LINEAR HEARING INSTRUMENT FITTING METHODS• Hearing instrument fitting methods have evolved over the past seventy years as a result of a mixture of hearing instrument technology available at any particular time; and the experimental trials and errors of scientific inquiry at that time period.
LINEAR HEARING INSTRUMENT FITTING METHODS • History Initial attempts to create fittingformulae began in 1935. The firstpremise was to “mirror” theaudiogram. In other words restoreone decibel of gain for everydecibel of measured hearing lossby frequency.
LINEAR HEARING INSTRUMENT FITTING METHODS• Mirroring the audiogram seems intuitively correct because we could make all thresholds 0dbHL for the patient/client’s hearing loss.• If their tolerance for loudness grew with the hearing loss, we could fit a sixty decibel hearing loss with a sixty decibel gain hearing instrument.
LINEAR HEARING INSTRUMENT FITTING METHODSMost of our patient/clients will nothave a conductive hearing loss;they will have a sensorineural loss—a damaged cochlea wheresensitivity to soft sounds is reducedwhile the tolerance for loud soundsremains unchanged or evenreduced, as well (recruitment).
LINEAR HEARING INSTRUMENT FITTING METHODS• In 1946 Ray Carhart developed a comparative clinical procedure by trying several different hearing aids upon patients to see which hearing instrument they liked the best.• His approach involved various speech test measures in aided and unaided conditions.
LINEAR HEARING INSTRUMENT FITTING METHODS• It soon became clear that this approach was difficult to teach and replicate from one clinic to another.• Clinicians began to feel the need for a systematic prescription approach to fitting hearing instruments.
LINEAR HEARING INSTRUMENT FITTING METHODS • HistoryThe next thought was to determinethe patient/client’s most comfortablelistening level (MCL) and createenough amplified gain to makesound audible and comfortable.For some reason, the dynamic rangevariations of speech energy were notconsidered.
LINEAR HEARING INSTRUMENT FITTING METHODS • History This investigation of MCL createdthe observation that most satisfiedhearing aid users were desiring thegain of their instruments to beestablished at approximately halfof the revealed threshold ofhearing loss.
LINEAR HEARING INSTRUMENT FITTING METHODS• In 1963, Sam Lybarger proposed a half-gain rule for linear hearing instrument gain.
LINEAR HEARING INSTRUMENT FITTING METHODS• During the 1960’s, 1970’s, and 1980’s, subsequent linear fitting formulae were developed. All with a similar premise of the half- gain rule. All concluded that mirroring the audiogram was not an appropriate hearing instrument gain formulae for cochlear pathologies.
LINEAR HEARING INSTRUMENT FITTING METHODS• Linear based fitting formulae offered an alternative to the comparative approach to hearing instrument fitting.• These fitting formulae created “prescriptive” gain targets rather than speech testing comparisons.
LINEAR HEARING INSTRUMENT FITTING METHODSLet’s review some of the morepopular fitting formulae asdescribed in Venema, chapterthree, pages #57--#59.
LINEAR HEARING INSTRUMENT FITTING METHODSIt is interesting that most fittingformulae are based upon thescientific information gained fromresearch upon non-pathologichearing ability.However, the majority of hearinginstruments are fit to pathologicconditions—mostly cochlearpathologies.
LINEAR HEARING INSTRUMENT FITTING METHODSThe importance of measuringthreshold by frequency anddetermining supra-thresholdloudness perception has existed forover fifty years.
LINEAR HEARING INSTRUMENT FITTING METHODSMost research into appropriatefitting formulae was based uponlinear hearing instruments wherethe increase in HI gain directlyincreased the HI output.
LINEAR HEARING INSTRUMENT FITTING METHODSThe target for any particularhearing loss and for any particularthreshold based fitting methodarises out of a compromisebetween the reduced dynamicrange encountered with thesensorineural hearing loss andlinear hearing instrumenttechnology.
LINEAR HEARING INSTRUMENT FITTING METHODS Finding a simple relationship between hearing loss and target gain has not been easy for four primary reasons. They are:1. The optimum target gain is dependent upon the input signal.2. The optimum gain by frequency may depend upon supra-threshold loudness by frequency.3. The optimum gain by frequency may depend on the perception formed by many years of hearing loss.4. The patient/client’s individual communication goals i.e. comfort in noise, speech intelligibility in noise, etc.
LINEAR HEARING INSTRUMENT FITTING METHODSHarvey Dillon has stated thathearing instrument candidacyshould not be solely based upon apure tone threshold audiogram.As Hearing Instrument Specialists,we will learn to not only measurehearing loss but, how to defineresidual hearing ability.
LINEAR HEARING INSTRUMENT FITTING METHODSHearing Instrument Specialists willalso find that the pathologic natureof one single frequency of yourpatient/client’s hearing loss candetermine the overall perceptualloudness of their hearinginstrument.
LINEAR HEARING INSTRUMENT FITTING METHODS • Fitting Formulae Most fitting formulae have beenusing targets based upon thethresholds of hearing loss and usinglinear amplification. Certainly some digital hearinginstruments may be programmedbased upon these same targets andresult in digitized analog/linearoutputs.
LINEAR HEARING INSTRUMENT FITTING METHODSAs dispensing professionals, we nowhave the ability to address loudnessby frequency discreetly thus,customizing the electroacousticinformation to our patient/client’spathologic ear.