principle.using the volley principle.• Volley principle Volley principle – like soldiers who – like soldiers who

alternate firing so that some can shoot alternate firing so that some can shoot while others reload, neural cells can while others reload, neural cells can alternate firing. alternate firing.

• By firing in rapid succession, they can By firing in rapid succession, they can achieve a achieve a combined frequencycombined frequency above above 1000 waves per second.1000 waves per second.

All the hairs vibrate but at

different speeds.

So which theory of pitch is right?So which theory of pitch is right? Like with research in theories of color vision, Like with research in theories of color vision,

researchers argued about these two competing researchers argued about these two competing theories for almost a century. theories for almost a century.

It turns out that It turns out that both are valid - in part. both are valid - in part.

The two were reconciled by Georg von Bekesy, The two were reconciled by Georg von Bekesy, 1947, with his traveling wave theory. Basically, 1947, with his traveling wave theory. Basically, von Bekesy said that the whole basilar membrane von Bekesy said that the whole basilar membrane does move, but the waves peak at particular does move, but the waves peak at particular places, depending on frequency.places, depending on frequency.• Place theory best explains how we sense high pitches, Place theory best explains how we sense high pitches,

frequency theory best explains how we sense low frequency theory best explains how we sense low pitches, and some combination of place and frequency pitches, and some combination of place and frequency (Bekesy’s theory) seems to handle the pitches in the (Bekesy’s theory) seems to handle the pitches in the intermediate range.intermediate range.

Hearing LossHearing Loss Conduction Hearing LossConduction Hearing Loss: caused by damage to mechanical system of ear.: caused by damage to mechanical system of ear.

• Ex: punctured eardrum, inability of the tiny bones of the middle ear to Ex: punctured eardrum, inability of the tiny bones of the middle ear to vibrate,vibrate, inability of the ear to conduct vibrationsinability of the ear to conduct vibrations, etc., etc.

Sensorinueral hearing loss (Nerve deafness): caused by damage to cochlea’s hair cell receptors or their associated auditory nerves.• More common• Usual causes are heredity, aging, and prolonged exposure to ear-

splitting noise or music.

DeafnessDeafnessConduction DeafnessConduction Deafness

Something goes Something goes wrong with the sound wrong with the sound and the vibration on and the vibration on the way to the the way to the cochlea.cochlea.

You can replace the You can replace the bones or get a hearing bones or get a hearing aid to help.aid to help.

Nerve (sensorineural) Nerve (sensorineural) DeafnessDeafness

The hair cells in the cochlea The hair cells in the cochlea get damaged.get damaged.

Loud noises can cause this Loud noises can cause this type of deafness.type of deafness.

NO WAY to replace the NO WAY to replace the hairs.hairs.

Cochlear implant is Cochlear implant is possible.possible.

Cultural Differences in Cultural Differences in Hearing Loss?Hearing Loss?

While the majority of 70 year olds living While the majority of 70 year olds living near the Sudanese-Ethiopian border could near the Sudanese-Ethiopian border could hear a whisper from 100 yards away, hear a whisper from 100 yards away, about 1 in 4 Americans over 65 needs a about 1 in 4 Americans over 65 needs a hearing aid to detect whispers across the hearing aid to detect whispers across the room.room.

The data from Africa suggest that hearing The data from Africa suggest that hearing loss may not be a physiological loss may not be a physiological consequence of aging but could be the consequence of aging but could be the cumulative effect of a lifetime’s exposure cumulative effect of a lifetime’s exposure to environmental noise.to environmental noise.

Why does our own voice sound Why does our own voice sound unfamiliar when we hear it on tape?unfamiliar when we hear it on tape?

The answer? Bone conduction!The answer? Bone conduction! When we listen to ourselves speak, we hear both the sound When we listen to ourselves speak, we hear both the sound

conducted by air waves to the outer ear and that carried conducted by air waves to the outer ear and that carried directly to the auditory nerve by bone conduction. The latter is directly to the auditory nerve by bone conduction. The latter is easily demonstrated by clicking the teeth or munching easily demonstrated by clicking the teeth or munching popcorn, or by striking the prongs of a fork on a table and popcorn, or by striking the prongs of a fork on a table and quickly applying its handle to the bone behind the ear. An even quickly applying its handle to the bone behind the ear. An even more resounding effect will be produced if the handle is more resounding effect will be produced if the handle is clenched between the teeth. The strictly air conducted sound clenched between the teeth. The strictly air conducted sound that others normally hear (like a sound we hear when our voice that others normally hear (like a sound we hear when our voice is on tape) is thinner. Students can hear the sound waves is on tape) is thinner. Students can hear the sound waves conducted by bone if they plug their ears and talk in a normal conducted by bone if they plug their ears and talk in a normal voice. voice.

You can also demonstrate bone conducted sound with a metal You can also demonstrate bone conducted sound with a metal coat hanger tied to the center of a thin string about four feet coat hanger tied to the center of a thin string about four feet long. long.

You should first press one end of the string into each ear with You should first press one end of the string into each ear with the tips of the index fingers while plugging your ears. Then ask the tips of the index fingers while plugging your ears. Then ask someone to tap the coat hanger with a knife or fork. John someone to tap the coat hanger with a knife or fork. John Fisher reports that the effect will sound like “Big Ben.” Fisher reports that the effect will sound like “Big Ben.”

Are all deaf people really deaf?Are all deaf people really deaf? People who are deaf due to a defect in either the People who are deaf due to a defect in either the

inner or middle ear may still be able to hear by inner or middle ear may still be able to hear by bone conduction. bone conduction.

When Beethoven became deaf, he could still hear When Beethoven became deaf, he could still hear a piano being played by placing one end of his a piano being played by placing one end of his walking stick against it and gripping the other walking stick against it and gripping the other end between his teeth. end between his teeth.

To determine the nature and degree of their To determine the nature and degree of their hearing loss, deaf violinists reportedly applied hearing loss, deaf violinists reportedly applied their teeth to some part of their vibrating their teeth to some part of their vibrating instruments. If they could not hear sound, they instruments. If they could not hear sound, they concluded that the auditory nerves were the concluded that the auditory nerves were the problem and the deafness was past cure. problem and the deafness was past cure.