Hearing Aids I Final Flashcards
occlusion
blockage of ear canal
cartilaginous outer 1/3 - vibrates & trapped making own voice louder
microphone
converts acoustic to electrical
- sound energy enters inlet
- creates vibrations on diaphragm
- creates movement to & from electrate
- generates electronic sinal
low cut
inbuilt imperfection of microphone
very low sounds will be dampened due to length of wave
resonance
characteristic of cavity
vibrate at max amplitude due to dimentions
4k-5kHz in mic (helmholz)
microphone limitations
low cut resonance easily damaged sensitive to noise wind noise internal noise adds distortions
omni vs directional mic
directional - more focus front, cardiod, reduces low freq sounds
omni - 360, sports, kids,
amplifier & parts
takes battery power & amplifies signal
diodes, integrated circut (IC), capacitors
peak clipping
done primarily by amplifier
sound input above max capacity of HA
adds distortions
(max amp range determined by battery power)
two types of peak clipping
symmetrical - odd harmonics clipped
asymmetrical - odd & even harmonics clipped
other distortions
complex sound
combination tones
digital hearing aids vs. analog journey
digital: M - AMP [analog - digital (numbers) - analog (electric)] R
analog all electrical signal
receiver
electric to acoustic transducer
resonance in cavities - all designs prone to resonance
solution - acoustic damper - smoothes peaks dur to resonance
cannot use damper in thin tube - moisture
ALD’s why?
hearing aid limits [ noise, distance, reverb ]
benefits everyone
classroom acoustics
unoccupied room <35dB SPL
reberb time
RECD
real ear coupler difference
use especially for pediatrics
could be dangerous
aural rehab
- case history
- candidacy
- HA fitting [verification or functional gain measurements]
- Validation
- Counseling/Aud training
cavities that simulate ears
coupler & ear simulator
limitations of ear simulators
delicate
expensive
good with low & high frequencies (better simulate ear)
ANSI Measurements
OSPL avg (HFA), OSPL 90, HFA at 50, THD, EIN
why OSPL 90?
90 - hearing aid gets saturated
EIN
equivalent input noise
only issue if TH in low f good
internal noise
THD
Percent not signal
Verification - insertion gain methods
- Speech mapping
2. Pure tones
dB SPL
dB HL+RETSPL+RECD
speech mapping
meet 90% of targets
pure tone verification (insertion gain)
REUG - measures gain of ear canal w no amp
REAG - with aid
REIG - REAG-REUG
limitations to insertion gain methods
- patient cooperation
- audiologist confidence
- cost
Functional Gain Measurements
unaided & aided gain through sound field testing
Advantage to FGA
patient’s voice heard
resonance of adult canal
2600Hz, higher in kids up to 4k
3 types of coupling systems
- earmold
- earshell
- domes (modulated)
earmold
deep impressions (fit) - beyond second bend & helix lock
severe to profound losses
good for pediatrics
good hold
advantages of deep fit
better retention closer to TM: better quality avoids leakage & feedback avoid occlusion effect better self perceived voice
deep fit limitations
- helix lock insertion with dexterity issues
- requires good impression skills
- pain insertion & removal
- curvy ear canals
modifications of coupling
- vent
- dampers
- sound bore
vent
any opening that allows air to move in & out of canal
smaller = lower frequencies can escape
larger= more higher frequencies can escape
adv to vents
- occlusion affect - allows sounds out (315Hz)
- helps high frequency HL - taking out low frequencies will prevent upward spread of masking
- moisture control
Disadvantages to vents
- feedback
- low frequency hearing loss not rec (low cut)
- less directionality
- lower OSPL 90
- direct sound
dampers
800-2500Hz removed for BTE
1500-3500 Customs
Sound bore
horn & reverse horn
horn effect
wider opening at canal - changes resonance, +6-+12 in high frequencies
reverse horn
narrower at canal
reduces amp in high frequency
avoid downward spread of masking (high masking low)
compression threshold
where compression begins (gain changes)
knee point
any point compression ratio changes
compression ratio
change in input : change in output
limiting
CR = 8:1 or more slowly increasing gain before saturation
peak clipping vs. limiting
peak clipping is linear at saturation
limiting is nonlinear lower distortion
expansion
less gain to soft sounds
CR: 1:1.1 or more
attack time
time required to apply compression
release time
time required to release compression
overshoot
slow attack time: beyond required output
TILL
Treble increase in low levels
more compression in high intensity high frequencies
BILL
Bass increase in low levels
more compression in low frequency high intensity sounds
types of compression
- low level - more compression in soft sounds, loud linear
- wide dynamic range - compression at all levels
- high level - more compression in loud sounds, soft sounds more linear
compression benefits
- safe, comfort
- SNR improvement
- reduces intesity difference bt sp sounds
- normalizes loudness
- may improve intelligibility
compression limitations
- distortion
- benefits not as high older users - linear - power
- AT/RT variation
bands
gain for specific frequencies
channels
compression ratio for specific frequency regions
open fitting issues for directionality
- low frequency leakage
- direct sound
- creates third source of sound
directional benefit factors
- directivity of hearing aid (internal & external delay)
- reverberation
- distance of noise & speaker
listening situations
- speech close, noise far
- speech far, noise close
- noise close, speech close
- speech far, noise far (same direction) - directivity not helpful
SNR benefit from directionality
2-3dB
directional processing happens when
first - affected by compression
compression affect on directionality
reduces front-back difference: reduces SNR benefit from front, helps back
localization & compression
cues are reduced by compression - reduces interaural time difference
directivity disadvantages
low cut responses target sounds from back or side greater internal noise reduced localization wind noise requires closed fitting
noise reduction
winer filter
spectral subtraction
noise characteristics
low f random low intensity less variation narrow modulation depth
winer filter
reduce gain in poor SNR channel
spectral subtraction
mic analyzes signal & estimates noise & eliminates
feedback reduction
closed fitting
- gain reduction
- phase cancellation
- path cancellation
gain reduction
reduce overall gain or in specific region
phase cancellation
detects where feedback happens, alternate wave generated to cancel
path cancellation
feedback detector cancels signal before getting to the mic
frequency lowering
- compression
2. transposition
frequency compression
reduces frequency separation
frequency transposition
uses existing frequencies
free field
no reverb
diffuse field
exert substantial effects on waves, equally distributed
quasi - free
minimal reverb moderate affect
signals in sound field
pure tones - standing waves
warbles (FM)
narrow band noise
