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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 the anvil (incus) the hammer (malleus) the stirrup (stapes) eardrum 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; tonotopic organization of the cochlea • Cells at the apex respond to low. 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 Table Band 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.