Audition (or, how we hear things)

April 7, 2009

Dirty Work• Final interim course reports to turn in.

• Final project report guidelines to hand out...

• On Thursday, we’ll talk about auditory (exemplar) models of speech perception.

• Recap: categorical perception homework.

How Do We Hear?• The ear is the organ of hearing. It converts sound waves into electrical signals in the brain.

• the process of “audition”

• The ear has three parts:

• The Outer Ear

• sound is represented acoustically (in the air)

• The Middle Ear

• sound is represented mechanically (in solid bone)

• The Inner Ear

• sound is represented in a liquid

The Ear

Outer Ear Fun Facts• The pinna, or auricle, is a bit more receptive to sounds from the front than sounds from the back.

• It functions primarily as “an earring holder”.

• Sound travels down the ear canal, or auditory meatus.

• Length 2 - 2.5 cm

• Sounds between 3500-4000 Hz resonate in the ear canal

• The tragus protects the opening to the ear canal.

• Optionally provides loudness protection.

• The outer ear dead ends at the eardrum, or tympanic membrane.

The Middle Ear

eardrum

the hammer (malleus)

the anvil (incus)

the stirrup (stapes)

The Middle Ear• The bones of the middle ear are known as the ossicles.

• They function primarily as an amplifier.

• = increase sound pressure by about 30 dB

• Works by focusing sound vibrations into a smaller area

• area of eardrum = .55 cm2

• area of footplate of stapes = .032 cm2

• Think of a thumbtack...

Concentration• Pressure (on any given area) = Force / Area

• Pushing on a cylinder provides no gain in force at the other end...

• Areas are equal on both sides.

• Pushing on a thumb tack provides a gain in force equal to A1 / A2.

• For the middle ear , force gain

• .55 / .032 17

Leverage• The middle ear also exerts a lever action on the inner ear.

• Think of a crowbar...

• Force difference is proportional to ratio of handle length to end length.

• For the middle ear:

• malleus length / stapes length

• ratio 1.3

Conversions• Total amplification of middle ear 17 * 1.3 22

• increases sound pressure by 20 - 24 dB

• Note: people who have lost their middle ear bones can still hear...

• With a 20-24 dB loss in sensitivity.

• (Fluid in inner ear absorbs 99.9% of acoustic energy)

• For loud sounds (> 85-90 dB), a reflex kicks in to attenuate the vibrations of the middle ear.

• this helps prevent damage to the inner ear.

The Attenuation Reflex• Requires 50-100 msec of reaction time.

• Poorly attenuates sudden loud noises

• Muscles fatigue after 15 minutes or so

• Also triggered by speaking

tensor tympani

stapedius

The Inner Ear• In the inner ear there is a snail-shaped structure called the cochlea.

• The cochlea:

• is filled with fluid

• consists of several different membranes

• terminates in membranes called the oval window and the round window.

Cochlea Cross-Section

• The inside of the cochlea is divided into three sections.

• In the middle of them all is the basilar membrane.

Contact

• On top of the basilar membrane are rows of hair cells.

• We have about 3,500 “inner” hair cells...

• and 15,000-20,000 “outer” hair cells.

How does it work?• On top of each hair cell is a set of about 100 tiny hairs (stereocilia).

• Upward motion of the basilar membrane pushes these hairs into the tectorial membrane.

• The deflection of the hairs opens up channels in the hair cells.

• ...allowing the electrically charged endolymph to flow into them.

• This sends a neurochemical signal to the brain.

An Auditory Fourier Analysis• Individual hair cells in the cochlea respond best to particular frequencies.

• General limits:

20 Hz - 20,000 Hz

• Cells at the base respond to high frequencies;

• Cells at the apex respond to low.tonotopic organization of the

cochlea

How does this work?• Hermann von Helmholtz (again!) first proposed the place theory of cochlear organization.

• Original idea: one hair cell for each frequency.

• a.k.a. the “resonance theory”

• But...we can perceive more frequencies than we have hair cells for.

• The rate theory emerged as an alternative:

• Frequency of cell firing encodes frequencies in the acoustic signal.

• a.k.a. the “frequency theory”

• Problem: cell firing rate is limited to 1000 Hz...

Synthesis• The volley theory attempted to salvage the frequency rate proposal.

• Idea: frequency rates higher than 1000 Hz are “volleyed” back and forth between individual hair cells.

• There is evidently considerable evidence for this proposal.

Traveling Waves (in the ear!)• Last but not least, there is the traveling wave theory.

• Idea: waves of different frequencies travel to a different extent along the cochlea.

• Like wavelength:

• Higher frequency waves are shorter

• Lower frequency waves are longer

The Traveling Upshot• Lower frequency waves travel the length of the cochlea...

• but higher frequencies cut off after a short distance.

• All cells respond to lower frequencies (to some extent),

• but fewer cells respond to high frequency waves.

• Individual hair cells thus function like low-pass filters.

Hair Cell Bandwidth

• Each hair cell responds to a range of frequencies, centered around an optimal characteristic frequency.

Frequency Perception• In reality, there is (unfortunately?) more than one truth--

• Place-encoding (traveling wave theory) is probably more important for frequencies above 1000 Hz;

• Rate-encoding (volley theory) is probably more important for frequencies below 1000 Hz.

• Interestingly, perception of frequencies above 1000 Hz is much less precise than perception of frequencies below 1000 Hz.

• Match this tone:

• To the tone that is twice the frequency:

Higher Up• Now try it with this tone:

• Compared to these tones:

• Idea: listeners interpret pitch differences as (absolute) distances between hair cells in the cochlea.

• Perceived pitch is expressed in units called mels.

• Twice the number of mels = twice as high of a perceived pitch.

• Mels = 1127.01048 * ln (1 + F/700)

• where acoustic frequency (F) is expressed in Hertz.

The Mel Scale

Equal Loudness Curves• Perceived loudness also depends on frequency.

Audiograms• When an audiologist tests your hearing, they determine your hearing threshold at several different frequencies.

• They then chart how much your hearing threshold differs from that of a “normal” listener at those frequencies in an audiogram.

• Noise-induced hearing loss tends to affect higher frequencies first.

• (especially around 4000 Hz)

Age• Sensitivity to higher frequencies also diminishes with age. (“Presbycusis”)

Note: the “teen buzz”

Otitis Media• Kids often get ear infections, which are technically known as otitis media.

• = fluid fills the middle ear

• This leads to a form of conduction deafness, in which sound is not transmitted as well to the cochlea.

• Auditorily, frequencies from 500 to 1000 Hz tend to drop out.

Check out a Praat demo.

Loudness• The perceived loudness of a sound is measured in units called sones.

• The sone scale also exhibits a non-linear relationship with respect to absolute pressure values.