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TITLE:   HEARING AID FITTING AND SELECTION
SOURCE:  UTMB Department of Otolaryngology
DATE:    1/25/95
RESIDENT PHYSICIAN:     Gregory Young, M.D.
FACULTY:                Deborah Carlson, Ph.D.
DATABASE ADMINISTRATOR: Melinda McCracken, M.S.
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"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." 

  Introduction:

       Approximately 22 million people in the United States report some
degree of hearing loss.  This number is expected to rise as the elderly
population continues to grow.  While hearing loss may sometimes be treated
with medication or surgery, most patients derive benefit only from
amplification.  Hearing aids are becoming more popular with improvements
in electronics and design.
  
       The first hearing aid was probably the hand cupped behind the ear.
This provides about 14 dB of cosmetically unappealing amplification.
Acoustic amplifiers such as horns and speaking tubes were used from the
seventeenth to nineteenth century.  Carbon hearing aids were developed at
the beginning of the twentieth century using telephone technology.  In
1938, the vacuum-tube hearing aid was introduced, offering greater
amplification, wider frequency response, and lower distortion.  Bell
Telephone Laboratories invented the transistor in the 1950s, which is the
basis of today's hearing aids.  With the transistor, hearing aids were
smaller and more flexible in design.
  
       A hearing aid is a device consisting of a microphone, an electronic
amplifier, and a receiver.  The microphone receives environmental sounds,
the amplifier enhances a few or several frequencies depending on the needs
of the user, and the receiver transmits the modified sounds to the middle
ear.
   
     There are typically three contributions that hearing aids can provide
the user.  The first is to amplify normal conversational speech to levels
that are maximally understandable, which leads to improved communication.
The second is to allow the patient to hear other environmental sounds,
such as warning signals and music.  The third contribution is to promote
education and development.  Hearing aid use in the hearing impaired child
may allow the development of  normal language and speech skills.
  
  
  Types of Hearing Aids:

     There are three common types of hearing aids, which differ based on
size, and location on the patient.  In general, the smaller the hearing
aid, the less electronic flexibility, the smaller and fewer the user
controls, the fewer the possible acoustical modifications , and the
smaller the battery.  However, smaller aids do offer a more natural sound
because the acoustic properties of the pinna and in some cases the canal
are preserved.   In addition, smaller aids have higher cosmetic appeal.
  
     The largest  of the common types of hearing aids is the behind the
ear (BTE) hearing aid. This device has a module that fits behind the ear,
which contains a microphone, an amplifier, a battery, and a receiver.  The
microphone connects to an earmold in the external auditory meatus via a
piece of clear tubing that travels over the top of the post auricular
crease.  Tapering the size of the earmold tubing from smaller at the
module(earhook) to larger at the external auditory canal allows for more
amplification in the higher frequencies, while venting the earmold reduces
amplification in the low frequencies.  These devices are rugged and easily
serviced, and have sufficient space for several electroacoustic
adjustments.
  
     The in-the-ear(ITE) device sits in the concha, helix and external
auditory canal.  The same  type of components that are found in the module
of the BTE device fit within the ear mold of the ITE.  Most ITE aids are
custom devices, with the components built into a shell made from an
impression of the user's ear.  Modular ITE devices are a standard shape
and fit into a custom made earmold.  They do not utilize space as
effectively as the custom ITE devices.
  
     The ITE aids are further divided into full concha, low profile, and
half concha, based on their location.  The full concha aid fills the
entire concha, and is the most common type.  Its relatively large size
allows for substantial flexibility in design, while its shape assists in
reducing internal feedback and holds the aid securely in place.  The low
profile aid is designed to protrude less from the concha.  While having
possible cosmetic advantages, it has less space for electroacoustic
adjustments.  The half concha aid occupies the concha cavum and the
lateral canal.  It has more restricted design options, and uses a smaller
battery.  Patients with greater than 70 dB hearing loss may experience
feedback problems with ITE devices, because the microphone is adjacent to
the receiver.
  
     The in-the-canal(ITC) hearing aid is similar to the ITE device,
except the components are small enough to fit entirely within the external
auditory canal.   These aids have more limited acoustic adjustment
selections, and require a smaller battery.  However, they do utilize the
natural acoustic properties of the concha and lateral canal.  ITC hearing
aids are custom-molded.  These devices have high cosmetic appeal, and have
acoustic advantages such as reduced feedback when using the telephone.
The microphone location within the ear reduces wind noise interference and
enhances high-frequency sounds in the 2 to 4-kHz region.
     
  
     In the United States in 1994, 57% of dispensed hearing aids were ITE,
25% were ITC and 22% were BTE.  Occasionally, the hearing aid components
are integrated into eyeglass frames(0.1%) or a body-worn device(0.3%).  In
1993, 1.67 million hearing aids were sold in the United States, and in
1994, approximately 1.57 million were sold.
  
     More specialized types of hearing aids also exist.  The
CROS(contralateral routing of signal) device uses a microphone on the
worse hearing side to transmit sounds to a receiver at the better hearing
ear.   Transmission occurs via a wire that runs around the back of the
neck, or in a wireless mode by radio frequency.  In this way, the wearer
can use the good ear to hear sounds from the impaired ear.
    
     The CROS unit may benefit those with no usable hearing in one ear and
minimal hearing loss or normal hearing in the other ear.  For example, a
cab driver with an impaired right ear may benefit from the CROS unit,
since the right ear is closer to the passengers.  The CROS unit is also
beneficial to those with bilateral high frequency hearing loss.  Only high
frequencies are amplified in the CROS unit because the external canal is
not occluded.  The non-occluding design allows low frequency sounds to
escape amplification.  Therefore, distortion resulting from unneeded low
frequency amplification does not occur.  An ipsilateral fitting known as
IROS(ipsilateral Routing of Signal) has been developed which utilizes a
non-occluding design without the contralateral routing.  IROS is used for
mild-moderate high frequency hearing loss.
    
     As technology advances, hearing aids can be packaged into
increasingly smaller housings.  Devices which fit entirely within the bony
canal have been developed, called completely-in-the-canal(CIC).  These
aids have high cosmetic appeal because they are virtually undetectable.
However, they also have several acoustic advantages.  These include
reduced occlusion effect, reduced gain requirements, and preservation of
the natural acoustic properties of the pinna and external canal.
  
     With traditional hearing aids, amplification of low frequency
vibrations in the patient's voice produces an echoing, hollow sound.  This
is called occlusion effect, and results from occluding the lateral portion
of the external auditory canal.  The CIC devices have little or no
occlusion effect because of their medial location in the canal.
  
     CIC devices have reduced gain requirements, because the volume of air
between the hearing aid and the tympanic membrane(V3) is reduced.  This
allows the hearing aid to operate with lower acoustic power,  resulting in
reduced distortion and improved sound quality.  As V3 is reduced by half,
the sound pressure at the tympanic membrane is increased by 6 dB.
  
     The CIC wearer is better able to use the natural acoustic cues
provided by the pinna, because the microphone is located deep within the
canal.  There is evidence that speech understanding in noisy conditions is
improved, and localization of sound is enhanced.
  
  
     Patients with irreparable, debilitating conductive hearing loss may
benefit from a bone conduction oscillator.  The oscillator may be worn as
a head band or may be implanted into the temporal bone. Patients with
chronic otitis or with malformation of the canal or external ear may also
benefit from a bone conduction hearing aid.
  
  
  Selecting a Hearing Aid:

     The choice of hearing aids for a particular patient sometimes is
limited by the amplification capabilities of the aid.  The maximum
amplification available with the ITE aid is somewhat less than that of the
BTE aid.  Usually, however, logistic considerations weigh more heavily in
the final decision.  For example, patients with limited dexterity may
benefit more from the BTE aid, while others may choose the ITC aid for
cosmetic reasons.
     
  
  Hearing Aid Circuitry:

     Hearing aids utilize analog, programmable, or digital circuitry.
Analog hearing aids change sound into electrical current.  Almost all
contemporary hearing aids are analog in nature, and these will likely
remain the most common type of hearing aid for the next few years.
  
     Programmable hearing aids have analog type amplifiers and filters
which are controlled by an external digital source.   These hearing aids
contain a memory module such as a   CMOS(complimentary metal oxide
semiconductor), RAM(random access memory), or EEPROM(electrically erasable
programmable read only memory).  An external microprocessor(computer)
accesses the memory locations within the chip to modify the hearing aid's
electroacoustical performances.  The memory module replaces the
conventional trimmer potentiometer functions in the analog hearing aid and
provides more precise control of the acoustic characteristics.  Some
programmable hearing aids allow patient control of electroacoustic
parameters with a remote control.  Programmable hearing aids will likely
become more prevalent in the next few years.

     Entirely digital hearing aids are not yet commercially available.  In
digital hearing aids, the input signal is digitalized, then processed with
digital signal processing circuitry.  These hearing aids are fitted using
software packages, and can be programmed to make changes in
electroacoustic performance based on the input signal.  While digital
hearing aids have several theoretical advantages, several obstacles remain
to be overcome before these devices are commercially available(e.g. power
consumption, size).
  
  
  Electroacoustic Characteristics of Hearing Aids:

     Several electroacoustic parameters are used to describe the
performance of hearing aids.  The three most important characteristics are
saturation sound pressure level(SSPL), gain and frequency response.
Electroacoustic measurements are performed by directing the output of a
hearing aid into a hard-walled cavity with dimensions of 2cm x 2cm x
2cm(2-cc coupler), in accordance with standards developed by the American
National Standards Institute(ANSI).
  
     As input to a hearing aid increases, output increases up to a certain
point, after which further increases in output do not occur.  At that
point, the aid is driven into saturation.  The saturation points at
various frequencies form the saturation sound pressure level(SSPL) of the
hearing aid.  This represents the maximum amplification of the device.
The SSPL is important because saturation that occurs too high could exceed
the user's threshold of discomfort, and that which occurs too low may not
provide enough signal to those requiring greater amplification.  The SSPL
of a hearing aid is typically obtained using a 2-cc coupler by delivering
a 90 dB sound to a hearing aid with its volume control full-on, then
measuring the output across test frequencies.  The response curve obtained
is known as the SSPL 90 curve.
  
     Gain is a measure of amplification.  The acoustic gain of a hearing
aid is the difference in dB between the input and the output at a
particular frequency.  Full-on gain is the amount of amplification
achieved when the volume control is turned maximally turned up, and the
input is adjusted to a suitable value(60 dB).  The high-frequency full-on
gain is an average of the gains at 1 kHz, 1.6 kHz, and 2.5 kHz.
  
     The frequency response is a measure of hearing aid output when the
volume control is in the normal operating range.  Specifically, the volume
control is set to the reference test position(reference test gain).  The
reference test position is the volume position obtained by adjusting the
gain downward so that the high-frequency full-on gain is 17 dB below the
SSPL90 across a range of frequencies.  The curve is obtained with an input
of 60 dB, and the gain set at the reference test position.  For aids with
milder gain, and for AGC aids, the reference test position is full-on.
  
  
  Automatic Signal Processing:

     Circuits have been developed which automatically change the gain or
frequency response of a hearing aid in response to a change in the input
signal.  This type of circuitry is called automatic signal
processing(ASP).
  
     Most types of ASP are circuits that modify only the gain of the
instrument, known as fixed frequency response(FFR), because the frequency
response remains constant.  FFR circuits are usually output limiting
systems.  These systems adjust the maximum deliverable pressure of the
aid, so that the output capability of the hearing aid is not exceeded, and
distortion is thus kept low.  In addition, these circuits keep the
wearer's uncomfortable level(UCL) of sound from being reached.
    
     Hard peak clipping is the simplest form of output limiting.  It
results in significant harmonic distortion at saturation levels, although
speech intelligibility is probably unchanged.
  
     Newer output limiting systems have a built-in monitoring circuit that
automatically reduces the electronic gain and compresses the dynamic range
of the output signal so that it better matches the dynamic range of the
impaired ear.  Since the gain of the hearing aid is automatically
controlled, these devices are called Automatic Gain Control(AGC).
    
     In general, AGC hearing aids deliver three types of amplification,
represented on the input/output curve as the linear section, the
compression section, and the limiting section.  In the linear section of
the curve, input and output are directly proportional and no compression
occurs.  In the compression section,  successive increases in input result
in diminishing increases in output.  In the limiting section, increases in
input result in no increase in output.
     
     The limiting level is the point between the compression and limiting
sections, and the knee point is the point between the linear and
compression sections.  The compression ratio relates to the slope of the
compression section.  The compression ratio and knee point can be modified
to accommodate various types of hearing loss.
  
     AGC action is achieved by a level detecting device incorporated into
a feedback loop which samples amplified sound in the form of AC voltage,
and feeds an appropriate amount of DC voltage back to the amplifier, which
reduces the amplifier gain.  AGC devices are classified as either
Output-Controlled or Input-Controlled, based on whether the sound sample
is obtained after, or before the gain(volume) control.  With
output-controlled AGC, the knee point remains constant on the output scale
as the volume is varied.  The knee point remains constant on the input
scale as the volume is varied in input-controlled AGC.  Therefore, the
volume control setting affects only the gain in output-controlled AGC
devices, but affects the gain and maximal output in Input-Controlled AGC.
    
     Recently, circuits have been developed which automatically change not
only the gain but also the frequency response of the hearing aid.  These
are Level Dependent Frequency Response(LDFR) circuits.  The main types of
LDFR are BILL, TILL, and PILL.
  
     Bass Increases at Low Levels(BILL) circuits provide more bass at
low-level input, and less bass at high-level input.  Noisy environments
frequently have predominantly low frequency noise.  If the lower
frequencies are amplified less, the speech range of sound may be more
easily detected.
  
     Treble Increases at Low Levels(TILL) circuits provide more treble at
low inputs and less treble at higher inputs.  This circuit is designed for
patients with high frequency hearing loss who require more high-frequency
gain for quiet sounds.
  
     Programmable Increases at Low Levels(PILL) circuits have programmable
processing bands such that high, intermediate, and low bands of frequency
can be independently controlled.
  
  
  Candidacy:

     The characteristics that determine hearing aid candidacy can be
divided into audiologic factors and motivational factors.  The audiologic
factors include the type of hearing loss, the degree of hearing loss, and
the dynamic range.  Motivational factors relate to the patient's lifestyle
and acknowledgment of a hearing problem.
  
     Those patients with intractable conductive hearing loss typically
retain normal cochlear function, so the pure tone audiogram is a good
predictor of which patients would benefit from amplification.  With
conductive hearing loss, the greater the loss, the greater the need for
amplification.
  
     In sensorineural hearing loss, the condition of the cochlea is highly
variable.  Therefore, sound processing capacity is not predictable from a
pure tone audiogram.  Two individuals with sensorineural hearing loss and
identical pure tone audiograms may have quite different sound processing
capabilities, such that one relies heavily on amplification, and the other
does well without it.  The person with mild to moderate sensorineural
hearing loss typically hears the louder portions of speech, such as
vowels, but not the voiceless consonants like t, p, k, f, s and ch.  The
result is a patient who can hear speech, but not understand it.
  
     The likelihood of benefit from hearing aids is increased if the
degree of hearing loss is moderate to severe.  Those with moderate to
moderately severe hearing loss need amplification in almost all social and
work situations, but typically still have good word recognition skills.
Word recognition is usually lacking in those with profound hearing loss,
which substantially reduces the derived benefits of amplification.
Patients with mild hearing loss are more difficult to assess.
  
     The dynamic range is the decibel range between threshold and the
point that the stimulus becomes uncomfortably loud.  The dynamic range is
often much reduced in patients with sensorineural hearing loss.  Elevated
thresholds in these patients shift the lower limit of the dynamic range
upward.  In addition, the upper limit can be shifted downward by
recruitment.  Individuals with a narrow dynamic range present greater
difficulty with fitting.  The dynamic range represents the target area for
amplification, with its midline usually being the most comfortable
listening level.
  
     Motivation for hearing aid use is influenced by degree to which the
individual's quality of life has been affected by hearing loss.  This is
referred to as the individual's hearing handicap.   A questionnaire may
help assess the patient's lifestyle, motivation toward hearing aid use,
and acknowledgment of the hearing loss.  Denial of a hearing problem
reduces motivation, which diminishes chances of a successful fitting.
Counseling may heighten the patient's awareness of a hearing problem and
improve his or her attitude towards hearing aids.
     
     In children, motivational factors usually play a lesser role in
determining candidacy.  With few exceptions, any child with significant
long term hearing loss is a candidate for amplification.  Evaluation of
young children involves both behavioral and electrophysiologic tests.
Behavioral tests provide frequency-specific information, and whereas tests
such as auditory brainstem response(ABR) provide individual ear
information.
  
  
  Monaural vs. Binaural Amplification:

     In most cases, binaural amplification is indicated, because patients
typically have improved speech understanding in noisy conditions, and are
better at localizing sounds.  Carhart's theory of "binaural squelch"
suggests that improved hearing with two aids in noisy conditions is due to
the phase differences of the signal and noise occurring between the two
ears.  In addition, the use of two aids eliminates head shadow effects,
which decreases the high-frequency cues needed for hearing many
consonants.  With binaural amplification, there is also an increase in
loudness of sound due to binaural summation.  Finally, studies have shown
that auditory deprivation in the un-aided poorly hearing ear may result in
loss of word-recognition which would not occur if a hearing aid were
present.

     If the patient decides on monaural amplification for financial or
other reasons, it is usually more beneficial to amplify the ear with the
wider dynamic range or the ear with better word recognition ability.
Occupational needs, handedness, or occupational needs may also influence
which ear is aided.
  
  
  Medical Evaluation:

     The medical examination of a hearing aid candidate should focus on
the pinna, external auditory canal, middle ear.  The earmold must fit
snugly within the external auditory canal in order to prevent acoustic
feedback.  Therefore, the canal must be healthy in order to accept the
resulting constant pressure.  However, even a severely malformed pinna or
canal does not rule out the possibility of amplification if the cochlea is
normal, because a bone conduction device may work well.  Contraindications
to occluding the external meatus include otitis externa and otitis media.
Elderly diabetics should be fitted with caution and counseled extensively
about their increased susceptibility to otologic infections.
     
     Excessive cerumen is a common problem with hearing aid users.  The
devices tend to push the cerumen towards the tympanic membrane, and hinder
the normal extrusion process through the external meatus.  Additionally,
even a small amount of cerumen adjacent to the receiver may lead to
feedback problems.  Cerumen removal may be required prior to fitting the
hearing aid, and periodically as long as the aid is used.
  
  
  Fitting Strategies:

     Fitting a hearing aid involves determining the output and gain
requirements for the hearing aid user.  Selective amplification is the
process of matching the frequency response of a hearing aid to the
audiogram of an individual.  One of the first examples of selective
amplification was the mirror-fitting technique, developed by Watson and
Knudsen in 1940.  Using this technique, the frequency response of the
hearing aid was set to the mirror image of the hearing loss, so the
poorest hearing regions on the audiogram received the greatest
amplification.
  
     The mirror-fitting technique has proven unsatisfactory as a fitting
strategy for hearing aids, mainly because it leads to overamplification.
In addition, the frequency response of the hearing aid is based on
measurements obtained in a coupler, and does not account for coupler/real
ear differences or insertion loss created by the hearing aid.
  
     Lybarger, in 1944, developed a fitting technique in which the optimal
frequency response curve would by about 1/2 of the audiogram curve.  This
is the origin of the one- half gain rule, which is often incorporated into
several of today's fitting strategies.
     
  
  Fitting Strategies - History:

     In 1946, Carhart described a fitting procedure in which the patient
would compare several hearing aids and select the one that provided the
best results.  This "comparative technique" was a lengthy process which
included training, counseling, hearing aid trial periods, and extensive
audiometric testing.  The hearing aid that provided the best results and
most satisfied the user was the instrument of choice.  The main
disadvantage of this approach is that it is too time consuming.  However,
variations of this approach are still used today for some fittings(e.g.
Texas Medicaid Program fittings).
  
     Recently, prescriptive procedures for fitting hearing aids have
become more popular.  These techniques attempt to assess an individual's
frequency response and gain requirements using a specific formula.  A
hearing aid is then selected which best matches the calculated
specifications.  There are several prescriptive procedures, and the
selection of a particular technique is due in large part to the
dispenser's familiarity with a specific technique.  These  techniques are
especially useful for ITE and ITC aids, which are usually custom made, and
therefore multiple instruments are not available for comparison.  The main
limitation of prescriptive procedures is that they were based on linear
hearing aids, and have more limited application for the currently used
non-linear aids.
  
     Lybarger's system is based on the one-half gain rule and utilizes the
patient's audiometric thresholds.  Lybarger's formula derives the
operating gain, defined as the gain used under most conditions.  The
operating gain roughly equals one-half of the puretone average of three
threshold values, and takes into account both air and bone conduction
hearing.
  
     Instead of using a puretone average to calculate the operating gain,
Berger in 1976 developed a formula which incorporated specific
frequencies.  The operating gain in the Berger technique is slightly
greater than one-half the hearing loss.  This technique also contains
formulas for output limiting devices when the individual's dynamic range
is limited.
  
     McCandless and Lyregaard developed the prescription of gain and
output(POGO) in 1983, which defines the insertion gain and maximum power
output requirements for individuals with sensorineural hearing losses of
less than 80 dB.  Compared to Lybarger's technique, POGO results in less
low frequency amplification, which may improve speech intelligibility in
noisy conditions.
  
     In 1986, the revised National Acoustics Laboratory(NAL) prescriptive
technique was developed by Byrne and Dillon.  The revised NAL is a
prescription for selecting the real-ear gain and frequency response of a
hearing aid.  This prescription uses speech spectrum data.
  
     A standard speech spectrum has been developed within which speech at
normal conversational levels is usually understood.  The spectrum is
represented as bands of hearing levels.  Without amplification, these
bands are between 35 and 70dB and vary somewhat among the different
frequencies(200 Hz - 6000 Hz).  With normal hearing, thresholds of holding
at the various frequencies are below or within the speech spectrum.
  
     The goal of fitting is to identify the hearing threshold at each of
various frequencies, and then raise the speech spectrum bands which fall
below the patient's threshold for hearing. The speech spectrum bands are
elevated by amplifing the sound in those frequencies.  In this way, the as
much of the speech spectrum as possible occurs above the person's
thresholds.
  
     Some prescriptive procedures are computer assisted.  Threshold data
from several frequencies are entered into the computer, which converts the
data from dB HL to dB SPL, and the required gain and frequency response
are calculated.  Some computer programs can also serve as data base
systems to help find a particular model hearing aid that approximates the
prescription.
  
     Digital and programmable hearing aids offer the potential for better
fitting to the individual hearing loss, but the complexity of the
adjustment procedure can make these hearing aids difficult to fit.
Frequency response and gain characteristics are usually programmed into
the memories of these hearing aids prior to shipment.  However, fine
adjustments usually need to be made by the dispenser, which requires a
programming unit from the manufacturer.  Most programming units are
devices that stay either at the manufacturer or in the dispenser's office.
However, there are some systems that utilize at hand held remote control
device which can be operated at any time by the hearing aid wearer.
  
  
  Hearing Aid Verification:

     Methods of verifying an appropriate hearing aid fit include speech
audiometry, functional gain, self-report procedures, and real-ear
measurements.
  
  
     Word recognition scores are the most common type of speech audiometry
when used in hearing aid evaluation.  Monosyllabic word lists are read or
tape recorded words are replayed at levels ranging between 40-50dB HL to
approximate normal conversational level speech.  The use of speech
audiometry for hearing aid verification is controversial, because the
ability of speech tests to predict how a hearing aid will perform in
everyday situations is questioned.  Additionally, there is an
unpredictable interaction between various types of speech stimuli and
various electroacoustic characteristics of hearing aids.
  
     Functional gain has also been used for hearing aid verification.  It
is defined as the difference between unaided and aided sound field
thresholds for narrow bands of noise.  Frequency specific values of
functional gain are obtained.  Limitations of this technique are that it,
like word recognition, depends on user participation, and its acquisition
is somewhat time consuming.
  
     Self-assessment questionnaires are helpful in verifying hearing aid
performance.  Improvement of an individual's subjective hearing handicap
is an important aspect of assessing the quality of hearing aid fit.
  
     Real ear measurements utilize a probe-tube microphone which is placed
medial to the hearing aid, approximately 5 mm from the tympanic membrane.
Coupling devices do not accurately account for the acoustic properties of
the pinna, head, and external auditory canal.  With real ear measurements,
the performance of the hearing aid can be measured from within the ear.
  
     The real-ear unaided response(REUR) is the open ear canal measurement
in dB minus the input signal in dB.  In effect, it is a measure of the
individual's ear canal resonance, external ear resonance and head
diffraction effects.
  
     The real-ear aided response(REAR) is the aided ear canal measurement
in dB minus the input signal level in dB.  It represents the total
response of the hearing system, including the REUR.
  
     The real-ear insertion response(REIR) is the REAR in dB minus the
REUR in dB.  This is called the real ear insertion gain(REIG) when
measured at a specific frequency.  The REIG represents the amount of
amplification  provided by the hearing aid system alone, and therefore
approximates the functional gain of the hearing aid.  The REIG is
especially useful in verifying the electroacoustic parameters of a device.
  
  Hearing Aid Orientation:

     In order to provide the patient with maximal benefit from the hearing
aid, it is important to explain how to operate and maintain the aid.
Follow-up appointments are critical for identifying potential problems in
fitting or the patients adjustment to the hearing aid.  Counseling and
encouragement may be necessary to promote acceptance of the device.
Elderly patients and those with severe or profound hearing loss may
benefit from enrollment in aural rehabilitation programs.
  
  
  Assistive Listening Devices:

     ALDs are amplification devices that improve the signal-to- noise
ratio(S/N) at ear level 15 to 20 dB in moderate noise and reverberation.
The S/N ratio expressed in decibels is the ratio of speech to ambient
noise reaching a person's ear.  Examples include the FM wireless system,
the infrared systems, alerting devices, and systems for radio and
television.

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  Cranmer-Briskey KS: Programmables: the options, variables and benefits.
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  Katz J: Handbook of Clinical Audiology, 4th ed. Baltimore, MD: Williams
& Wilkins; 1994
  
  Kirkwood DH: After a two-year slump, hearing aid market shows first
signs of recovery. The Hearing Journal 47: 7-13, 1994
  
  Letowski T: Nonlinear signal processing: classification of
amplitude-compression systems. The Hearing Journal 46: 13- 16, 1993
  
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specification. The Hearing Journal 46: 19-24, 1993
  
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