outer and middle ear

SLHS 302, Section 2, Purdue 2026

peripheral auditory system

outer ear to the middle ear to the inner ear to the CN VIII

temporal bone

skull bone containing organs of hearing and balance

main functions of the outer ear

protect middle ear structures

creates binaural cues for sound localization

amplifies sound pressure before it reaches the tympanic membrane

pinna/auricle

flap made out of cartilage

acts as a resonator

pinna anatomy

helix

tragus

cavum concha

cymba concha

anti-helix

antitragus

lobe

triangular fossa

helix

outer rim/edge

tragus

flap partically covering ear-canal opening

cavum concha

upper portion of concha

anti-helix

ridge above cymba concha

anti tragus

bump above the lobe

lobe

most inferior part

triangular fossa

depression between helix and antihelix

external auditory canal

about 2.5 cm in adults

s-shaped

lined with skin

ends at the typanic membrane

outer 1/3 of external auditory canal

cartilaginous

hair follicals

ceruminous glands

sebaceous glands

inner 2/3 of external auditory canal

passes through temporal boone

no glands

no hair

tympanic membrane

eardrum

cone-shaped

funnels acoustic energy towards its center

boundary between outer and middle ear

pinna amplification

about 3-4 dB increase, around 3-4 kHz

concha amplification

about 10-15 dB increase, around 5-6 kHz

Exernal Auditory Canal

about 10-15 dB increase, around 2.5 kHz

interaural level difference

difference in sound level/amplitude between 2 ears

most important for highfrequencies

why are high frequencies more important for interaural sound difference

high frequency wavelengths are shorter than the head, so the head creates a sound shadow. teh ear from the sound recieves a lower level

overal maximum amplififcation of outer ear

about 20 dB

Interaural level difference calculation

1. calculate the level at each ear

2. subrtact the two levels

interaural time difference

difference in arrival time between 2 ears

most important for low frequencies

middle ear main structures

tymapnic membrane

malleus

incus

stapes

eustatition tube

ossicle order

TM to Malleus to Incus to Stapes to Oval window

tympanic membrane

two regions: Pars flacicida and Pars tensa

Pars Flaccida

superior portion of the tympanic membrane

smaller

does NOt contain fibers that make the tympaic membrane tight

Pars Tensa

inferior ¾ of the tympanic membrane

larger

very tight/tense because of fibers

eustachian tube

opens into the nasopahrynx

normally closed

opens using muscles including: tensor veli palatini and levator veli palatini

equalizes pressure between middle ear and environment

why do we need the middle ear

the middle ear transforms sound from air to fluid

the problem is the air and fluid have different acoustic impedances

acoustic impedance

resistance to movement

high impedance

difficult to move

low impedance

easy to move

without the middle ear the would be a ______ dB loss of gain

60

3 ways the middle ear overcomes impedance mismatch

area transformer (18 dB)

lever action (13 dB)

buckling of TM (30 dB)

area tranformer

+18 dB

tympanic membrane has a large area, stapes foot plate has a small area

force is concentrated onto smaller area

pressure= force/area

smaller area=greater pressure

lever action

+13 dB

malleus and incus act as a lever

trade movement distance for greater force

stapes moves a short distance but with great force

buckling of tympanic membrane

+30 dB

tympanic membrane in conical/curved

different parts of the tympanic membrane move differently

this increases force delivered to malleus

types of impedance

mass, stiffness, and damping

impedance of mass

requires force to move mass

most relevant at high frequencies

impedance of stiffness

resitsance caused by springiness

most relevant at low frequencies

impedance of damping

resistance caused by friction

most relevant at middle frequencies

ear pathologies

microtia

anotia

cerumen impaction

otistis externa

otosclerosis

otistis media

cholesteatoma

microtia

pinna is under-developed/partically developed

anotia

pinna is completely absent

cerumen impaction

earwax blocks external auditory canal

outer-ear problem

otistis externa

infections/inflammation of the external auditory canal

also called swimmers ear

redness in external auditory canal

otosclerosis

bone growth around stapes footplate

locks/restricts stapes

increases stiffness

causes low-frequency hearing loss

otistis media

fluid in middle ear

increases stiffness

cholesteatoma

skin growth in middle ear space

can destroy ossicles

can cause 60-80 dB hearing loss