Sven- Hearing Part 2

Question 1

The power of out hearing in location

Answer 1

Location can grab visual attention (sirens, people), can distinguish sounds from two heads next to each other

Question 2

Azimuth and elevation

Answer 2

Horizontal plane varies in azimuth (0 is straight ahead). Medium plane is vertical and varies in elevation positive to negative

Question 3

Minimum audible angle MMA

Answer 3

Experiment in sound proof booth, start W reference sound at az 0, second to the l/r and have to decide. Find threshold of spatial separation which becomes the MMA. when reference is 0, MMA is 1 degree for tones at low freqs, as freq increases, MMA becomes higher, worst at 1500hz and larger for sounds not 0/straight ahead

Question 4

How we know where sound is coming from - diff between ITD and ILD

Answer 4

There’s a temporal diff in arrival between ears (sound hits left ear first from left side). 343m/S is speed of sound. The diff is called the interaural timing diff ITD. When source on left, head shields right ear due to head shadow- creates interaural level diff ILD

Question 5

ITD

Answer 5

When sound is az 90 (from one side) ITD is 0.65ms. As az reaches 0, OTD gets smaller as when it’s 0, sound reaches ears at same time. To find threshold: play tones through earphones, one in each ear and adjust until just tell sound moved l/r. Threshold is 10ms (1 degree)

Question 6

ILD

Answer 6

when sound comes from az 90, ILD is 20db at 6000hz but 0 at 200hz (can’t tell at low freqs) so sound has to be high freq as they bounce off but low aren’t attenuated in the same way, reason why you only need one subwoofer as low freq. threshold is 1db across headphones as no head shadow

Question 7

How ITD is affected by frequency

Answer 7

Sound hits ear at 2600 us, time delay between eats is 6500 us. Brain detects which comes first as brain pairs similar co occurring patterns. ITD ambiguous are 770hz as waves hit at same time. They are misleading above 770hz as heard left before right even tho sound on right . So only useful at low freqs

Question 8

Lord rayleighs duplex theory

Answer 8

Listeners use ITDs at low freqs and ILD at high. But theory only valid at pure tones as ITD work all the time for complex tones- more EV so theory not useful

Question 9

Measuring ITDs

Answer 9

Delay lines are axons, one connected to each ear, and aps travel down them. They are connected by neural coincidence detectors, which are neurones that only fore if both delay lines activate at the same time. Each codes for specific ITD, if sound hits ears at same time, aps travel down at same time so coincidence detectors don’t fire until the middle- codes as 0. Method in birds but mammals use opponent process

Question 10

Opponent process

Answer 10

There are two channels, one tuned to left half of space, other to right. System calcs the diff between responses to work out where sound is coming from in horizontal plane

Question 11

Front back ambiguity

Answer 11

Can’t tell diff between ahead and behind at ILD and ITD the same (0). Pins used to distinguish as creates a shadow so some freqs bounce off back of ear, not front. High freqs reflected more from behind, so less from ahead. Sound W less high freq more likely to come from the back but need to be familiar W the sound. By rotating head, create ITD and ILD diffs, if clockwise, OTD favours left ear so sound from front

Question 12

Elevation

Answer 12

ITD and ILD will be the same so use the pinna: reflection of sound boosts energy at some freqs and reduces others, depending on elevation as shape produces unique resonance patterns. So if disputed should have disrupted elevation. Ps had mould of pinnas to change shape, found judgements of elevation impaired but improved W experience and no after effects

Question 13

Vr

Answer 13

ILDs and ITD limited to horizontal plane. Need to externalise acoustic sound space using head related transfer functions HRTF. Need to record sounds from manikin point of view W pinnas. Take head shadow into account. Ps played sounds W generic pinnas, individual and ground truth (no headphones, just speakers) and counted errors in elevation. Most in generic, then individual then ground truth.

Question 14

Localisation in rooms

Answer 14

You hear direct sound and echos but brain ignores them to calc direction. Echo suppression precedence effect: ps hear click from l speaker before r, r is suppressed so hear single click from l but only when played in quick succession. Release from informational masking via precedence: can cancel out info by playing it later- creates an echo

Question 15

Ohm and helmoltz

Answer 15

Said lowest freq you can hear is perception of pitch. When the first harmonic is removed, you can hear the melody.

Question 16

How we perceive pitch

Answer 16

Based on largest common factor, the one you hear. If fundamental removed and harmonics go down, you hear pitch going up- proof . Phone audio 300-3300hz, male 100hz and female 200hz on reality so misses fundamental frequency. Accuracy greater when fundamental differs in target from competing voice but lower again when fundamental 200hz as double competition

Question 17

Impaired hearing prevalence and testing

Answer 17

0.1% at birth, 11% 61-70, 30% 71-80. Measure w pure tone audio gram, start W audible pure tone, and turn down to find threshold. Repeat for diff freqs and plot against 0db (normal healthy). Anything below 0 + as need to increase to hear it. Goes from straight line to down curve as get older, struggle to hear anything above this e.g. derivative sounds. Pure tone average clinical bands up to 20 is normal, 21-40 mild, 41-70 mod, 71-95 severe and 95+ profound . Mod most prevalent

Question 18

Types of loss

Answer 18

Loss originating in inner ear called sensorineural/cochlear loss and loss in middle ear is conductive as ossicles affected

Question 19

Conductive loss

Answer 19

Loss of mobility of middle ear bones, fluid in middle ear is otitis media so bone movement impaired and fixation of the shapes is otosclerosis where stapes becomes porous and doesn’t move much like arthritis. Otitis media occurs in ear infections in young children and fluid builds in so report pain. Treated W antibiotics/grommets. Affects all freqs, can use hearing aids or surgery

Question 20

Cochlear loss

Answer 20

Loss of cochlear hair cells, caused by genetics, ototoxicity (repeated drugs), noise and decreased 02 (recreational noise now overtaken work noise). After affected high freqs as affects hairs on basilar, hearing aids can help. Most common form of impairment. Outer hair cells affects : amp the basilar means loss of sensitivity (to low so pure tone threshold raised), loss of freq selectivity (can’t discriminate freqs close together)

Question 21

Cochlear loss and tuning

Answer 21

If you disable outer hair cells in rodents and use interferometry, you find basilar membrane movement decreases at high freqs. For ppl with one bad ear, found tuning sharp in good ear but broader in bad ear (played test tone along masking and had to adjust until makes it hard to detect test tone). V vs u shape so more freqs mask test tone

Question 22

Loudness recruitment

Answer 22

Play a tone of one intensity in one ear, ps have to adjust intensity in other ear to match . Normal ps increase test in line with normal ear in a linear way. For hearing loss in one ear, they katch loudness of tone in bad ear W less intense tone in good e.g. 60db in bad, 30db in good. But diff decreases W intensity so loud sound perched the same and auditory pain threshold the same so can’t use same amp level for all tones in hearing aids or uncomfortable

Question 23

Fine structure and temporal envelope

Answer 23

Both are changes in amp and freqs. Temp changes responsible for perception, complex sounds are decomposed by auditory systems by narrowing into bands which convey info at time scales. Slow temporal envelope cues and faster temporal fine structures

Question 24

TFS

Answer 24

With hearing loss, lose fine structure sensitivity but not evolope. Used for pitch perception so makes it harder to hear what one person is saying if another is speaking at the same time. Female voice has to be x amount louder than male to be able to tell 50% of what is spoken-called signal to noise ratio. If provide TFS, performance goes up for normal hearing but for impaired, need better signal to noise ration and less benefit from TFS

Question 25

Hearing aids

Answer 25

Used to amp sound to eardrum, fitted into ear canal/outer ear. Helps loss of sensitivity and loudness recruitment but not freq selectivity or TFS. Didn’t use to do loudness recruitment as used to amp everything, now function of initial intensity called auto gain control. Used for mod to severe , cochlear for profound

Question 26

Profound hearing loss - cochlear implant structure

Answer 26

Both outer and inner hair cells affected. Minimal benefits from hearing aid. External part has batteries, microphone to pick up spin and process then transmit to coil via radio wave connected to implant. Implant against coil using magnet behind call, goes to electdoes inserted into cochlear so bypasses middle ear but not too far in or brakes cochlea

Question 27

Cochlear implant function

Answer 27

Outer bit filters acoustic signal into bands, 350-540phz. Temporal envelope not fine structure in each band extracted and conveyed to auditory fibres by varying amp of electrical pulses. Recreates normal relationship between place of stim and freq but not complete resolution, can’t hear music. Understand 80%, use phone but variability. Best for late deaf, some hearing, good lang and memory, skills and motivation

Question 28

Prelingually deafened children

Answer 28

Best outcomes for those who receive implants at youngest age as before 4 did best at word rec compared to after 8 although same for first year. For expressive lang: younger implantation, closer to chronological age, never equal. Same for reading age

Question 29

Bi or unilateral

Answer 29

+ of bi: guaranteed to implant better ear, insurance if one fails, creates potential for skills in spatial hearing (timing diffs, ILD). -: operation on both ears, cost 20-30k per ear. Children born deaf can get 2, adult who become can get 1 unless deaf and blind