Audiology hearing aids

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A common strategy for selecting hearing-aid gain involves the"half gain rule" The rule specifies that gain should be equal to one-half of the a)distance from the hearing threshold to the loudness discomfort level b) hearing loss when the thresholds: E because that is the half gain rule
Problems faced by people with hearing impairment?: -decreased audibility
-decreased dynamic range
-decreased frequency resolution
-decreased temporal resolution
What does decreased audibility affect?: -inability to hear some things at all
-especially softer consonant sounds
-when parts of speech sounds are missing, words become difficult to recognize
-high freq components are softer than low & most HL is in the High freq
(hearing aids are good at providing different amounts of amplification for high vs low freq)
History of hearing aids divided into 5 eras: 1. acoustic- horns
2. carbon
3. vacuum
4.transistor
5 digital
Carbon hearing aids: -carbon mics
-single mic
-very small amount of gain ~ 20dB
-with 2nd battery got 30-35 dB gain
-noisy
-sharp resonant peaks
-distortion
-if user bent over or sat down the HA might not work because carbon granules lose contact with diaphragm
By mid 30's what type of HAs were available?: Vacuum tube HA
-carbon mics were replaced with vacuum tube
-required 2 batteries, larger HAs
Parts:
-microphone and amplifier
-battery pack
ear reciever
had better freq response than carbon aids and less distortion
An improvement to vacuum tube HAs?: Pieqoelectric microphones
-Rochelle salt crystal, air pressure fluctuation on the diaphragm cause a compression of the crystal which creates a flow of current that is proportional to input
-flat freq response from low freq
couldn't function in dry climates or in high humidity
Battery improvement in the 1940s: development of the mercury battery
- great advantage becuase allowed much smaller device.
-1 piece HAs
By early 1952 HAs used?: transistors
-large reduction in power needed
-no longer needed 2 batteries
-cost of operation decreased
-transistor was the most important contribution to small size until todays digital technology
Body and eyeglass aids gave way to ?: BTEs
ITEs
ITCs
CICs
Improvements to transistor HAs: -chip amplifiers
-integrated circuits
-smaller and smaller batteries
-volume controls
-more reliable transducers with better freq response
-improvement in filter technology
Developments other tha digital technology: 1936 - telecoil
1946 - wearable telecoil
developments other than digital technology (cont): 1964- integrated circuit
late 60s - FET = field effect transistor
1968 - ceramic microphone
1st directional HA
1971 - tiny electret/Fet microphones
Before the 1980s what were hearing aids mostly like? After 1980's how did they change?: Amplifiers with linear gain only
-gain, freq response, output limiting were the only parameters adjusted

Became adaptive frequency response multi channel compression digital signal processing instruments
-fitting process a bit more involved
-one target was not enough
Hearing aid goals: -Audibility - to hear important sounds
-comfort - sound "comfortably loud"
-safety - sounds prevented from being too loud
-maximize the intelligibility of speech sounds
-maximize the perceived quality of sounds (as little distortion as possible)
What is auditory deprivation: After a person with symmetrical HL is fittd with HA in only 1 ear, the speech recognition ability in the unaided ear may progressively deteriorate over time.
-can occur as soon as 1 year after aiding (important to monitor speech discrim annually)
-occurs for losses 35dB and up
-effects may be greater for severe losses
-recovery after months to years of subsequent bilateral aiding can be:
-absent, partial, or complete
3 reasons why people can more easily understand speech with 2 ears than 1: 1. head diffraction effects
2. binaural squelch - noise presented at 0 deg & 90 deg, the auditory system can combine wave forms to produce a central representation of the signal
3. binaural redundancy
-both signal & noise presented at 0 deg
-identical wave form to both ears
-binaural summation- ability to combine intensities of 2 ears thus reducing the volume needed for each ear.
-results in less feedback, less battery drain
Binaural interference: -may arrise from cochlear distortion
-small portion of elderly patients (about 10%) will have poorer discrim when listening binaurally then when listening monaurally
-loss of interhemispheric transfer (right ear advantage)
Considerations for HA candidacy?: - case history
-clients perspective on the effects of HL
-assessment tools
-impact of auditory impairment on everyday listening
-clients unique circumstances
get their opinions
What conditions require immediate medical referral?: -visible congenital or traumatic deformity of the ear
-history of active drainage within previous 90 days
-history of sudden rapidly progressive hearing loss within previous 90 days
-acute chronic dizziness
-visible evidence of significant cerumen or foreign body in ear canal
-audiometric air bone gap greater than 10dB
must have medical clearance for wearing a prosthetic device or sign a waiver
A block diagram shows what?: -the operations a hearing aid carries out on a signal
-within the diagram the order of operations
-the location of filter & user controls
definition of transducers what are they in a HA?: A device that transforms energy in one energy system to energy in another
-microphone (input)
-telecoils
-receivers (output)
What is signal processing?: -all the things we need to do to get a hearing aid to do what we want
-amplyfing, filtering, peak clipping, compression, freq shifting
common core assumptions of most prescriptive fitting strategies: 1. Audibility of speech sounds is critical
2. Restoration of audibility takes place within the constraints of patients area of dynamic range
3. Sound quality & distortion must be considered. Slope of freq response too steep or with some high freq amplification may be detrimental
What have been common components of HA throughout history: -Microphone
-amplifiers
-Batteries
-volume control
-recievers
What elements should be considered in the audiologic rehabilitation?: -hearing and fitting
-assistive device fitting
-speech perception training
-communication strategies training
-counseling
-outcome evaluation
--verification of benefit
--evaluating aided performance
Advantages of binaural HAs: -localization
-increased signal to noise ratio
-binaural squealch
-binaural summation
-no head shadow effect
-no auditory deprivation
-access to directional microphones in both ears
-masking of tinnitus
-lose one, still have one
-less gain for same loudness
-avoidance of late-onset auditory deprivation
6 stages of fitting & rehab (formalized): 1.assesment - is there a HL?
2. treatment- What are the options?
3.Selection - take in all parameters
4. verification - does the product do what it is intended to do?
5. orientation- instruct on usage. Realistic expectations
6. validation - is choice justified?
4 non electroacoustic characteristics during HA selection stage: -cosmetic considerations
-volume control options
-remote control options
-programmable options
-telecoil/DAI
Elements involved in HA selections: -electoacoustic characteristics
-freq gain characteristics
-expressed in 2cc coupler measurements
-incorporation of individuals real ear differences
-desired freq response for different input levels
-binaural fitting considerations
-conductive HL considerations
-linear vs non linear signal processing
and also non electroacoustic considerations:
HA style
earmold configurations
number and size of controls
telecoil capacity
vol control pref
remote control
programmable options
What goes into audiological assessment?: -Case history
-otoscopic inspection
-audiologic assessment to determine type & extent of HL
-speech recog at conversational levels, in quiet and in noise
-need for med treatment or referral
-counseling on results & recomendations to client and family/caregivers
-determination of candidacy & motivation
-determination of medical clearance
Effects of reduced dynamic range: -SNHL increases threshold of hearing more than it increases threshold for loudness discomfort= reduced dynamic range
-all meaningful sounds including weak & loud need to be compressed into this reduced range
-most hearing aids can compensate reasonably well for a decreased dynamic range
How does decreased frequency resolution effect HL: SNHL decreases the ability to separate sounds of different frequencies
-OHCs loose their ability to increase sensitivity of cochlea
-the brain is unable to untangle the signal from the noise
-upward masking can occur - low Hz speech sounds mask weaker high Hz consonants
-the need to listen to sounds at elevated level (as with HA) can contribute to decrease in freq resolution
-HAs offer several solutions but mixed speech and noise is still a problem
Effects of decreased temporal resolution on HL: Temp Resolution - the ability to process the order of sounds and to tell when one sound stops & another begins
-this give the ability to hear weak sounds in the gaps of more intense sounds
-decrease in this ability makes it more difficult to use the pieces of information in those gaps that would otherwise be audible
-HAs do not offer solutions to this kind of problem
What is upward spread of masking?: When low frequency parts of noise cause parts of high frequency speech to be masked
-low freq are louder than high freq
-low freq contain less critical info for intelligibility so can lower noise without too much effect on intelligibility
-spread of masking increases with increasing noise level
Mounting the microphone causes an acoustic resonance called: Helmholtz resonance
-a resonance peak in the freq response occurring at 4-5kHz
LTASS =: Long term average speech spectrum
Disadvantages of binaural HAs: -costs
-self perception
-wind noise
-binaural interference
W/loss of transfer in the corpus callosum
Binaural HAs and localization: -for speech itelligibility...2 are better than one
-bilateral better when sounds are inaudible in the unaided ear
-horizontal localization will improve but vertical will no
A hearing impaired person needs a better ___ -____ ____ than a normal hearing person in order to understand speech: Signal to noise ratio

HAs will never give back unimpaired hearing skills but they do make it better
Transducers used in HAs: -Microphones - acoustic sound field, electret. Sounds of 70dBSPL produce voltage of 1 microvolt.
-Amplifiers - turn minute electric current into larger current that is passed on to the main amplifier
-Receivers - converts amplified electric signal to an acoustic output
-telecoils - small coil of wire that produces voltage when an alternating magnetic field flows
In choosing an ear to aid in unilateral fittings it is important to consider: -not just the audiogram
-consider speech recognition scores
-consider preference and dexterity
Ultimately it is the patients decision to wear one or two hearing aids but cautions:: -be clear about explaining the advantage & disadvantages of bilateral amplification
-certain patients should be strongly encouraged to wear 2 (mod to severe high freq sloping loss)
-waiver: if after counseling the patient they decide for unilateral fitting, they should sign a statement that they are aware of your recommendation and have decided not to take it.
Alternate to a telecoil? [a way to get an audio signalinto a hearing aid]: DAI
-direct audio Input
-the signal is delivered via an electrical cable that is connected to the hearing aid
-signal can originate from any equipment, IPOD, Mic, FM wireless
improves S/N ratio
How does linear circuitry reach goals?: -by amplifying input sounds by the same amount
-problem if louder sounds become too loud
-solution- use limiting strategies ie peak clipping (causes distortion)
What is compression?: The range of input sound intensities is reduced to fit a smaller range of output intensities
What does digital circuitry do in HAs?: -converts the input signal to a numerical form before processing it. (easier to manipulate)
-analog info --> digital
-next it is converted into binary digits or bits
-now it can be manipulated mathematically for filtering
-then numbers converted back to analog using DAC (digital to analog) converter
Advantages of digital HAs?: -they can perform more complex processing than is possible in analog
-many operations can be completed with less power & circuit size
-dig aids can make decisions on how to process a sound based on over all acoustic environment
-cannot provide CD quality sound
When the loudness of one sound is clearly affected by the cessation and /or commencement of another sound...?: called Pumping
what are 3 types of level dependent frequency responses in signal processing?: BILL = Bass increase at low level

TILL = Treble increase at low level

PILL = Programmably increase at low level
TILL: TREBLE INCREASE at low level -Part of classification schema describing multichannel processing
-when the degree of compression is greater in the high frequency channel, than in the low freq channel
there will be a greater high freq emphasis at low input levels than at high input levels
BILL: BASS INCREASE at low levels
-when degree of compression is greater in the low frequency channels than in the high freq channels
-there will be less high freq emphasis at low input levels than at high input levels
Linear Prescriptive strategy?: -single gain target
-goan does not vary as function of input level
-designed to optimize conversational levels with the understanding that soft levels will be inaudible
-if average conversational speech is at comfortable listening level, fitting is typically successful
Any control that follows the sensing point in the signal chain has what effect on compression: does not effect the amount of compression but does affect the final level of compression
Any control that precedes the sensing point in the signal chain has what effect on compression?: does affect the compression threshold and hence the amount by which a signal is compressed
-varying these controls causes the I-O curve to shift horizontally
2 reasons why maximum output must be limited?: -excessively intense signals will experience discomfort
-excessively intense signal can cause further damage
-Compression is the preferred method because peak clipping causes distortion
so does compression limiting but not as objectionable
Two kinds of copression kneepoint controls: Conventional - affects kneepoint, output but not gain
-useful for individuals with severe/profound HL, volume control early on in circuitry
-aid goes into compression only after sufficient signal reaches the compressor
Threshold Kneepoint Control (TK) affects kneepoint of compression & gain (most commonly used)
-vol cont is before compressor
-TK adjusts kneepoint over a range of low input levels
-40-55
-gain booster for soft sounds
-gain minimal or not audible for loud sounds.
-appropriate for mild-moderate HL
5 examples of fitting strategies: POGO -
LIBBY - 1/3 gain
BERGER
NAL-R (National acoustics laboratories revised)
DSL (Desired sensation level)
for children
NAL-R based on: Byrne & Tonnison 1976
-based on loudness equalizaton, making all bands equally loud
-most popular fitting method in the US
-original NAL-R was geared to linear fittings providing only a single gain target
-now we have non linear versions NAL - NL
DSL based on: -aim is to provide audible & comfortable signal in each frequency regions
-RE aided gain target
-does not make speech equally loud in each frequency but attempts comfortable loud
-for any hearing level provides a target or desired sensation level
non linear fitting strategies: -non linear hearing aid fitting with compression signal processing is 3 dimensional
-gain varies as a function of freq and input level
-prescriptive procedures include at least 3 targets soft 45dB, med 65dB and loud 85dBSPL input levels. Gain for soft input level > than for med or loud input
-input/output function for WDRC, will have a linear response up to a knee point (compression threshold) and then change in input will not produce identicle change in output.
-Expansion - providing an increasing amount of gain with a decreasing input to reduce ambient noise
NAL-NLI: -nonlinear fitting procedure, version is an extension of NAL-R
-Designed for nonlinear signal processing
-provides different prescriptive targets as function of input level
-goal is still to maximize speech recognition for a given speech input level within constraints that HI should percieve speech no louder than a normal hearing individual
-speech judged to be soft, comfortable, & loud by normal listeners. Should be percieved similarly by hearing impaired user
The opposite of compression?: expansion - gain decreases as input level decreases
-also called squelch or noise gating
-useful in some aids for decreasing the audiblility of very low level sounds (HA internal noises)
5 broad catagories in the history of methods of hearing aid fitting: 1. Common frequency response approach
2. Threshold based prescriptive formulas
3. Supra threshold prescriptive techniques
4. Carhart selection & comparative evaluation procedure
5. first fit targets based on threshold & suprathreshold prescriptive techniques with adaptive adjustment
reasons to use non-linear compression: -limit the output of the HA w/o creating significant distortion
-fit output of HA into patients dynamic range (compress range of useful sounds)
we need some way of automatically turning down the gain of the HA as the input intensity increases
-AGC = automatic gain control
-provide "relaxed listening" in background noise(significant compression in low frequencies)
Carhart method of prescription: He compared HAs across 4 dimensions
-sensitivity to sound (thresh)
-tolerance limits (suprathresh)
-efficiency in background noise (s/n)
-efficiency in distinguishing small sound differences (intelligibility)
used audiologics tests
-functional gain
-aided loudness discomfort
-examination of dynamic range
-speech recognition tests in quiet and noise
this was the profile before "REAL EAR" entered the scene
What is multichannel compression?: -split the incoming signal into different frequency bands
-each band of signal passes through a different amplification channel
-each channel contains its own compressor
What is a compression amplifier and when is it needed?: -an amplifier that turns down its own gain as the input to the amplifier increases.
-also know as an automatic gain control (AGC) or automatic volume control
-used when dealing with a person who has a reduced dynamic range because less amplification is required for intense sounds than for softer sounds
When compression limiting is used to control the SsPL of the HA it must be an: output controlled compressor
-a high compression ratio is needed
-attack time must be short so gain decreases rapidly enough to prevent loudness discomfort
-release time must be short or adaptive
In HAs, the gain that is needed to make weaker sounds of speech audible will at the same time make the stronger sounds uncomfortably loud. What 3 things can be done to prevent this?: 1. Peak clipping -limits the output of an amplified signal whenever they exceed a critical level but causes nonlinear distortion
2. Compression Limiting - reduce gain when amplified sound reaches critical output level
3. Wide Dynamic Range Amplitude Compression (WDRAC) - can be use to match characteristics of the HL not only with respect to threshold shift & max loudness but also with respect to the increase of loudness with intensity within the dynamic range of hearing
ASHA recommendation of prescribed gain change when fitting binaurally?: - gain for each ear should be reduced by 3-6 dB to compensate for binaural summation
ASHA guidelines for HA fitting: 1. assessment
2. treatment
3. selection
4. verification
5. orientation
6. validation
What is NOISE?: Any unwanted signal that interferes with a desired signal. speech is the signal of primary interest
three types of noise: 1. Random noise -with intensity freq spectrum similar to speech
2. a second interfering voice - or many = speech babble
3. Substantial room reverberation - produced by sound being reflected off walls, floors, ceiling & other hard surfaces. some amount is good but too much reduces intelligibility
The ideal amplifier would have...: -the gain frequency response required
-would not generate internal noise
-would not distort the signal

most HAs live up to this to a certain degree
All HAs (analog and digital) made since early 80s use ___ microphones: electret
-sound pressure enters through the inlet port which moves the diaphragm. The electret is a precharged membrane. Movement of the diaphragm creates and increase in it charge which results in a change in output voltage
Electret mic imperfections: -eventually they breakdown due to chemical agents (perspiration)
-they generate a small amount of random electrical noise which may be audible to the use
-they are sensitive to ANY vibration not just sound
-they are subject to wind noise even random particles hitting the diaphragm will be transduced into electrical noise
-displacement of the microphone or receiver can cause vibration (more likely in CIC, ITC, & ITE)
Directional microphones are important because?: -they improve the intelligibility of speech
-they are the only form of signal processign in the HA that can improve the S/N ratio
-most directional microphones today are part of a duel-microphone system
-when ambient noise is not a problem the use can switch off one of the microphones and reverts back to omnidirectional
The best class amplifies is ?: Class "D"
also know as switching amplifiers
-output is a smooth audiosignal
-much more efficient than the others
The ideal receiver has...: -a wide flat frequency response
-most receivers roll off at high freq
-to reduce rolloff you need either a larger receiver or greater electrical power (bigger battery)
-poorly made receivers are prone to peak clipping
Possible benefits of conducting loudness scaling?: -provides information regarding the appropriate gain for average input levels which cannot be accurately precicted from threshold LDL data
- provides information regarding appropriate compression knee points and compresson ratios
- provides more accurate LDL which will improve the overall fitting of some algorithms
What is cross modality matching?: Loudness is matched to visual modality
-loudness matched to line length
What is an earmold?: -individually fabricated ear insert that channels sounds reproduced by the hearing aid through the ear canal to the eardrum
Important functions:
-couples the sound from the receiver to the users ear via a sound bore
-affects the gain freq response of the HA
-controls the extent to which the residual volume of the ear canal is open to outside air
-comfortably retains the hearing aid in the ear
Ear shell styles: For ITE:
-full concha or full shell-extends above the crus-helix
-half concha or half shell - contained below the crus-helix
For ITC:
-also known as mini canals
-extend only part of the way along the posterial medial wall of the tragus
CIC
-little or no material in the concha
3 functions/objectives of HA coupling (shell or earmold): -provide a sound bore that couples the sound from the receiver to the aid wearer's ear canal and consequently affect the gain freq response of the HA
-they control the extent to which the inner part of the ear canal is open to air outside the ear (venting)
-they retain the hearing aid in the ear in a comfortable way
Freq regions affected by each of the components of a HA: 1.Sound Bore dimensions - affect above 2000 hz for BTE
above 5000 for custom aids
2.Damping affects - the shape of the response of mid frequencies. 800-2.5Hz for BTE, I.5 -3.5 for custom
3.Venting - affects low frequency response range 0 - 1kHz, depending on size and HA gain
Vents and acoustic mass: A vent is a column of Air surrounded by walls of a tube
- air has mass therefore inertia
-a long and thin vent will have the greatest weight of acoustic mass
-for a vent to transmit sound, the inertia must be overcome or air will not move
-more efficienctly done by low freq than high freq
- a vent provides a low cut to freq response of amplified
-extent depends on vent size
Effects of vents on Gain & OSPL90: -Low freq gain & OSPL90 are affected
-vent allows low freq sound to escape out of the ear canal
-allows low freq sound into the residual canal w/o passing through HA
goal of venting?: to select a vent size that allows the target gain to be achieved w/o creating excessive occlusion or feedback
-also to prevent excessive moisture
a sound bore is?: the sound path from the receiver to the ear canal
What is a vent?: Vent is a second sound path between outside air & inside ear canal.
-when no vent is present (as in high gain hearing aids) the mold or shell is said to be occluding
The shape of the sound bore affects what?: the high frequency gain & output
Bores that widen as they progress inward are? and do what?: called horns and increase high frequency output
Bores that narrow do what?: Decrease high frequency range
The shape of the gain freq response is affected by?: the way the hearing aid is coupled to the ear
3 aspects
-sound bore
-damping
-venting
What is an IROS vent?: Ipsilateral routing of signals
-path is formed by removing some of the shell at the medial or lateral ends of the vent
-leaves only a short wide path
Describe HA test box: Test box
-acoustic chamber
-measures a coupler or simulator response
-generates calibrated sounds at the HA microphone
Includes
-tone or noise generator, amplifier, and loud speaker
-control microphone or ref. mic
-measurement mic
-chamber to attenuate ambient noise
Ear simulators do what?: -mimic the characteristics of a normal real-ear over a wide freq range
-have multible cavities with different volumes that adjust to combinations required for different frequency testing
Zwislocki coupler is an ear stimulator
Purpose of doing REAL EAR measurements: -to characterize the performance of HA in place on ear
-allow individualized correction factors for maneuvering among various dB measureing factors used in audiology
What is a coupler?: -a cavity that allows a HA to be connected to a microphone
-provides a repeatable way for the HA to be attached to a sound level meter
-ANSI stds use 2cc couplers
IEC standards use ear simulators
2 types of 2cc HA couplers?: HA1 - has no ear mold simulator and is used for ITE and ITC hearing aids

HA2 - includes an earmold simulator & is connected to a BTE via tubing and to a a receiver of a body aid
What is RECD: Real ear to Coupler Difference
-represents the spl generated in the average real ear minus the spl generated in a 2cc coupler.
-how close the 2cc coupler is in approximating the average adult ear
-most offices use 2cc couplers
-should be performed on every adult and kept for reference
Purpose of the control mic: -is placed very close to the hearing aid microphone within the test box while measurement is taking place
-it monitors the SPL reaching the HA from the loudspeaker
-it automatically adjusts the volume of sound coming from the loud speaker during measurement
What measurement signals are used when testing linear HAs?: -puretones or broadband noise
-both will give the same response
What kind of signals used for testing Compression HAs: NOT PURETONES
-broadband signals give a better estimate of the response of a compression HA
-many HAs allow a disengage of compression feature and can then be tested with puretones
What is involved in verification of fit?: -can the HA do what it is supposed to do?
-is it in good working condition?
-is it comfortable?
-have you met your target?
-listening check to rule out circuit noise or intermittancy
-cosmetic appeal
-ease of insertion & removal
-security of fit
-ease of control operation
Validation measures?: - did you improve the persons hearing in the way sought?
is disability reduced?
-re assess patient PB max to confirm success
What is the average OSPL90?: -the average gain at 1k,1600, 2500Hz
-ansi specifies that HFA OSPL90 should be within plus or minus 4 dB of manufacturer specs
what is total harmonic distortion: the ratio of the power of all the distortion products summed relative to the power of the wanted ouput signal component
What is EIN: Equivalent Input noise
-components generate their own noise usually orignating in the microphone and microphone amp
-ein is expressed relative to the input of the HA
Total harmonic distortion is measured at?: -with volume control at RTS
-measured at 70dBSPL at 500 and 800 Hz
-measured at 65 DBSPL at 1600 Hz
-THD reported as a% (power of all harmonics is summed)
the measurement for THD using a pure tone can under-estimate the amount of dist in high freq and over-estimate the amount of dist in low freq
why test for THD?: 1. to ensure that the HA meets published specs
2. to compare the fidelity of 2 diff HAs
3.to establish if the HA uses compresson limiting or peak clipping
THD in compression limiting HAs should be below 10% at every freq
When does distortion occur?: -Distortion occurs when the output of a device contains frequencies that ar not in the input
-Harmonic distortion occurs when the input is a puretone and the distortion occurs at integer multiples of the input freq
-Intermodulation distortion occurs when the input is a complex signal (multiple puretones) and distortion is equal to the harmonics of the input signal and products of the input freq at whole number multiples
-peak clipping is most common cause of distorion
How is reference test gain calculated?: The HFA output has to be 17dB below OSPL90plus 60 dB input
What does an compression ratio of 4:1 mean: for every 4 dB in in put there is a 1 dB in output
What measurements are done at the RTS setting?: -freq response curve with 60dBinput
-harmonic distortion
-equivalent input noise level
-battery current
-t-coil measurements
-input output curves AGC
-attack and release (AGC)
What is the purpose of HA standards?: -to specify which parameters (on HAs) should be measured
-to describe how HA characteristics should be measured - equipment, procedures, calculations
-state what the accepted tolerances for each hearing aid characteristic measured
What is the electroacoustic Test for HAs required according to ANSI stds: -OSPL90
-Gain freq response curve
-Harmonic distortion
-Internal noise
-Battery current drain
-magnetic response (telecoil)
Special tests for HA
- AGC input/output
-AC attack & release time
What is the half gain rule for selecting HA gain?: Rule specifies that the gain should be equal to 1/2 of the HL in dBHL

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