1. Audition (or, how we hear things)
April 8, 2013

2. 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

3. 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

4. The Ear

5. 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  cm
Sounds between  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.

6. The Middle Ear the anvil (incus) the hammer (malleus)
the stirrup (stapes)
eardrum

7. 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 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...

8. 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

9. 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

10. Conversions Total amplification of middle ear  17 * 1.3  22
increases sound pressure by dB
Note: people who have lost their middle ear bones can still hear...
With a dB loss in sensitivity.
(Fluid in inner ear absorbs 99.9% of acoustic energy)
For loud sounds (> dB), a reflex kicks in to attenuate the vibrations of the middle ear.
this helps prevent damage to the inner ear.

11. The Attenuation Reflex
Requires msec of reaction time.
Poorly attenuates sudden loud noises
Muscles fatigue after 15 minutes or so
Also triggered by speaking
tensor tympani
stapedius

12. 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.

13. Cochlea Cross-Section
The inside of the cochlea is divided into three sections.
In the middle of them all is the basilar membrane.

14. 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.

15. 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.

16. 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

17. Hair Cell Bandwidth
Each hair cell responds to a range of frequencies, centered around an optimal characteristic frequency.

18. 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:
Low frequency: 100
Sequence: 200, 210, 190
High frequency: 1000
Sequence: 2010, 2000, 1990

19. 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.

20. Equal Loudness Curves
Perceived loudness also depends on frequency.

21. 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)

22. Age
Sensitivity to higher frequencies also diminishes with age. (“Presbycusis”)
Note: the “teen buzz”

23. 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.

24. 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.

25. 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.

26. 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 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.

27. Bark Table Band Center Bandwidth Band Center Bandwidth

28. Spectral Differences
Acoustic vs. auditory spectra of F1 and F2

29. 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.

30. 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.