2. Audition (or, how we hear things) April 7, 2016

3. Last Call. Course project report #5 is due today! I have some homeworks to hand back to you. Mystery spectrograms? We will have course evaluations at the end of class today. Today, we’ll start learning about audition; We’ll probably wrap up this lecture on Tuesday… and then talk about Exemplar Theories of speech perception, as well. Let’s start things off today with another perception experiment!

4. 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

5. The Ear

6. 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.

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

8. 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 cm 2 area of footplate of stapes =.032 cm 2 Think of a thumbtack...

9. 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 A 1 / A 2. For the middle ear, force gain .55 /.032  17

10. 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

11. 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.

12. 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

13. 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.

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

15. 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.

16. 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.

17. 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

18. 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...

19. 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.

20. 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

21. 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.

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

23. 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:

24. 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.

25. The Mel Scale

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

27. 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)

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

29. 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.

30. 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.

31. 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.

32. 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.

33. Bark Scale of Frequency The Bark scale converts acoustic frequencies into numbers for each critical band

34. Bark Table BandCenterBandwidthBandCenterBandwidth 15020-1001318501720-2000 2150100-2001421502000-2320 3250200-3001525002320-2700 4350300-4001629002700-3150 5450400-5101734003150-3700 6570510-6301840003700-4400 7700630-7701948004400-5300 8840770-9202058005300-6400 91000920-10802170006400-7700 1011701080-12702285007700-9500 1113701270-148023105009500-12000 1216001480-1720241350012000-15500

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

36. 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.

37. 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.

38. Cochlear Implants A Cochlear Implant artificially stimulates the nerves which are connected to the cochlea.

39. Nuts and Bolts The cochlear implant chain of events: 1.Microphone 2.Speech processor 3.Electrical stimulation What the CI user hears is entirely determined by the code in the speech processor Number of electrodes stimulating the cochlea ranges between 8 to 22.  poor frequency resolution Also: cochlear implants cannot stimulate the low frequency regions of the auditory nerve

40. Noise Vocoding The speech processor operates like a series of critical bands. It divides up the frequency scale into 8 (or 22) bands and stimulates each electrode according to the average intensity in each band. This results in what sounds (to us) like a highly degraded version of natural speech.

41. What CIs Sound Like Check out some nursery rhymes which have been processed through a CI simulator:

42. CI Perception One thing that is missing from vocoded speech is F0. …It only encodes spectral change. A former honors student, Aaron Byrnes, put together an experiment testing intonation perception in CI-simulated speech for his honors thesis. Tested: discrimination of questions vs. statements And identification of most prominent word in a sentence. 8 channels: 22 channels:

43. The Findings CI User: Excellent identification of the most prominent word. At chance (50%) when distinguishing between statements and questions. Normal-hearing listeners (hearing simulated speech): Good (90-95%) identification of the prominent word. Not too shabby (75%) at distinguishing statements and questions. Conclusion 1: F0 information doesn’t get through the CI. Conclusion 2: Noise-vocoded speech might not be a completely accurate CI simulation.

44. Mitigating Factors The amount of success with Cochlear Implants is highly variable. Works best for those who had hearing before they became deaf. Depends a lot on the person Possibly because of reorganization of the brain Works best for (in order): Environmental Sounds Speech Speaking on the telephone (bad) Music (really bad)

45. Critical Period? For congentially deaf users, the Cochlear Implant provides an unusual test of the “forbidden experiment”. The “critical period” is extremely early-- They perform best, the earlier they receive the implant (12 months old is the lower limit) Steady drop-off in performance thereafter Difficult to achieve natural levels of fluency in speech. Depends on how much they use the implant. Partially due to early sensory deprivation. Also due to degraded auditory signal.

46. Practical Considerations It is largely unknown how well anyone will perform with a cochlear implant before they receive it. Possible predictors: lipreading ability rapid cues for place are largely obscured by the noise vocoding process. fMRI scans of brain activity during presentation of auditory stimuli.

47. One Last Auditory Thought Frequency coding of sound is found all the way up in the auditory cortex. Also: some neurons only fire when sounds change.