The anatomy of the ear

Ling 205

Acoustic vs. auditory

● “Acoustic” -- refers to raw physical properties of soundwaves as detectable by a microphone

● But we don't have direct access to the acoustic signal; for humans, soundwaves are always filtered through our ears before our brain perceives them. This process transforms the signal in various ways

● “Auditory” -- refers to properties of soundwaves as detected by the ears

Outer ear

● The visible earlobe, plus the ear canal.● serves to catch soundwaves and funnel

them into the canal● other mammals have more sophisticated

outer ears, can be oriented towards sound independent of head movement.

● ear canal leads to tympanic membrane (eardrum)

A note on sound localization

● Differences in amplitude, and delay of sound onset, between left and right ear are used by the brain to compute location of the sound source relative to the hearer

Middle ear● Soundwaves cause vibrations of tympanic

membrane● These vibrations cause movement of a chain

of 3 small bones (ossicles) in the middle ear– malleus (hammer)

– incus (anvil)

– stapes (stirrup)

● The middle ear thus serves as a mechanical tranmitter of sound energy to the inner ear

Middle ear

Inner ear

Inner ear

● A snail-shaped organ, filled with fluid, surrounded by hardest part of skull

● Semicircular canals -- fluid levels in here help us maintain balance

● Vestibule – connects to stapes● Cochlea – coiled “snail-shell” part, encloses

basilar membrane

Basilar membrane within the cochlea

Basilar membrane and hair cells● Tapered, thinnest at base (closest to stapes), thickest at apex● Thinnest region vibrates more in response to highest frequency

components of the waveform. As membrane thickens along its length, it responds to increasingly lower frequencies– in effect performing a Fourier analysis on the sound; same frequency

breakdown information as in a spectrogram.● “Hair cells” are arrayed all along the membrane. A hair cell emits a neuro-

electrical impulse (“fires”) when the corresponding region of the membrane is vibrating.

● Hair cell firing patterns are transmitted to the auditory nerve, which carries the information up to the auditory cortex of the brain.

Sensitivity

● The human auditory system is sensitive to frequencies from about 20 Hz to a maximum of around 20,000 Hz (although the upper hearing limit decreases with age).

● Within this range, the human ear is most sensitive between 200 and 5000 Hz, largely due to the resonance of the ear canal and the transfer function of the ossicles of the middle ear.

● This range coincides with the typical range of speech formants

Equal loudness contours

Auditory frequency scaling

Masking

● Forward masking: because the auditory nerve takes time to recover from excitation, a later sound may be somewhat masked by another sound that comes right before it.

● Downward masking: because higher frequency sounds excite a wider area of basilar membrane (extending down into lower frequency region), lower frequency sounds may be somewhat masked by nearby higher frequency sounds.

Spectrogram vs. cochleagram