Audition (or, how we hear things)

April 8, 2013

Lest We Forget• First off: I am going to post the notes on obstruent acoustics

• Read them!

• Wednesday: we’ll do a brief perception experiment at the beginning of class…

• At the end of class, you can fill out USRIs!

• Friday:

• Jacqueline will say a few things about speech synthesis

• Next Monday:

• Jessi will give a presentation of her work

• I’ll wrap up a discussion of speech perception and exemplar theory

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 20-25 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 - 25 dB

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

• With a 20-25 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

Hair Cell Bandwidth

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

Frequency Perception• There are more hair cells that respond to lower frequencies…

• so we can distinguish those from each other more easily.

• The Mel scale test.

• Match this tone:

• To the tone that is twice its frequency:

• Now try it for a high frequency tone:

The Mel Scale

• Perceived pitch is expressed in units called mels.

• Note: 1000 Hz = 1000 mels

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

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.

Masking• Another scale for measuring auditory frequency emerged in the 1960s.

• This scale was inspired from the phenomenon of auditory masking.

• One sound can “mask”, or obscure, the perception of another.

• Unmasked:

• Masked:

• Q: How narrow can we make the bandwidth of the noise, before the sinewave becomes perceptible?

• A: Masking bandwidth is narrower at lower frequencies.

Critical Bands• Using this methodology, researchers eventually determined that there were 24 critical bands of hearing.

• The auditory system integrates all acoustic energy within each band.

• Two tones within the same critical band of frequencies sound like one tone

• Ex: critical band #9 ranges from 920-1080 Hz

• F1 and F2 for might merge together

• Each critical band 0.9 mm on the basilar membrane.

• The auditory system consists of 24 band-pass filters.

• Each filter corresponds to one unit on the Bark scale.

Bark TableBand Center Bandwidth Band Center

Bandwidth

1 50 20-100 13 1850 1720-2000

2 150 100-200 14 2150 2000-2320

3 250 200-300 15 2500 2320-2700

4 350 300-400 16 2900 2700-3150

5 450 400-510 17 3400 3150-3700

6 570 510-630 18 4000 3700-4400

7 700 630-770 19 4800 4400-5300

8 840 770-920 20 5800 5300-6400

9 1000 920-1080 21 7000 6400-7700

10 1170 1080-1270 22 8500 7700-9500

11 1370 1270-1480 23 10500 9500-12000

12 1600 1480-1720 24 13500 12000-15500

Spectral Differences

• Acoustic vs. auditory spectra of F1 and F2

Cochleagrams• Cochleagrams are spectrogram-like representations which incorporate auditory transformations for both pitch and loudness perception

• Acoustic spectrogram vs. auditory cochleagram representation of Cantonese word

• Check out Peter’s vowels in Praat.

Hearing Aids et al.• Generally speaking, a hearing aid is simply an amplifier.

• Old style: amplifies all frequencies

• New style: amplifies specific frequencies, based on a listener’s particular hearing capabilities.

• More recently, profoundly deaf listeners may regain some hearing through the use of a cochlear implant (CI).

• For listeners with nerve deafness.

• However, CIs can only transmit a degraded signal to the inner ear.