--------------------------------------------------------------------------- TITLE: HEARING AIDS: SELECTION, FITTING, AND DISPENSING SOURCE: Dept. of Otolaryngology, UTMB, Grand Rounds DATE: February 27, 1991 RESIDENT PHYSICIAN: Jeff S. Chimenti, M.D. FACULTY: Francis B. Quinn, M.D. DATABASE ADMINISTRATOR: Melinda McCracken, M.S. --------------------------------------------------------------------------- "This material was prepared by resident physicians in partial fulfillment of educational requirements established for the Postgraduate Training Program of the UTMB Department of Otolaryngology/Head and Neck Surgery and was not intended for clinical use in its present form. It was prepared for the purpose of stimulating group discussion in a conference setting. No warranties, either express or implied, are made with respect to its accuracy, completeness, or timeliness. The material does not necessarily reflect the current or past opinions of members of the UTMB faculty and should not be used for purposes of diagnosis or treatment without consulting appropriate literature sources and informed professional opinion." I. General Considerations A. Population 1. 20 million adults and 4 million children in U.S. have some measure of hearing impairment 2. 65 years and older--1 in 4 have hearing impairment; 3rd most common problem of elderly is hearing impairment; elderly population is increasing with improvements in health care 3. Less than 1 million Americans are hearing aid users B. Hearing Loss 1. Greater than 90% of hearing aid fittings for SNHL (1987) 2. SNHL--most common peripheral dysfunction of auditory system (presbycusis, noise-induced most common etiologies); amplification is single best treatment 3. Recruitment (abnormal growth in perception of loudness with more intense stimulus) is an important factor C. Hearing Aid Sales 1. In-the-ear (ITE) hearing aid sales accounted for 80% of market (1987); ITE was less than 1/3 market in 1977 2. Major boost to ITE market in 1984 when President Reagan publicly acknowledged his use of an ITE II. Hearing Aid Components A. Modern Hearing Aid 1. Amplifier whose function is to increase intensity of sound energy and deliver it to ear with as little distortion as possible 2. Acoustic energy cannot be amplified directly--must convert it to electrical energy which is amplified and changed back to acoustic energy (analog system) B. Components 1. Microphone a. energy transducer which converts acoustic energy into electric current 2. Amplifier a. increases voltage of electric current from microphone b. greater amplification with more transistors and circuitry 3. Receiver a. converts amplified electrical energy into acoustical energy and transmits it to the ear b. can be air conduction transducer (sound waves to external canal) or bone conduction transducer (vibratory signal to mastoid) c. internal receiver (within the ear level or ITE aid) or external receiver (separate from the case of body aid) d. considered "weakest" link--narrower frequency response than microphone 4. Power Supply a. battery (1.3-1.5 v.) provides power for amplification 5. Volume Control a. variable resistor (potentiometer) which adjusts amount of amplification of input signal (gain) b. relatively little gain once volume control is beyond 50% of total range with increase in harmonic distortion past this point 6. Other Components a. Tubing or Cord (1) ear-level and eye glass hearing aids require plastic tubing connecting elbow or receiver nozzle to earmold (2) body aids require cord to deliver amplified signal to receiver b. Tone Control (1) can provide high frequency emphasis (HFE) or low frequency emphasis (LFE) by frequency supression or filtering c. Telephone Pick-up (1) magnetic induction pick-up coil (telecoil) picks up magnetic field from telephone receiver leakage and converts to electric signal (2) telecoil takes the place of hearing aid microphone; advantage is no interference from environmental sounds d. Output Control (1) adjustment to reduce maximum output intensity that can be generated III. Selecting Hearing Aid Candidate A. Degree of Hearing Loss 1. General a. normally weighted heaviest of variables in candidacy b. pure tone audiogram used as screening method c. poor candidates--hearing sensitivity too good or too poor 2. Severe losses a. may be successful even with loss up to 90- 100dB; patient should be given opportunity to try HA b. unilateral loss exceeding 50-60dB or with impaired speech recognition associated with opposite normal ear--limited success for monaural fitting; if no benefit with amplification, then use CROS (contralateral routing of signal) HA c. bilateral loss--likelihood of successful fitting increases since patient does not have better ear (always attempt aiding) 3. Mild losses a. flat audiometric configuration--easiest to fit and may be successful even with borderline impairment (20-30dB); mild gain amplification can restore symmetry b. high frequency losses (>2000 Hz)--usually normal hearing at 2000 Hz with 30-50dB impairment at >3000 Hz; communication difficult in noisy environment; may be able to provide more gain at 3k-4k than 2k by using acoustically tuned ear molds (horn type molds or open earmold configuration)--not adaptable to ITE aids c. low frequency losses (<2000 Hz)--usually 30- 60dB hearing sensitivity at 500 Hz and 1000 Hz and normal >2000 Hz; problem is that to obtain max. gain in low frequencies, a close fitting must be used which attenuates high frequencies; recently (1985) KBASS hearing aid introduced which is a low frequency emphasis HA that can be fit with special non-occluding earhook- tubing arrangement B. Motivation for Assistance 1. Commonly there is a discrepancy between need for HA and action to obtain one 2. Younger people tolerate less hearing loss before seeking assistance 3. Motivation must be evaluated on person-to-person basis with larger likelihood of success if patient is highly motivated C. Acceptance of Loss 1. Unlikely that any rehabilitation will be successful unless patient has admitted need for assistance 2. Reluctance to accept loss may occur when patient attributes communication difficulties to other factors or when there is anger associated with the loss 3. Self-report inventories may help clinician to gain insight into patient's acceptance of loss D. Cosmetic Concerns 1. Rapid increase in ITE (in-the-ear) and ITC (in-the- canal) sales in recent years represents cosmetic component 2. Many patients believe wearing HA will elicit negative view from observers--"hearing aid effect"- -studies have shown that some negative bias does exist and is related to size of HA 3. May need to compromise between desired electroacoustic characteristics and HA size 4. Must ensure patient has manual dexterity to operate smaller controls on ITE and ITC aids IV. Theoretical Hearing Aid Selection A. Comparative Approach 1. Hearing aids compared to one another using an objective measure such as speech recognition 2. HA selection process usually incorporates comparative and theoretical (prescriptive) approaches B. Theoretical (prescriptive) Approach 1. Based on presumption that there is a predictable relationship between audiometric char. of hearing impaired person and electroacoustic char. of HA which would provide optimal amplification 2. Several mathematical formulae used to target ideal insertion gain--half gain rule: apply gain equal to half the hearing loss at each frequency to determine ideal insertion gain; some reduction of gain in lower frequencies needed to address upward spread of masking (low frequency background noise interfering with low, as well as, higher freq. speech recognition) 3. Amplified signal should fall between patient's threshold of sensitivity and discomfort (comfortable listening range); attempting to "mirror" the audiogram results in overamplification of higher frequencies 4. Probe microphone measurements (discussed later) are used to determine if target gain has been reached 5. Flexibility from different HAs and earmold configuration when conducting a BTE (behind-the-ear) fitting cannot be utilized in an ITE fitting--this places importance on clinicians prescriptive fitting (gain, frequency response, saturation sound pressure level (SSPL)) V. Preselection Considerations A. Hearing Aid Styles 1. Behind-the-ear (BTE) aids a. ear level microphone circumventing need for unsightly cord of body instrument b. most flexible of all styles--can use earmold modifications for acoustic affects; have potentiometers for controlling power and freq. response c. most commonly dispensed for severe HL or if not good candidate for ITE due to HL configuration, poor manual dexterity, or other factors 2. Body-worn aids a. BTE aids have cosmetic advantage and acoustic advantage by locating microphone at ear level b. body aid candidate: (1) if extra gain from body aid is critical (2) poor dexterity--need larger controls (3) less chance of loss/damage (4) pinna cannot support BTE 3. Eyeglass aids a. fitting, adjustment, and repair is greater problem because hearing and vision accounted for in single device b. no advantage over BTE or ITE--clinicians discourage its use for new HA users c. best candidate--patient previously using this type of aid and satisfied d. CROS, BICROS, or bone conduction aid may be worn using eyeglasses 4. CROS and BICROS aids a. CROS (contralateral routing of signal)--for individuals with one unaidable ear and opposite normal ear; microphone at ear level on bad side routes signal (by wire or wireless FM transmitter) to receiver on good side b. BICROS--adds microphone on side of better ear; preferred over CROS when there is hearing loss in better ear c. Wired system--usually fitted into eyeglasses; wireless--electronics usually require larger BTE case 5. In-the-ear (ITE) aids a. Popularity increasing--cosmetic concerns play major role b. In-the-canal (ITC) is smallest type--some have difficulty (39%) with #1 complaint being occlusion caused by aid c. If desired electroacoustic char. can be met equally with BTE and ITE then use patients preference B. Bandwidth 1. Microphones today can produce response from 20Hz- 20,000Hz ("wide-band") 2. High frequency--speech components exist above 4000Hz; studies support that 4000-6000Hz amplification improves aided speech intelligibility; limitations--lesser sensitivity, more feedback (problem if HL exceeds moderate category) 3. Low frequency--low frequency amplification may improve speech intelligibility; limitations--amplify backgrd. noise therefore must use reduced gain in lower frequencies C. Earmold Selection 1. General a. acoustic theories that apply to earmold technology also apply to ITE fittings (hearing aid case) b. frequency response altered by earmold plumbing (earhook, tubing, earmold) c. must choose correct combination of factors to appropriately distribute amplification: 1) venting (primarily affects low freq.), 2) damping (mid frequency), 3) horning (high freq.) d. Hard plastic molds (Lucite)--durable, used with most ITE hearing aid shells, can be used when acoustic gain less than 55 dB e. Soft molds (Polyvinyl chloride)--used when gain exceeds 55 dB because of need for better ear canal adherence which reduces feedback; young children--protect concha and meatus 2. Venting a. Vent: opening in earmold from tip to lateral surface; oriented parallel or diagonal to sound bore; venting valve may be ordered to alter vent b. Similar acoustic effects with leakage from loose mold (intentionally with ITE's) c. Non-occluding earmold (open, CROS) is very large vent d. Use of venting: (1) attenuation of lower frequencies (<1500Hz)--greatest in open mold (2) pressure relief--use small vent (0.6mm) (3) allow input of unamplified sound (normal hearing in certain frequencies) (4) decrease "hollow" or "reverberant" sound e. Limitations--feedback increases with larger vent, may decrease gain in high freq. as well as low (diagonal vent), may increase gain in low freq. (tight fitting earmold) f. Venting or open earmold (acoustic modification) results in better speech intelligibility than electronic modification when employing "high- pass" fitting 3. Damping a. Resistance added to transmission line of HA which smooths peaks created by receiver-earmold system and reduces output esp. in mid-frequency range b. Frequently placed in tip of earhook with several choices for each BTE model (ie., 680 ohm, 1500 ohm, 3300 ohm) 4. Horn Effects a. Horn molds--stepped bore or tapered diameter transmission system to replace energy in 2500- 3000Hz region lost due to earmold placement (insertion loss) b. Libby horn--most popular; one piece tapered horn with damped tone hook (3mm or 4mm) c. Majority of pts. fitted with HA have downward sloping audiogram--horn molds considered for most BTE fittings d. small or abnormally shaped ear canals--poor candidates 5. Earmold Impression a. Ethyl methacrylate or silicone-base polymer used to make impression b. First examine ear to ensure no discharge, infection, or excess cerumen c. Position otoblock just beyond isthmus of canal, position syringe into canal and inject material with nozzle of syringe embedded in impression material,allow material to fill helix and concha bowl areas, wait 10 min. then remove impression D. Microphone Type 1. Directional--less amplification of sounds of sounds originating behind the ear than in front (vs. Omnidirectional) 2. BTE hearing aid--directional microphone is superior; despite this directional used in only 20% BTE's; may provide significant improvement in situations where HA user is able to understand only portions of conversation secondary to background noise 3. ITE hearing aid--directional microphones not commercially available E. Binaural Amplification 1. Evidence suggests that most hearing impaired individuals should be fitted binaurally 2. Advantages: a. Localization--depends on intensity, time, phase; binaural is superior to monaural b. Binaural summation--binaural threshold approx. 3 dB better than monaural, may require less gain with less likelihood of exceeding loudness discomfort level (LDL) c. Elimination of head shadow effect--monaural aided pt. will have sound from side opposite aid attenuated by the head (greater in high freq.) d. Binaural squelch--improvement in signal to noise ratio due to ability of interactive mechanisms of auditory system to "squelch" undesired noise 3. Anyone having 2 aidable ears is a candidate for binaural amplification; must also consider financial situation, manual dexterity F. Output Limitation 1. General a. Often the shape and degree of SSPL90 (saturation sound pressure level) is more critical than gain curve; Output maximum of HA at different frequencies (determined by SSPL90) should not exceed patient's loudness discomfort level (LDL) b. SSPL90 setting should be just below LDL to give widest dynamic range without exceeding users LDL c. Methods of output limitation: (1) peak clipping (2) compression (input/output) d. Linear amplifier--one to one relationship between intensity changes of signal (input) and intensity changes produced by amplifier (output); amount of amplification applied to input is constant (unless changed by volume control manually); desirable with conductive hearing loss; output limit set by peak-clipper device e. SNHL--degree of amplification required for higher intensity inputs is less than that required for less intense inputs (recruitment), therefore linear amplifier is not acceptable and peak-clipper introduces much distortion of input signal; compression amplifier is preferable f. Compression amplifier--automatic voltage regulating device which monitors voltage of either preamplifier (input compression) or power amplifier (output compression); amount of amplification automatically changes (decreases) as a function of increasing input intensity VI. Hearing Aid Selection A. General 1. Consists of formalized testing to determine the final hearing aid fitting 2. Testing can include: speech-based procedures, functional gain, and probe-tube microphone measures 3. If sound theoretical approach used with consideration of preselection factors, this testing may be used more for verification than selection B. Speech-based Procedures --previously most widely used method; popularity decreasing in past decade with introduction of computerized real-ear probe-tube microphone measures 1. Speech recognition procedures a. Unaided and aided speech recognition testing (usually monosyllabic words) b. Small differences between HA's with this testing may be due to variability of speech instead of HA performance c. May get more reliable measurements with longer word lists but this is time consuming d. Most appropriate role appears to be in comparing unaided from aided performance by demonstrating to patient and clinician that HA improves recognition 2. Judgements of speech quality a. HA users opinion (judgement) concerning quality of amplified sound--found to relate poorly to speech intelligibility; often depends on low freq. amplification (more LF amplification-- better rating) b. Should construct listening tasks similar to real life--reverberant room with background noise c. Best to use bipolar adjectives to describe sound quality (i.e., clear vs. hazy); informal opinion has little value 3. Judgements of speech intelligibility a. Relate better to measured intelligibility (speech recognition) than judgements of speech quality b. Sensitivity for differences between HA's increases as time elapsed between presentations decreases--usefulness of acoustic valve fitted at ear level for attachment of 2 BTE aids (rapid switch between aids) C. Functional Gain Measures 1. Functional gain = difference between aided and unaided frequency specific thresholds with HA adjusted to a `use gain' setting 2. Has surpassed speech recognition testing as most commonly used selection procedure; development of formula based prescriptive methods encouraged use of functional gain measures 3. Like speech recognition, many sources of variability a. must assure that non-test ear is not participating by masking with an earphone b. head movement can alter thresholds as can standing waves which may occur with pure tones in a sound field c. may be affected by masking from internal circuit noise of HA and ambient room noise-- makes aided threshold appear poorer for freq. region where patient has hearing at near normal levels d. reliability--at least 10dB difference must be observed between aids before considering it significant D. Probe-Tube Microphone Measures 1. 2cc coupler--electroacoustic measures of HA performance have centered on this but it does not have a response similar to a real ear 2. Zwislocki coupler--reproduces response of adult ear more accurately 3. KEMAR (Knowles Electronics Manikin for Acoustic Research)--uses Zwislocki coupler; used to take into account head and body interaction on an acoustic signal which influences HA performance; used in applying correction factors for prescriptive fittings but otherwise limited clinical application 4. Probe-Tube Microphone Measures (1981)--computerized self-calibrating system using a microphone connected to a soft tube which is placed in the ear a. most accurate method for predicting HA performance on an individual b. calculates real-ear insertion gain (difference between unaided and aided SPL measured in ear canal); this is electroacoustic corollary to behavioral measurement of functional gain c. helpful when selecting SSPL90 values because it can measure aided SPL values in ear canal; especially helpful in children d. Advantages (over functional gain): (1) eliminates subject threshold response variability (2) no contamination by HA circuitry or ambient roon noise (3) time efficiency (4) obviates need for audiometric test room e. will become routine part of HA selection f. should use some combination of formula based procedure (i.e., probe-tube measures) verified by speech based testing VII. Special Considerations in Children A. Selecting Hearing Aid Candidate 1. Early identification of hearing loss in children is crucial and once identified, fitting of appropriate amplification must proceed promptly--mitigate against effects of sensory deprivation on development and ensure adequate audition during critical language period of early childhood (no child is too young for a HA 2. Degree of hearing loss a. Mild hearing loss--recent support for thought that even mildest hearing losses in children deserve HA trial (i.e., 15-20dB); includes those with recalcitrant middle ear effusion, unilateral hearing loss, and mild bilateral hearing loss (all may lead to communication handicap) b. Unilateral hearing loss--difficulties encountered may result from effects of head shadow and loss of binaural squelch; options include monaural HA, CROS aid, assistive listening devices (FM wireless systems) in classroom; may have significant classroom communication problems and preferential seating may not be sufficient c. Profound hearing loss--best to proceed with selection and fitting until it is conclusively shown that there is no benefit (always treat child as if he/she is aidable) d. Classroom only amplification--part-time use of HA or use of FM system; possible with children who have minimal or unilateral hearing loss B. Theoretical Approach to Selection 1. Selection based on combo. of prescriptive and comparative methods but prescriptive approach has increased importance 2. Methods a. Attempt making aided threshold as normal as possible (problem: HA setting may exceed LDL or not be appropriate for maximum speech intelligibility b. Desired sensation levels for amplified speech for different frequencies within speech spectrum determined by computer (desired sensation level decreases in non-linear fashion as hearing threshold increases); based on sound field audiometric thresholds, the desired real- ear SSPL values selected C. Preselection Considerations 1. General--must decide between HA, assistive listening device (ALD), vibrotactile system 2. Hearing aid style a. BTE--standard for bilateral hearing impaired child is binaural BTE; advantage over body aid by providing ear level microphones b. ITE/ITC--frequent recasing because of changes in canal and pinna size which is more costly and time consuming than fabricating new earmolds; limited as to ability for direct audio input (FM device) or telecoil which are important for child; may be alternative with mild hearing impairment because tight fit not as critical if less gain required; also preferable if concha or pinna shape make BTE use difficult; 1985--ITE 15%, BTE 75% c. Body aids--used with very young child because more durable and easier to secure to child d. CROS/BICROS--use same as in adults e. Classroom--HA vs. FM system or classroom amplification system--FM system alone has significant improvement in signal to noise ratio over HA alone; probably best to use FM receiver coupled via direct input to child's HA with option to use HA alone, FM system alone, or aid + FM system 3. Earmold Selection a. Often governed by HA style, degree and configuration of hearing loss, geography of pinna/ear canal b. Earmold material--usually soft vinyl for protection during falls; also provides efficient acoustic seal important for high gain HA c. Earmold modifications to alter frequency response similar to adults but horn mold limited in younger children due to size of ear canal d. Ill fitting earmold is poorly tolerated so must frequently inspect fit and comfort 4. Microphone Type--directional is preferred 5. Binaural--best with bilateral hearing loss 6. Output Limitation a. Crucial in children who are less able to verbalize discomfort from excessive SSPL b. Probe-tube microphone measurements allow direct measure of SPL delivered in child's ear canal c. Concern of hearing loss from excessive SSPL or "overamplification"--cause permanent threshold shift over time (must monitor hearing sensitivity closely) D. Hearing Aid Selection 1. Behavioral Approaches a. Speech recognition testing--monosyllabic tests appropriate for pediatric patient; large variability; best suited for demonstration of aided vs. unaided performance b. Functional gain--measurement of unaided and aided sound field thresholds for narrow bands of noise 2. Objective Approaches a. General--avoids necessity for interpreting behavioral response to make decisions regarding amplification characteristics b. Acoustic reflex methods (1) acoustic reflex thresholds (ART) under aided and unaided conditions (2) real ear gain measured as difference between aided and unaided ART's at a particular frequency (3) advantage: frequency specific, inexpensive instrumentation (4) problem: need for pure sensory hearing loss of mild to moderately severe degree so ART's are evident c. Auditory evoked response measures (1) auditory evoked responses (AER)-- compare aided and unaided responses in sedated child (2) may be used with CHL (3) many ABR parameters are targeted for improving with amplification (wave V latency, wave V threshold, latency/intensity function of wave V); in general, the more normal an ABR can be made the more satisfactory is the HA system (4) problems: limited frequency specificity, electronic HA delay may alter latency, brain potential recordings may be contaminated by radiant energy from HA d. Probe-tube microphone measures (1) value may be greatest for pediatric patient (2) only objective approach which is not dependent on hearing status of child for response (3) insertion gain determined by measuring actual HA output in ear canal (4) after determination of gain and maximum power output desired (based on prescriptive procedure), probe-tube measures are preferred objective measure for HA selection VIII. Hearing Aid Dispensing A. Hearing Aid Dispensers 1. Prior to 1970's: 3 tiered system including physician, audiologist, and hearing aid retailer; retailer was responsible for HA sales, fitting, and counseling; pt. could go directly to retailer and bypass system 2. During 1970's: some clinical audiologists began dispensing HA's after breaking affiliation with the American Speech and Hearing Association which would not allow members to dispense for profit; 1978-- Academy of Dispensing Audiologists formed; 1977-- FDA issued regulations governing HA labeling and conditions for sale making HA a medical device rather than consumer product--adult buyers to obtain medical exam prior to purchase or sign waiver 3. The American Speech/Language and Hearing Association no longer prohibits members from dispensing for profit--many audiologists sell HA's and more physicians are including sales of hearing aids as part of medical practice B. Modular Hearing Aids 1. Contemporary custom hearing aid delivery system requires the dispenser to send an impression of user's ears and audiologic test results to a custom HA manufacturer who constructs "custom" HA in terms of shell construction and electroacoustic performance; if repair needed--must be returned to manufacturer; system depends heavily on the mail 2. Recently some manufacturers have preassembled electronic "modules" that can be inserted by the dispenser into a custom or semicustom shell 3. Advantage--no need to wait for the manufacturer assembly, economical, most service issues handled at dispenser office 4. Disadvantage--dispenser must maintain large inventory, and still must rely on custom HA for selected cases IX. New Approaches A. Noise Suppression Technology 1. Background noises make speech perception more difficult and if background noise of signal is louder than speech portion of signal then speech unintelligible (signal to noise ratio) 2. Noise with same frequency as speech sounds interferes with perception of those sounds 3. Low frequency noise can interfere with low, middle, and high frequency speech (upward spread of masking) and higher freq. consonants contribute more to speech discrimination than lower freq. vowels; backgrd. noise is predominantly low freq. 4. Low frequency amplification reduction--triggered when low freq. input becomes loud (decrease upward spread of masking due to noise) B. Digital Technology 1. Recently much research to incorporate microprocessor digital technology into HA's 2. Digital/Analog Hybrids a. presently available b. basic analog HA functions (preamplification, filtration, power amplification, output limitation) under digital processor control which will actively adjust each analog component in response to acoustic environment 3. All Digital Signal Processor a. active research b. acoustic signal amplified and then converted into digital numeric "code" via an analog-to- digital converter--digital signal processing computer alters code as directed by program-- code converted into analog signal by digital- to-analog converter which goes to receiver c. presently only body style due to size and power requirements C. Implantable Hearing Aids 1. Cochlear implant--ideal candidate has profound SNHL with no measurable improvement with conventional HA or vibrotactile device 2. Implantable bone conduction device--vibrational stimulation via implanted accelerometer driven electromagnetically and transcutaneously via external hearing aid device; useful with moderate to severe CHL with good nerve function in at least 1 ear 3. Implantable canal aid--external auditory meatus surgically altered to accept conventional HA device; cosmetic advantage and acoustic advantage due to depth of insertion of HA speaker ----------------------------------------------------------------------------- BIBLIOGRAPHY 1. Alpiner, J.G., and McCarthy,P.A.; Rehabilitative Audiology: Children and Adults; Williams and Wilkins, 1987. 2. Aungst, H.L., and Gitles,T.; In-The-Ear Fitting Applications; Audecibel; 39(4):16,1990. 3. Chasin, M.; A Clinically Adaptive Hearing Aid Fitting Procedure; Seminars in Hearing; 9(3): 207,1988. 4. Johnson, E.W.; Binaural amplification-naturally!; Hearing Instruments; 38(12):19,1987. 5. Pollack, M.C.; Amplification for the Hearing Impaired; Grune and Stratton, Inc.,1988. 6. Silverman, C.A., and Silman, S.; Apparent Auditory Deprivation from Monaural Amplification and Recovery with Binaural Amplification; J. Am. Acad. Audiol.; 1:175,1990. 7. Smriga, D.J., et al; Developments in Hearing Aid Fitting and Delivery; Otolaryngol. Clin. N. Am.; 22(1):105,1989. 8. Zelnick, E.; Update on the necessity of binaural amplification; The Electrone Communicator; 7(2):April, 1990. ----------------------------END----------------------------------------------