MNSR 30 -Hearing Impairment and hearing aids Flashcards
why can’t we use interaural intensity difference for sounds of a frequency < 1kHz
calculate wavelength of sound = wave velocity/frequency
wavelength is large - when it is larger than an obejection
a phenomenom called defraction occurs
no shadow as sound moves around head
why do we have 2 ears
localisation occurs via interaural intensity difference
heads casts an acoustic shadow
how have animals perfected localisation of sound
barn owl perfected localisation of sound from prey.
ears are at different heights and are asymmetrical allows both lateral and vertical localisation of prey in darkness
how are sounds of a frequency less than 1 kHz localised
localised through interaural time difference between sounds travelling different path lengths
how is hearing function and hearing loss detected
measured in units of dBHL using an audiometer - produces audiogram
patient wears headphone - audiometer sends a series of tones of fixed frequency and increasing intensity
air conduction
detects sounds through the usual auditory route
air propagates through the ear and is detected
how can the ear hear sounds other than air conduction
sound is conducted directly into the cochlea through bone in the skull
where is the sound in the larynx produced transmitted
transmitted to cochlea via bone conduction
why do recordings of our voice sound different
the voice misses the bone conduction portion
what is the limitation to testing hearing loss via standard air conduction route
we won’t know where the problem lies
where can hearing loss may be caused
problems along the conductive path or in the cochlea
how do we test neural hearing
placing small transducers on the bone behind the ear
bone conduction
any vibration in the tympanic membrane or ear drum is transmitted to oval window
oval window is in bony structure which is connected to bone part off the skull behind the ear
2 types of hearing loss
conductive or neural
difference between conductive or neural hearing loss
conductive - physical damage or obstruction in outer or inner ear
neural - permanent damage to nerves in inner ear
how is conductive hearing loss detected in audiogram
low hearing level (y axis)
normal bone conduction but impaired air conduction
how is neural hearing loss detected in audiogram
high hearing level at low frequency but drop in hearing level at high frequency in both bone and air conduction
causes of conductive hearing loss
Ear infection
Earwax
Hole in eardrum
Fluid in middle ear
causes of neural hearing loss
Damaged hair cells
Damage to the auditory nerve
Neurodegeneration
noise exposure
notch defect
rapid decline in hearing threshold level at a specific frequency range
causes of neural loss of high frequency sensitivity
comes with age - presbycusis
the dBHL and Hz range between a conversation between people 1 metre apart
60dBHL
100-3000 Hz
when are hearing aids required
if audiogram shows more than 55dBHL loss in this frequency
why must hearing aids be carefully designed
threshold of pain is usually same for normal hearing
~120dBHL -130dBHL
how do hearing aids work
digital electronics enables frequency response of aid to be exactly tailored to individual
what is the purpose of the transducer being placed in the bone behind the ear?
vibration is released from transducer and the bone is connected to the bony structure in the ossicles
interaural time difference
difference in arrival time of sound between the two ears - used for localisation of sound
for sounds of a frequency <1 kHz
interaural intensity difference
difference in sound intensity between right ear and left - for localisation of sound
if sound intensity is greater in right ear than left ear - source must from the right side
for sounds of a frequency > 1.5kHz
