Hearing Flashcards
what are the principles of sound conduction
- an oscillating object will cause air to become more and less dense
- compression and rarefaction
- radiating waves
- wavelength = velocity / frequency
how would you define sound amplitude (decibels)
- sound amplitude is generally expressed as a ratio
- intensity in decibels = 10log10(intensity of unknown / intensity of standard)
- the ‘standard’ is the mean hearing threshold: 10^-12 Wm^-2
- corresponds to 10^-11m movement of air molecules: less than diameter of hydrogen atom
what is the range of hearing
- frequency: 20-20000Hz
- amplitude: 0-140dB
what is lower sensitivity to low frequencies
sometimes sound pressure levels are adjusted using the ‘A weighting’. This effectively down-scales low frequencies to acknowledge out lower sensitivity
what are the frequencies of sound
- low bass (20-80Hz) includes the first two octaves (bass, tuba)
- upper bass (80-320Hz) includes the third and fourth octaves (cello, trombone)
- midrange (320-2560Hz) includes the fifth through seventh octaves (guitar)
- upper mid range (2560-5120Hz) is the eighth octave
- treble (5120-20000Hz) includes the ninth and tenth octaves
what is the role of the ossicles (middle ear)
- middle ear acts as a lever
- converts high amplitude / low force motion at ear drum into low amplitude / high force motion at oval window
- impedance matching
what is the stapedius reflex
- two muscles act on ossicles
- contraction of these muscles pulls strapes away from oval window
- decreases transmission of vibrational energy to cochlea
- stapedius reflex occurs in response to very loud sound
- also occurs during speech
- crucial for preventing hearing damage
what is the make up of the cochlea (inner ear)
- 3 scalae
- vestibuli
- media
- tympani
- helicotrema
- round window
what is the organ of corti
- outer hair cells motois (prestin)
- inner hair cells sensation
- tectorial membrane
- basilar membrane
- small up-down movements of the basilar membrane cause a large relative shear of the tectorial membrane, thus activating the hair cells
what is fourier analyser
- any waveform can be decompressed into sine waves of various frequencies
- even a square wave can be made up of sine waves
- combine the fundamental frequency with progressively smaller harmonics
- decomposes time-based sound signals into their frequency components
what is sharpening of the tuning curve
tuning of hearing is far more sharp than explained by the passive mechanisms of the basilar membrane alone
how can the outer hair cells generate sound
- otoacoustic emissions are ‘echoes’ in response to clicks delivered to far
- absence indicates problem of inner ear
- used to asses hearing in newborns
- possible mechanisms of some types of timmitus
what is the frequency of speech
400-3000Hz
what causes noise induced hearing loss
- concerts can cause temporary loss notches
- damage depends on level and duration
- anything over 85dB is potentially damaging
what is the auditory pathway to the brain
- sound activates many areas from cochlear nerve to auditory cortex
- sound related brain activation measured using EEG
- Broca’s speech area - ‘expressive’
- Wernicke’s speech area - ‘receptive’
what is sound localisation
distance (range) and bearing (azimuth and elevation)
what is the ‘cocktail party’ effect
- we can understand conversation from an individual speaking among many others
- involves sound localisation cues (as well as vision)
- judging distance
- high frequencies travel less well; far away sounds dominated by bass
- expectation (e.g. intensity of voice)
- relative attenuation (timbre)(bass travels best, sibilants worst)
- echoes (multiple echoes - reverberation - less attenuation)
- judging direction
- inter-aural timing / phase differences
- inter-aural volume differences
- spectral colouring (by head and pinna)
how does inter-aural volume differ
- the head provides a sound ‘shadow’
- attenuates volume in ear opposite sound source
- greater attenuation for high frequency sounds
- hence, far away speech more bassy (vowels emphasised, sibilants attenuated)
- better for high frequencies
how does inter-aural timing differ
- better for low frequencies
- inter aural time delay (ITD): a click from the left will arrive at the left ear first, then right ear soon after
what is Jeffress theory of ITD detection
- neurons in superior olive (brainstem) act as coincidence detectors
- when action potentials arrive simultaneously from both ears, MSO neuron more likely to fire
- relies on differing lengths of axons
- provides a neuronal map of sound location
how does inter-aural phase differ
- if sound is a continuous tone, use phase difference to localise
- not very useful when wavelength is shorter than the head (i.e. high frequency sound)
- L-R difference of 360 degree sounds the same as 0 degree ambiguous
does head size impact hearing
- big head = large L/R time difference
- small head = not much L/R time difference
- however, smaller heads better for detecting high frequency sounds
what is the cone of confusion
- sounds emanating from different locations can produce identical ILD and ITD profiles at the two ears
- e.g. sounds emanating from behind can sometimes sound in front of the head
- tilting and turning the head alters the cone of confusion
what is spectral colouring by pinna and head
- the outer ear (pinna) attenuates sounds from certain directions, amplifies sound from others
- invisible to low frequency sounds
- therefore, the spectral content of sound combined with prior experience, can be used for localisation
what is binaural reading
- high fidelity recording studios often use binaural microphones
- mimics interaural difference in humans
- produces a more pleasing sound
what is the auditory reaction time
- typically 140-160ms
- visual reaction time slower (180-200ms)
- sound takes about 3ms per metre
what are startle responses
- evoked by loud sound ~120dB
- evokes blink at ~40ms
- neck contraction at ~80ms
- too fast to involve cerebral cortex
- mediated by brainstem
how can startle responses improve reaction time
- when signal is accompanied by loud sound (120dB) reaction time is much faster (from ~170 to 80ms in extreme case)
- <100ms reaction time considered false start in olympics
