The Ear - Pathway of Hearing

Auditory PhoneticsHow to Handle Speech

UBielefeld

What is Sound?

“ ... a sound is any disturbance of air that could cause a displacement of the eardrum which, after transmission by the bone chain, could affect the liquid in the inner ear in such a way that the auditory nerves are stimulated.”

(Ladefoged, Elements of Acoustic Phonetics, pp.2-3)

How Sound Travels“The source of sound causes movements

of the air particles in its immediate neighborhood; these movements cause disturbances in the air a little farther away from the source; these air particles in their turn affect their neighbors which are still farther away from the source; and so the disturbance spreads outward.”

(Ladefoged, ibid, p.3)

Just Another Sound: The Human Voice

• voice: fluctuations of air pressure > caused by rapid opening and closing of vocal folds

• pressure rises to a maximum, then falls steadily to a minimum before rising again to repeat the cycle, all within a small fraction of a second

Varying Air Pressure

• maxima: peaks above 0• minima: valleys below 0• peaks of air pressure

<=> air particles close together

• moments of low pressure <=> air particles furthest apart

THINK: What about sound in a vacuum?

Hearing vs. Perception

• ear (peripheral auditory organ): hearing > sound reception & sound transformation

• auditory organ (includes central nervous system; central auditory organ): perception > interpretation of semantic contents

Our Auditory Field

Our Auditory Field

http://home.schule.at/teaching/Hoeren_und_Hoerprobleme/vorgang/hoerfeld.htm

The Ear

• outer ear – protection, – collection of sound

• middle ear – transmission of mechanic vibrations

• inner ear – transformation of vibrations into neural

impulses

The Ear

outer earmiddle ear

inner ear

The Ear Cont‘d.

The Path of Sound

• external canal• vibrates eardrum• vibration moves through ossicles

– mallet (malleus), anvil (incus), stirrup (stapes)

• stapes vibrates oval window of cochlea• creates pressure wave in the fluid inside

The Outer Ear • Pinna / auricle:

– protects the entrance to the ear canal

– contributes to our ability to locate sounds, esp. at higher frequencies

• Meatus/outer ear canal:– pathway for the acoustic signal– protection of the inner ear– tube resonator, i.e. favors

transmission of high-frequency sounds (2000-4000 Hz) and generally between 500 and 4000 Hz

The Middle Ear

The Middle Ear Cont‘d

• Eardrum:– is deflected by air pressure variations (= sound)– converts air pressure variations into mechanical movement– transmits this movement to the auditory ossicles

The Middle Ear Cont‘d

auditory ossicles: mallet, anvil, stirrup:– pick up the mechanical movement of the eardrum and

convey it to the inner ear/oval window – transformation of air pressure variation into equivalent

mechanical movements– contraction between anvil and stirrup can reduce amplitude

(protection for inner ear)

The Middle Ear Cont‘d

• Eustachian tube:– narrow tube connecting

pharynx and middle ear – equalizes air pressure

differences between outer and middle ear

• oval window:– interface between middle

and inner ear – transmits mechanical

movement

The Inner Ear

• cochlea:– coil-like structure terminating in a window with a

flexible membrane at each end– internally divided by two membranes, filled with

cochlear fluid– converts mechanical movements into neural

signals

The Inner Ear Cont‘d -Cochlea

The Inner Ear Cont‘d

• basilar membrane:– stiffer at one end than at the other

> sensitive to particular frequencies at particular points because of the different resonance which is due to different stiffness

• high frequencies picked up at the stiff end• lower frequencies at the more flexible end> conducts spectral analysis of incoming sound

(see animation later!)

The Inner Ear Cont‘d

cross section of onesingle turn of the cochlea

(no 5. = basilar membrane)

The Inner Ear Cont‘d

• Organ of Corti:– attached along the basilar membrane – hair cells which get moved and excited – responsible to transform membrane displacement into neural

signals

1. inner hair cells2. outer hair cells3. tunnel of Corti4. basilar membrane

The Inner Ear Cont‘d -Hair Cells

• about 16,000 in each cochlea

• located on the basilar membrane• do not regenerate• inner (~ 3,500) hair cells

vs. outer hair cells (~ 20,000)• stimulated by bending of the basilar membrane (length a wave travels on the basilar membrane depends on frequency > first frequency (spectral) analysis)• amplitude: lower & medium amplitudes affect outer hair cells, higher amplitudes affect inner hair cells

Anatomy of the CochleaAnimated

http://www.hhmi.org/biointeractive/animations/cochlea/coc_frames.htm

The Auditory Nerve

• Auditory nerve:– picks up the neural signals from the organ of Corti– sends signals to the brain where the information is

processed and interpreted > analytical effects of the human auditory system

The End