HEARING

The Nature of Sound

• Sound, like light, comes in waves

• Sound is vibration

• Features of sound include:

– Pitch

– Hertz

– Decibels

Pitch• A sound’s highness or lowness

• Dependent on the frequency of the sound wave

• Is measured as hertz (Hz)

High pitched sounds

Low pitched sounds

Hertz (Hz)

• A measure of the number of sound wave peaks per second; measures “frequency”

• Determines the pitch of the sound

• Human hearing goes from 20 Hz to 20,000 Hz

Frequency of Sound Waves

• The frequency of a sound wave is measured as the number of cycles per second (Hertz)– 20,000 Hz Highest Frequency we can hear– 4,186 Hz Highest note on a piano– 1,000 Hz Highest pitch of human voice– 100 Hz Lowest pitch of human voice– 27 Hz Lowest note on a piano

Decibel (dB)• A measure of the height of the sound

wave

• Determines the loudness of the sound

• Sometimes called amplitude

Loud sounds

Soft sounds

• Above are examples of Frequency & Amplitude/decibels• Timbre – distinctive quality of a sound determined by the

complexity of the wave and its different combinations of frequencies. (Figure C is more complex than Figures A or B)

Who hits the higher pitch? Christina or Mariah?

The Structure of the Auditory System

Hearing: Sound Waves

• Auditory perception occurs when sound waves interact with the structures of the ear

• Audition (sense of hearing) results in sound waves being collected in the outer ear, amplified in the middle ear and converted to neural messages in the inner ear.

Anatomy of Ear

Purpose of the structures in the ear:– Measure the frequency (pitch) of

sound waves

– Measure the amplitude (loudness) of sound waves

Parts of the Ear – Sound Wavesenter through the Pinna

Pinna

Hearing: Sound Localization

Localization of Sound

• Locating where sound is originating from

• Done through two cues:

– Which ear hears the sound first?

– Which ear hears the louder sound?

Localization of Sound

Parts of the Ear – Auditory Canal

Auditory Canal

• Sound Waves enter through the pinna then travel through the auditory canal.

• The opening through which sound waves travel as they move into the ear for processing

• Ends at the tympanic membrane (eardrum)

Parts of the Ear – Tympanic Membrane (Eardrum)

Tympanic Membrane (eardrum)

• The tissue barrier that transfers sound vibration from the air to the tiny bones of the middle ear

• Can be damaged by objects in the ear or exceptionally loud noises

Parts of the Ear - Ossicles

Ossicles• Three tiny bones that transfer sound waves

from the eardrum to the cochlea• Hammer, anvil and stirrup• In old age they may become brittle or

damaged resulting in conduction deafness

Parts of the Ear - Cochlea

Cochlea

• A hearing organ where sound waves are changed into neural impulses

• The major organ of hearing

• Filled with fluid; a snail shaped body tube

Parts of the Ear – Oval Window

Oval Window

• The point on the surface of the cochlea which receives the sound vibration from the ossicles

• As the oval window vibrates, the fluid in the cochlea vibrates moving hair cells along the basilar membrane.

Hair cells along the Basilar Membrane move as the fluid vibrates

Anatomy of the CochleaAnother View

Outer ear Middle ear Inner ear

A sound causesthe basilarmembrane to waveup and down.

Basilarmembrane

Hair cells

Tectorialmembrane

Round window

Eardrum

Oval window

Cochlea,partiallyuncoiled

Stirrup

AnvilHammer

Soundwaves

Auditorycanal

Hair Cells• The receptor cells for hearing in the cochlea

that change sound vibrations into neural impulses. When they move they trigger action potential in the base of the hair cell (transduction).

• Similar to the rods and cones within the eye except hair cells are sensitive to vibrations rather than light.

• If these are damaged (due to prolonged loud noises) then you have nerve deafness which cannot be helped by a hearing aid.

Parts of the Ear – Auditory Nerve

Auditory Nerve

• The nerve that carries sound information from the ears to the thalamus then to the auditory cortex in the temporal lobes of the brain

• The auditory nerve is stimulated by the hair cells in the basilar membrane of the cochlea.

Major Divisions of the Ear

• Outer Ear—acts as a funnel to direct sound waves towards inner structures

• Middle Ear—consists of three small bones (or ossicles) that amplify the sound

• Inner Ear—contains the structures that actually transduce sound into neural response

Divisions of the Ear

Divisions of the Ear

Divisions of the Ear

Anatomy of the Ear:

A final look

Transduction of Sounds• Sound waves are captured by the Pinna and sent down

the ear canal where they stimulate the eardrum.

• The eardrum’s vibrations are amplified by the ossicles (hammer, anvil, stirrup).

• These vibrate the oval window on the cochlea which in turn vibrates the fluid around the basilar membrane.

• The fluid bends the hair cells on the basilar membrane triggering action potential in the base of the hair cells.

• This message is transmitted to the auditory nerve which carries the info to the thalamus and then to the auditory cortex of the temporal lobe.

• Review using this Nobel Prize site on Hearing

How Can I Remember This?• Please – Pinna• Eat – Ear Canal• Everything – Eardrum• Offered – Ossicles• On – Oval Window• Cuisine – Cochlea• Buffet – Basilar Membrane• Helpful – Hair Cells• Attendants – Auditory Nerve• Take – Thalamus• Away – Auditory Cortex• Trash – Temporal Lobe

Or Maybe This Works Better…Portenga- [Pinna]evaluates- [ear canal]exams- [eardrum]on- [Ossicles]ominous- [oval window]charts- [cochlea]between- [basilar membrane]hungry- [Hair Cells]architects- [Auditory nerve]that- [Thalamus]always- [Auditory cortex]titter- [Temporal lobe]

Distinguishing Pitch• Frequency theory—basilar membrane vibrates at the same

frequency as the sound wave/oval window– The higher the frequency wave the faster the firing of

hair cells– Theory used to explain how you hear low frequencies

• Place theory—different frequencies cause larger vibrations at different locations along the basilar membrane– Different pitches stimulate different areas on the basilar

membrane– The brain receives these messages and interprets them as

different pitches.– Theory used to explain how you hear high frequencies.

• Use both theories when you listen to sounds with both high and low frequencies. See this website to see how it works.

Frequency Theory

The Basilar Membrane vibrates according to the same Frequency of the sound wave

Basilarmembrane

Distalend

Proximalend

Oval window

Direction of traveling wave

Place Theory: Different Frequencies stimulate different areas of the Basilar Membrane

Coding and Auditory Masking

• The way in which waves travel down the Basilar Membrane causes some sounds to interfere with (or mask) our ability to hear other sounds

• Low frequency sounds provide better masking than high frequency sounds

Auditory Masking

• Low frequency sounds effectively mask high frequency sounds

• High frequency sounds cannot effectively mask low frequency sounds

Piccolo, soft

Bassoon, loud

Piccolo, loud

Bassoon, soft

Distance along basilar membrane

(a)

Distance along basilar membrane

(b)

Effect of bassoon on basilar membrane

Vibration amplitude of basilar

membrane

Vibration amplitude of basilar

membrane

Effect of piccolo on basilar membrane