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Hearing Aids Intro Handout
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Card number_____ 1
card fld "Info"
Syllabus, Hearing Aids, CD 5703
Instructor: Robert de Jonge, Ph.D.
Office:
Martin 58, 660-543-8809, [email protected]
[email protected]
Required Texts: See reading assignments
Exams: There will be two exams, a mid-term and a final.
Each will be multiple choice. The final will not be
comprehensive.
Attendance:
Class attendance policy is consistent with
University policy. In addition, four absences are allowed for whatever reason (approved or not, at your discretion).
Beyond this the final grade is reduced by 1/4 of a letter grade for each additional absence. The final grade will be
increased by 1/4 for each of the allowed absences that is not used. Perfect attendance improves performance by one full
letter grade.
The second half of the class will focus upon hearing aid measurement and selection. The main idea is to get you
comfortable with the process of converting an audiogram to an appropriate hearing aid prescription, including venting,
special earmolds, etc. To facilitate this we will work with a HyperCard stack, "Hearing Aid Selection," which runs on the
Macintosh computer. The final exam will include a practical section where you will use this stack to fit a hypothetical
client. It will help you a lot if you become familiar with this stack. There's lots of useful and practical ideas within it.
Consider it a "reading assignment." There is also a take-home self assessment assignment targeting your understanding
of hearing aid selection and verification. The hearing aid lab will also include a practical assessment..
Topics (a wish list)
1. Introduction/Overview
1.1 Overview of major issues involving amplification
1.2 History of hearing aids
1.3 Federal Regulations & state licensure
1.4 Private practice, dispensing
1.5 Types of hearing aids
-body
-ear level or behind-the-ear (BTE)
-canal (ITC), completely in the canal (CIC)
-programmable ITE, BTE, ITC, CIC
1.6 Components/functions/controls of hearing aids
-microphone
-amplifier
-receiver
-gain control
-tone control
-SSPL control
-noise suppression
-telecoil
-battery types
-compression and AGC
-in-the-ear (ITE)
-CROS
-eyeglass
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-earmold types
-dampers
-feedback
-analog/digital programmables
-multiband, channels
-memories
-remote controls
-programming software
1.7 Troubleshooting/routine maintenance
1.8 Taking an earmold impression
2. Measuring hearing aid performance
2.1 Test equipment
2.2 HA-1 and HA-2 couplers
2.3 Specification of hearing aid characteristics:
(ANSI S3.22-1982, 1987, 1996 or ASA STD 7-1982, 1987, 1996)
-OSPL90
-HF-ave OSPL90
-HF-ave full-on gain
-Full-on gain
-reference test gain
-frequency response
-equivalent input noise level
-harmonic distortion
-battery current
-induction coil
-AGC aids
-chart paper
2.4 Real-ear measures of hearing aid performance (ANSI S3.46-1997):
-REUG, REIG, REAG, RESR, RECD, REDD, etc.
3. Hearing aid coupling/acoustics
3.1 Receiver characteristics
3.2 Earhook
-damping
3.3 Tubing
-length/diameter
-damping
-smoothing the response
3.4 Venting earmolds
-parallel/side branch
-open mold (Jansen)
-length/diameter
-damping
-PVV/SAV
-insertion depth
-vent response
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-feedback control (Macrae)
3.5 Stepped diameter earmolds
-effects of diameter
-damper placement
-Libby horn
-belled
-high freq cavity
3.6 Real-ear effects
-KEMAR, probe-mic measurements
-Zwislocki vs. 2-cc coupler gain
-middle ear effects, middle ear pressure
-effects of varying ear canal dimensions
-venting large canals (resonance effects)
-modeling (predicting) hearing aid performance
-sound field-to-eardrum transform (Shaw curve)
-in-situ vs. insertion (etymotic) gain
-microphone placement, body baffle
-Fry's test box (Fonix 6500)
4. Hearing aid selection/evaluation
4.1 Candidacy for a hearing aid
-degree of loss
-speech recognition ability
-tolerance problems
-unilateral losses
-subjective characteristics
4.2 Preselection process
-ear(s) to fit
-type of aid
-specifications (gain,etc.)
-coupling
4.3 Selective amplification
-Mirroring the audiogram
-Gain equal to a proportion of hearing loss
-Mirroring the MCL curve
-Bisecting the dynamic range
-Master hearing aid
-Harvard report
-MCL and LDL measurements and speech recognition
-Software (IHAFF, DSL, Fig6, HAS)
4.4 Hearing aid evaluation
-Carhart (1946) method
-Tournament strategies
-Measuring functional gain
-Real-ear measures
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4.5 Reliability/validity of selection/evaluation process
-reliability of speech testing
-problems with measuring functional gain
5. Binaural amplification
5.1 Advantages of binaural amplification
-objective benefits
-subjective evaluation
5.2 Reported disadvantages
5.3 Candidacy
5.4 Selection and evaluation
5.5 Dichotic, split band amplification
5.6 Varieties of CROS hearing aids
6. Management
6.1 Counseling
6.2 Follow-up
6.3 Adjustment
6.4 Measures of user satisfaction (eg., APHAB, COSI)
card fld "Reading Assignments…"
Assignments
•The hearing aid lab will meet for approximately 2 hours per week. You will be able to document skills you have
developed.
Readings
Pluvinage, V., "Rationale and development of the ReSound system," in Understanding Digitally Programmable Hearing
Aids, R. Sandlin (ed.), 1994.
de Jonge, R., "Hearing aid evaluation and selection: A perspective," Corti's Organ, Fall newsletter of the American
Auditory Society, 1989. (See next card)
"Hearing Aids: Standards, Options, and Limitations," Michael Valente (ed), Thieme, 1996.
oChap 6, Valente, Valente, Potts, and Lybarger, "Options: Earhooks, Tubing, and Earmolds."
"Strategies for Selecting and Verifying Hearing Aid Fittings," Michael Valente (ed), New York, Thieme, 1994.
•Chap 9, de Jonge, "Selecting and verifying hearing aid fittings for symmetrical hearing loss."
•Also, check out the other chapters, there are lots of good ones
Hearing Aids: Standards, Options, and Limitations, M. Valente (Ed.), New York, Thieme, 1996.
•Chap 2, de Jonge, R., "Real-Ear Measures: Individual Variation and Measurement Error"
•Also, look at the other chapters
"Probe Microphone Measurements: Hearing Aid Selection and Assessment," H. Gustav Mueller, David B. Hawkins, Jerry
L. Northern, Singular Publishing Group, 1992.
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•Chap 3, Mueller, "Terminology and procedures."
•Chap 5, Hawkins, "Prescriptive approaches to selection of gain and frequency response."
•Chap 6, Mueller, "Insertion gain measurements."
•Chap 7, Hawkins, "Selecting SSPL90 using probe-microphone measurements."
•Chap 13, Hawkins & Mueller, "Test protocols for probe-microphone measurements."
"Handbook of Hearing Aid Amplification, Volume I and II," Robert E. Sandlin (ed), College Hill Press, 1988.
Volume I
•Chap 1, Lybarger, "A historical overview."
•Chap 5, Dillon, Compression in hearing aids."
√Chap 7, Libby & Westermann, "Principles of acoustic measurement and ear canal resonances." (Scan for exposure to
info)
Volume II
•Chap 1, Reiter, "Psychology of the hearing impaired and hearing aid use: The art of dispensing."
•Chap 3, Pascoe, "Post-fitting and rehabilitative management of the adult hearing aid user."
"Hearing Aid Evaluation," Margaret W. Skinner, Prentice Hall, 1988.
•Chap 2, "Effects of hearing impairment on the identification of speech sounds."
•Chap 10, "Counseling and hearing aid orientation."
"Amplification for the Hearing-Impaired," 3rd Edition, Michael C. Pollack (ed.), 1988.
•Chap 2, Pollack, "Electroacoustic characteristics."
•Chap 11, Glaser & Pollack, "Private practice and hearing aid dispensing."
Assignments for mid-term
1. Introduction/Overview
•Pollack, Chap 2, 3 (p. 105-114, impression technique), 11
•Sandlin, Volume I, Chap 1
•Sandlin, Volume II, Chap 1
•Skinner, Chap 2
•Hearing aid selection and evaluation: A perspective.
Assignments for final
2. Measuring hearing aid performance
•User's manual for the Fonix hearing aid test system.
•Sandlin, Volume I, Chap 5, 7
3. Hearing aid coupling/acoustics.
•Valente, et al. Chap 6 (p. 252-326)
4. Hearing aid selection/evaluation.
•Mueller et al., Chap 3, 5, 6, 7, 13
•HyperCard stack: Hearing Aid Selection.
5. Binaural amplification.
•Valente, 1994, Chap 9.
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6. Management.
•Skinner, Chap 10.
•Sandlin, Volume II, Chap 3
7. Programmable hearing aids
•Pluvinage
9. Individual variation.
•Valente, 1996, Chap 2.
card fld "Major Topics…"
You should understand…
•What a hearing aid is; i.e., anatomy of the aid
◊what are the major components
◊what controls are available
◊earmold coupling
•How the hearing aid functions; i.e. physiology of the aid
◊how do the major components function
◊what effect does each of the controls have
-objectively, electroacoustically
-perceptually
◊functions of earmold "plumbing"
•How to measure the characteristics of a hearing aid
◊ANSI specs
◊you will learn how to use the Fonix system
•How to select a hearing aid
◊prescriptive procedures
◊relate ANSI specs to user's needs
◊HyperCard stack, "Hearing Aid Selection"
◊DOS/Windows based selection software:
-IHAFF, DSL[i/o], Fig6, NAL-NL1
◊Manufacturer's software for fitting programmables; e.g.:
-Siemens Connexx (part of Unity)
-Oticon's OtiSet
-Micro-Tech's Meridian
-Phonak's Professional Fitting Guide (PFG)
-ReSound's Resource
-Starkey's ProConnect
-Argosy's Quadrasound
-Danavox's Danafit
•How to evaluate, verify the selection
◊mainly, real-ear probe mic measures
◊the practical part of final exam will assess the selection/verification process…
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-begin with an audiogram
-use Hearing Aid Selection stack to…
√select REIG
√convert REIG to 2-cc FOG response
√Select RESR
√convert RESR to SSPL90 curve
√select a matrix
√select a manufacturer's hearing aid
◊use Fonix system to verify the selection
•The dispensing environment
◊legal, ethical issues
◊audiologist & dealer relations
◊state licensure, FDA guidelines
•Historical perspectives
◊acoustical aids
◊electrical aids (carbon mic)
◊electronic aids (vacuum tube, transistor, integrated circuits)
◊programmable aids (ReSound, Phonak)
◊fully digital aids (Widex Senso, Oticon Digifocus)
•Taking the ear impression
◊you will order your own mold
•Troubleshooting hearing aids
◊electroacoustical, mechanical problems associated with the instrument
-evaluate and solve common problems
◊using the dremel tool, bench grinder to modify earmold, ITE casing
•At the end of the class you should have a good beginning understanding of how to:
◊develop prescriptive targets based upon hearing loss
◊select and adjust a hearing aid to meet those targets
◊how to take an ear impression and order the hearing aid
◊how to perform basic shell modification and troubleshooting
◊what issues to discuss with the patient in hearing aid counseling (APHAB, COSI)
card fld "HAS Study Questions…"
Final Exam Questions
Study Questions for the "Hearing Aid Selection" stack…
Approach the following questions in the same way that you would an "open book" exam, except that the open
book is the hearing aid selection program. For example, you don't have to remember what the exact dimensions of a 6R12
earmold are, just how to find the information.
1.
Describe how the program treates the relationship between MCL and insertion gain, UCL (or LDL) and SSPL90.
Under what conditions will the insertion gain never be less than 0 dB, why?
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2.
The AI is 50%. What would you expect the intelligibility to be for monosyllabic words, sentences? What is the AI
weight for the 1/3 octave band surrounding 1000 Hz? Is speech presented at a 40 dB SL more intelligible than speech
presented at a 30 dB SL. Why?
3.
At 4000 Hz the real-ear gain (REIG) is 10 dB. What coupler gain would produce this REIG for an ITE aid, a BTE
aid? Why is there a difference between the ITE and BTE aid?
4.
You have an audiogram threshold of 40 dB HL. This would correspond to what SPL in a) a 6 cm3 coupler b) an
average ear canal c) the sound field?
5.
At 2000 Hz the REAG gain is 40 dB, the insertion gain (REIG) is 28 dB. What is the REUG?
6.
Threshold is 50 dB HL at 2000 Hz. How much insertion gain would Berger prescribe? Byrne and Dillon if the
PTA was 55 dB?
7.
What happens when you increase diameter and/or reduce the length of the vent? What happens to the REIG
when the canal/vent system resonates?
8.
For a closed earmold, enter a mild gradually falling hearing loss. Use NAL to calculate an REIG. Using the "trace
curve" effect, demonstrate the effects upon the REIG of a) using a 1.5 x .3 cm vent b) adding an undamped 8CR earmold c)
changing the shape of the canal resonance curve (REUG). Should you use ITE or BTE corrections with the 8CR mold?
9.
What is the Zc of a 3 mm ID tube? What are the dimensions of a 6C10 earmold. If you want a high frequency
emphasis would you choose a 6C10 or 6B10 mold? What kinds of problems occur with dampers?
10.
Describe the earmold that compensates for the loss of the canal resonance. What does it mean to have a reverse
slope hearing loss? Which earmold would be appropriate for a reverse slope hearing loss? Compared to the standard
closed mold configuration, how much extra gain does a 3 mm horn supply at 4000 Hz?
* * *
Real-ear and 2 cm3 Coupler Measurements
1.
Can you perform all the ANSI measurements using the Frye 6500 test box? Doing the testing is a lot easier than
understanding what the information means. After you get these measurements, can you explain what they mean? You
should be able to, both for BTE and ITE aids.
2.
Can you use the Quick Probe II to measure REUG, REAG, REIG, RESR, RECD and REOG? Can you enter an
audiogram and calculate a target gain curve? Can you enter your own, customized target gain curve? Can you
determine whether the hearing aid matches the target? Can you make adjustments in the trim pot settings of the hearing
aid to more closely match the target? Can you explain what you are doing in such a way that your patient can
understand it?
3.
Can you use the Quick Probe II option to measure the spectrum of noise generated by a hearing aid while it is in
the ear canal of a listener?
4.
How would you use the Quick Probe II to measure the occlusion effect, produced by wearing an ear mold, at a
frequency of, say, 500 Hz? By venting the hearing aid, would this occlusion effect be eliminated? If so, by how much, and
how would you measure it?
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5.
How would you use the Quick Probe II to measure the real-ear attenuation of a hearing protector device, such as
an EAR plug?
6.
Based upon a particular volume control wheel (VCW) setting for a hearing aid, the use gain setting, could you
predict the hours of battery life for a particular size battery?
7.
People will cup their hand behind their ear when trying to listen. Does this really help? If so, by how much?
How would you answer this question?
8.
Can you use the Frye box to demonstrate 1/4 wavelength resonances (say, for a "der der" tube)?
9.
The impedance of a tube increases with frequency. Demonstrate this using 1 inch of #13 tubing attached to the
HA-2 coupler.
10.
Does the inherent noise of a hearing aid increase, or decrease as the VCW setting is increased? Can you use the
Quick Probe II to measure this for a particular listener.
11.
If you are performing ANSI measurements for a hearing aid, how do you "let the equipment know" that you are
measuring a linear vs. ITE aid?
12.
During 2 cm3 coupler measurements you can display the input-output function for an AGC aid. How would
you do this? Normally, the display is dB SPL out vs. dB SPL in. Instead, can you get a display of gain vs. input?
card fld "Hearing Aid Lab…"
Topics for Hearing Aid Lab
by
Deb Galley, MS, CCC-A
(Arranged Times)
•Ear impression techniques and earmold selection
-impression materials
-techniques
-styles and materials
-venting
-modification and repair
•Hearing aid troubleshooting
-basic checking and repair
•Programmable hearing aids
-fitting techniques, ReSound, 3M (Sonar), Phonak
•Hearing aid selection
-style
-determining the prescription
-utilizing the HAS program
•Use the Fonix 6500 to measure the performance characteristics of hearing in an artificial ear (the 2-cc coupler) using the
ANSI standards.
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•Use the Fonix 6500 to measure the real-ear performance of hearing aids; i.e., probe-mic measurements.
•Fitting protocol and follow-up
•Counseling
Card number_____ 2
card fld "Selection Perspectives 1"
Hearing Aid Selection and Evaluation: A
Perspective
Robert de Jonge
Central Missouri State University
Warrensburg, MO 64093
Since many of our readers have had
little formal coursework in the area of hearing
aids, the purpose of this paper is to give a
general introduction to some of the issues
involved in providing a hearing aid for an
individual. For a comprehensive and more detailed review of hearing aid selection, please see a text such as Dr.
Skinner's, Hearing Aid Evaluation (1988).
How, to begin with, is a particular hearing aid selected? Once selected, how is it evaluated to make certain that
the original choice was a wise one? Is there, currently, a particular protocol for evaluation/selection that is generally
accepted by professionals to accurately predict how successful a hearing aid user will be? These are questions that, you
will see, don't really have definite answers. But, hopefully, exploring both past and present ideas and opinions
surrounding these issues will provide at least some form of understanding.
What has happened and is still evolving in the area of hearing aid selection is a curious hodgepodge of ideas,
some of which on the surface seem to have little to do with each other. Put them all together, though, and a coherent
pattern emerges. The concepts I would like to discuss are:
•Master hearing aids.
•Real-ear versus 2 cm3 hearing aid performance.
•The idea of selective amplification and prescriptive methods of hearing aid fitting.
•Carhart's method of hearing aid evaluation.
•The "Harvard Report", test-retest reliability of speech discrimination testing, and the 30-day trial.
•Real-ear measurement devices.
•Digital and programmable hearing aids.
Master Hearing Aids
The idea behind master hearing aids is a wonderful one, in theory. Unfortunately, in practice master hearing aids
have left a lot to be desired. To illustrate what a master hearing aid should do, imagine the following. You are a potential
candidate for a hearing aid, you are sitting in the dealer's office, and are listening to the output of a machine (the master
hearing aid). This machine is accurately simulating the different listening conditions you have difficulty with in every
day life. And you are dismally aware of how difficult it is for you to understand what others are saying, warning signals
may be inaudable, the quality of music is poor, and you find yourself tense with the strain of trying to respond
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appropriately to what is happening around you. Then, the dealer flips a switch, and the simulation changes. You are
now listening to one of the 1.6 million different hearing aid permutations and combinations that this dealer's company
has to offer. You notice that the speech sounds a little clearer, the music a bit richer, and the noise is not quite so
bothersome. The dealer flips another switch, and another, and you are amazed by how the quality of what you are
listening to improves. Finally, as the last switch is thrown, you realize that this is it. This is the one you want. Sound just
couldn't be any better than this. So, the selection and evaluation session is over. You and the dealer shake hands over the
deal. A week later you come back to the office and you are fitted with a hearing aid. As you leave the office you notice
that the sounds you are listening to on the street are just as clear and distinct as you remember them from the week
before. In fact you are amazed that everything sounds just like the dealer said it would. As you walk away you are
unaware that you a humming a little tune, and that bounce in your step.
Well, what's wrong with this picture? Nothing ideally, except for the week delay (more about this later), it is almost a
perfect illustration of what hearing aid selection/evaluation should be. But, practically, there are at least two major flaws.
The first problem is that typically the dealer would use the master hearing aid to talk to the client in quiet on a one-to-one
basis, this being the easiest possible listening situation. The listening environment would not be an accurate simulation
of the difficulties associated with real every day listening. The variety, complexity, and richness of human
communication behavior might almost seem to preclude this. The second problem is with the hearing aid itself. The
simulation inevitably would not be an accurate representation of what the real hearing aid would do. Typically, the
circuitry in the master hearing aids would not be the same, most often it would be quite superior to that in the actual
hearing aid. So, the master hearing aid turns out to be little more that a sales tool. The sound produced by the hearing
aid purchased bore little resemblance to that promised.
Real-ear vs. 2 cm3 Hearing Aid Performance
As obvious as it may seem, let me say that hearing aids, as they are ultimately used, are worn on real individuals.
So, how the aid behaves in this situation would be the real-ear performance of the hearing aid. So, you might think that if
a hearing aid was being selected for another to wear, the hearing aid specifications would be referenced in real-ear terms.
This is, however, not usually the case. More commonly, specifications for a hearing aid are given in terms of how it
performs in the 2 cm3 coupler of a hearing aid test box. And the two measures (real-ear vs. 2 cm3) can at times be quite
different (see Hawkins and Haskell, 1988). The problem is that the dealer may be prescribing a hearing aid, and not know
exactly how it will function on the person.
The Test Box
How to measure hearing aid performance is a subject addressed by an ANSI standard (ANSI 3.22-1982).
According to the standard, the hearing aid's performance is supposed to be evaluated under free-field conditions: as if the
hearing aid were suspended in the atmosphere in empty space. No objects would be close enough to the hearing aid's
microphone to interfere with propagation of the sound wave. Nothing would be present to cause reflections, or echoes.
So, to simulate these conditions, the hearing aid is placed inside a (fairly small) test box. Ideally the box should simulate
an acechoic (no echoes) chamber. Unfortunately, hearing aids are not worn under these conditions. Hearing aids are
worn in the reverberant field, which is filled with echoes, and the microphone of the hearing aid is located close to a large
obstacle (the head). The size of the head interacts with the wavelength of the sound wave. Longer wavelengths are
relatively unaffected since they bend (diffract) around the head, but the shorter frequencies associated with higher
frequency sounds are reflected. This occurs in such a way that the microphone of a hearing aid "sees" a different signal in
the test box, when compared to real-ear listening. And real-ear listening is different depending upon whether the
microphone of the hearing aid is located above the pinna (behind-the-ear aid), in the concha (in-the-ear aid), or in the
canal (canal aid). So, even at this elementary level the hearing aid specifications are going to be different from real-ear
performance.
The Signal
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The ANSI standard specifies that the signal input to the hearing aid (from the loudspeaker in the test box) should
be a pure tone. First one pure tone is delivered. A short time interval elapses while the output of the hearing aid "settles
down," then another pure tone, another wait, and so on until, minimally, the range from 200 to 5000 Hz is covered. The
purpose of this is to find out how the hearing aid "shapes" the spectrum; to demonstrate how much some frequencies are
amplified relative to others. So we can get some idea of how the hearing aid might help compensate for a hearing loss
which can be greater for some frequencies, less for others. A pure tone is a simple sound to define and control, so it is not
surprising that it emerges as the signal of choice in the standard. However, unless the person you are listening to is
whistling, one note at a time, slowly, this type of signal is not representative of what typically goes into a hearing aid.
A great variety of different sounds, warning signals, speech, noise, etc. are encountered in everyday situations.
Any complex sound can be completely described by how it varies in frequency, amplitude, and phase as a function of
time. Any real-life signal can be synthesized by combining enough individual pure tones at the correct level, phase and
time. Large powerful amplifiers, with wattage to spare, can respond to each pure tone in essentially the same way:
whether they are input individually, one at a time, or all at once. So, how these amplifiers shape the spectrum can be
determined using the above (ANSI) method. However, milliwatt hearing aid amplifiers, do not respond this way. They
respond differently to broad-band vs. discrete frequency signals, and they can be very sensitive to transients, signals that
vary rapidly in time. So, unfortunately, the ANSI method for determining frequency response (i.e., how the hearing
responds to different frequencies) may tell us little about how the hearing aid responds to the steady-state formant
structure of a vowel, or the formant transitions and transient noise bursts so critical to consonant intelligibility. The
problem is immense considering the tremendous variety of sound a hearing aid must respond to. And, remember, people
do whistle. Sometimes slowly, one note at a time.
The 2 cm3 coupler
After the signal leaves the loudspeaker in the test box, the hearing aid microphone transduces it to an electrical
signal. This is amplified, shaped, clipped, compressed (and whatever), and finally sent to a miniature loudspeaker, the
receiver of the hearing aid. If the aid is an ITE (in-the-ear) type, the acoustic signals exits the hearing aid via a small tube
connecting the receiver to the tip of the shell of the mold. For a BTE (behind-the-ear) aid, the arrangement is a little more
complicated. The receiver connects first to an earhook, and the earhook is connected to the earmold via a (typically one
inch) length of tubing. In either case the shell of the ITE or the earmold of the BTE is the endpoint, where the output of
the aid is coupled to the ear canal of the listener. So, in the real-ear the "load" that the hearing aid has to "drive" can be
represented by the combined load impedance of the ear canal/middle ear system. The sound pressure level that the
hearing aid can provide to the entrance of the auditory system is determined (in a complicated way) by the physical
properties of the ear. This includes the length and diameter of the canal, the compliance of the eardrum, volume of the
middle ear cavities, mass of the incus, and so on. The output that the hearing aid is capable of offering can even change
depending upon whether the user has, say, a normal middle ear vs. an otosclerotic ear as opposed to an ossicular
discontinuity. So, just as no two snow flakes are exactly the same, a hearing performs uniquely on each individual
wearing it.
The ANSI standard requires that the output of a hearing aid be measured in a 2 cm3 coupler. Although the
precise construction of the coupler varies for an ITE or BTE aid, the essence of the coupler is a cylindrical cavity
terminated by a measuring microphone. The cavity has a volume of about 2 cm3. This cavity would ideally simulate the
load impedance of a typical, or average (median), real ear. But, it doesn't. So, the SPL measured at the output of the
hearing aid in a 2 cm3 coupler is different from that in the ear canal (at the drum) of a real-ear.
So, to summarize, the situation confronting us is that the information used to put a hearing aid on Mr. John Doe
comes from a set of measurements determined by an ANSI standard. This standard specifies:
•an environment that is not similar to true hearing aid listening;
•an input level to the hearing aid different from that experienced while the aid is worn;
•signals input to the hearing aid that are not representative of what the individual will actually be hearing.
And, we don't really know how the hearing aid is performing in shaping the signal for the hearing-impaired user because
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•the response of the hearing aid to these real-life signals is not entirely predictable from the test signal;
•the output of the hearing aid is not related to average real-ear performance; and
•we do not know exactly how the hearing aid will behave on any given individual.
Selective Amplification and Prescriptive Methods
What should a hearing aid do for a person? Ideally, it should allow them to hear better: understand what people
are saying, be able to respond appropriately to environmental sounds (footsteps, warning buzzers, a baby crying, etc.),
enjoy the richness and quality of music, and so on. Typically, since communication is so important, the greatest focus has
been upon hearing and understanding speech.
The term "selective amplification" generally means that people with different audiograms require different
hearing aid specifications. "One size" does not fit all. Usually, the focus is upon the audiogram thresholds and
uncomfortable loudness levels (UCL, or LDL for loudness discomfort level) for the listener; frequency response and
SSPL90 curve of the hearing aid. Whoever is "prescribing" (more about this term later) the hearing aid tries to choose a
frequency response that is based upon the audiogram thresholds, and an SSPL90 curve that is based upon UCL. This is,
of course, a gross simplification that ignores the tremendous variety of prescriptive methods that have evolved over the
past 40 some years (Pascoe, 1985). On the other hand, this is what most procedures do, basically.
Frequency Response and SSPL90 Curves
The frequency response curve is selected so that the hearing aid will amplify each portion of the speech spectrum
to a desirable level. This level should be within the user's residual dynamic range: at least above threshold, yet below the
UCL. Ideally, this level should be at the user's most comfortable level (the MCL). So, the goal is for the hearing aid to
shape the entire speech spectrum (from, say, 200 Hz to 5000 Hz) so that most of it will be at MCL. Intelligibility will be
maximized by making as much (as possible) of the speech spectrum audible. The frequency response curve should be
chosen to allow the hearing aid wearer to hear and understand speech.
The SSPL90 curve is another one of the hearing aid specifications. This curve (dB output of the hearing aid
plotted as a function of frequency) is obtained using tones input to the hearing aid at a very high level, 90 dB SPL. In
addition, the gain control of the aid is set to "full-on." This combination is sufficiently high as to drive the aid to
"saturation." This means that the output SPL is maximized. For example, if at 1000 Hz the SSPL90 is 112 dB SPL, then the
aid will never give an output higher than this. Ideally, the person's UCL at 1000 Hz should be somewhat greater than 112
dB SPL, otherwise sound entering the aid could be amplified to uncomfortable levels. To avoid causing the user
discomfort, the SSPL90 of the hearing aid should be chosen to avoid exceeding the UCL.
Prescriptive Methods
How do we obtain the desired result of amplifying the speech spectrum to MCL? Prescriptive methods. Over the
years a number of different schemes have evolved for choosing an ideal hearing aid frequency response (see Skinner,
1988) for a good review of the better known methods). By far the most common (and popular) methods predict gain
based upon audiogram thresholds. The audiogram threshold at a particular frequency, Tf, is multiplied by a coefficient,
Cf, to estimate the required gain at that frequency Gf:
Gf = Tf * Cf
Originally, a coefficient of 1.0 was chosen so as to "mirror" the audiogram; i.e., gain was chosen to equal the hearing loss.
Although this might be a reasonable assumption for pure conductive losses, cochlear hearing loss (with recruitment)
would be greatly over amplified: normal average conversational level (ACL) speech would be amplified to levels
exceeding UCL. So, a half-gain rule, and more recently, a third-gain rule was used instead. The half-gain rule specified a
coefficient of 0.5 so that gain equalled half the hearing loss. A coefficient of about 0.3 would be consistent with a thirdgain rule. Certain procedures would increase the value of the coefficient with frequency, emphasizing high frequency
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gain. Others would employ an additional correction factor; often to reduce the amount of low frequency gain, or add a
certain "cushion" of reserve gain. But, basically, the idea was pretty much the same: gain is based upon a percentage of
the hearing loss.
Except for more recent procedures, prescriptive methods often ignored important issues such as differences
between real-ear and 2 cm3 coupler gain, whether actual use gain or full-on gain was being specified. Also ignored
would be calibration differences between earphone listening vs. sound field listening. The audiogram thresholds used to
predict gain were obtained under earphones calibrated to output levels developed in artificial ears. The hearing aids
would be worn, however, in the sound field. And earphone thresholds are quite different from sound field thresholds.
Also, virtually none of the prescriptive methods recognized individual ear differences. So, the prescription was not very
accurate, and some researchers felt that the term "prescription" should not be used since it implied a degree of precision
not realized.
Hearing aids are worn, mainly, for making speech more clearly audible. The aid should take the signal and
amplify it to a suprathreshold level, close to MCL. Why base a prescription method upon threshold? Practically, even
though the threshold is imprecisely related to MCL, the amount of gain prescribed from threshold measures often comes
surprisingly close to equalling the gain needed to amplify ACL speech to MCL. But, not always. Ironically, sometimes
the prescriptive methods recommend an amount of gain insufficient to reach even threshold! So, portions of the speech
spectrum are totally inaudible.
So, the situation still exists now as it did in the mid 1970's when the proposed Federal Trade Commission
guidelines recommended a 30-day trial period for evaluating hearing aids. Then it was felt that there was no universally
approved method which could select a hearing aid and guarentee the user's success. No selection procedure had been
shown to demonstrate predictive validity.
Carhart's method of hearing aid evaluation
Carhart in 1946 published a method for evaluating and comparing hearing aids. The method has proven to be
extremely popular over the last 40 years or so, mainly with audiologists. It could be argued that his ideas have
completely dominated the topic area of hearing aid evaluation. However, the manner in which some of his main ideas
have been implemented is seriously flawed.
Carhart's procedure is not a prescriptive technique. It is
mainly a method for evaluating hearing aids that have been already selected. The idea is to find out which of a group of
aids is the very best. Remember that the selection process is still important. Instead of finding the "cream of the crop"
you could be identifying the "lesser of three evils." Without a good method for choosing which aids are to be evaluated,
Carhart's procedure is meaningless. Usually three (maybe four) hearing aids are chosen that are thought to be
appropriate for the user. Each of the aids (in addition to the unaided condition) is subjected to the same set of tests. The
particular aid that performs the best is the aid recommended for dispensing. The tests consist of listening tasks designed
to determine how well the aid:
•improves the ability to hear speech;
•improves the ability to understand speech;
•performs in noise;
•allows the listener to tolerate loud speech.
Over the years details of the Carhart procedure have been changed or modified slightly, but the basic underlying
philosophy has not changed much. And, indeed, it is not easy to argue with his goals. One would expect a hearing aid to
make speech more audible and intelligible, allow the listener to tolerate the masking effects of noise, and yet not to be
overamplified so as to be disturbed by intense speech.
By far the greatest emphasis has been placed upon how the aid affects the user's ability to understand speech.
The typical test protocol involves presenting a list of 50 monosyllabic words for the listener to repeat. How well the aid
improves intelligibility is then estimated by the percentage of words correctly identified. Sometimes, in the interest of
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saving time (also having mercy upon the patient), only 25 words (half word lists) would be used. In a busy practice it
may not be possible to spend more than an hour in a hearing aid evaluation. In any case the average person soon
becomes bored, or hoarse, with repeating word list after list for aid after aid. After awhile the clinician begins to question
the quality of the average adult's responses. Of course this procedure cannot be used effectively with non-verbal children,
and in only a limited sense with even the most cooperative young verbal children.
A few problems with Carhart's procedure
So, there were problems evaluating children and other non-cooperative patients. There were (and still are)
problems in terms of time and energy for both the clinician and the client. There were other problems as well. Carhart's
procedure developed in an era of body, eyeglass, and predominantly behind-the-ear aids. The procedure does not fit so
well into the current age of the popularity of custom ITE aids: approximately 4 out of every 5 aids sold. A basic premise
of Carhart's method was that the aids could be evaluated prior to purchase. Although it might be necessary to resort to
using a stock clinic mold (since the user's custom mold probably did not yet exist), at least the mold could be coupled to
the BTE that would actually be purchased. Or, if not the same BTE, at least another instance of the very same model. It
could be argued that, with good enough quality control, the aid received would be virtually identical to the one
evaluated. Now, however, most all ITE aids come with the electronics built into a custom-made shell. The user has to
purchase the instrument to try it. Ever heard of the expression, "Buying a pig in a poke?"
So, if the user must buy the aid to actually try it, it becomes more important to make a good initial selection. Both
the dealer and the client have a lot of time, energy, and emotion invested what with the deal consummated, money
exchanged, the wait for the aid, etc. So, what is the dealer to do? Say the evaluation is performed and an aided speech
discrimination score of 74% is found. Should the client be told to return the aid to the manufacturer, and wait a week or
two in hopes of eventually getting 86%? What if the results of the evaluation come out rotten, but the client is thrilled
with the new aid. Will the dealer take the aid away from a happy customer? What if the test results come out great, but
the client is completely dissatisfied? Will the dealer "force" the client to keep the aid? You know the answers to these
questions, and they all point to the impracticality of Carhart's procedure within this context.
When Carhart's procedure first developed, audiologists were not involved in the direct retail sale of hearing aids.
Audiologists dispensing was a phenomenom beginnining mostly in the late '70s and early 1980's. Prior to this most
people needing a hearing aid probably went to the dealer for initial testing, fitting, and followup. The audiologist was
usually eliminated from the cycle. If the occasional patient went to an audiologist, the audiologist performed the hearing
evaluation. The audiologist would try a few aids on the person (i.e., do the Carhart evaluation) and make a
recommendation. The recommendation might be that the client purchase a particular make-model hearing aid, maybe
with specific tone control settings, earmold venting, etc. The ethical audiologist would give the prospective user a choice
of dealers who would sell the aid. The client would be encouraged to come back before the 30-day trial was over, for
counseling and for aural rehabilitation. Most of the time this is that last the audiologist would ever see that person.
Now, it is not productive to become embroiled in old issues involving the relative competencies of audiologists
vs. dealers. Was the hearing aid the audiologist recommended a good choice? Did the experienced dealer know of a
better fit, or was the motive to "move" a certain model aid purchased at discount? None of this is relevent. It is relevent
that often the aid recommended (by the audiologist) was not the aid purchased (from the dealer). The point is: should the
audiologist have the authority to make a recommendation when the dealer would be responsible for the consequences? If
the audiologist's choice turned out to be a poor one, it would be the dealer who would have to spend time counseling,
making adjustments, returning the aid to the manufacturer, refunds, and so on. While ASHA's code of ethics encouraged
the audiologist to be unsullied by crass materialism, it also encouraged authority without responsibility.
The Harvard Report, Reliability of Speech Discrimination Testing, and 30-day Trials
Each of these topic areas is interesting because they point out flaws in some cherished beliefs about how hearing
aids are selected and evaluated. Lets consider each in turn.
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The Harvard Report
The so-called Harvard report (Davis, et al., 1946) has often been cited when criticizing the concept of selective
amplification. Although it really has not proven to be a "death blow," it did serious damage to the way people thought
about recommending hearing aids. Should a dispenser spend a lot of time trying to carefully match the shape of the
hearing loss to a frequency response curve? Probably not, based upon one of the major conclusions that is usually drawn
from this study: one type of hearing aid frequency response will produce the best speech discrimination for a variety of
different audiometric configurations; that is, "one size fits all." For subjects with either flat, high frequency, or even low
frequency losses, a hearing aid with a mild high frequency emphasis would produce the best speech discrimination
scores.
So, is this conclusion justified? No, not really. The experimental and theoretical evidence is overwhelming in
demonstrating that good intelligibility is a direct consequence of the amount of the speech spectrum that is audible.
Especially the amount that is above threshold in the 1000 to 4000 Hz region. So, it is not surprising that an aid giving
amplification in this region (like the mild high frequency emphasis aid of the Harvard report) would produce good
speech discrimination. But, could not a careful adjustment of the sensation level of all portions of the speech signal do an
even better job? Probably yes, in most cases.
All hearing aid manufacturers offer a variety of hearing aids with many "different" frequency responses. Yet on
closer inspection most of these responses are just minor variations on a theme of mild high frequency emphasis. No
manufacturer offers a hearing aid that is capable of independently adjusting the band sensation level of speech
throughout the spectrum. So, the dispenser cannot finely tune (say, within a ‹-octave band) the aid's output to adjust for
irregularities in the audiometric contour. It seems as if the manufacturers commitment to selective amplification is there,
but only somewhat so. Perhaps the impact of the "one size fits all" reasoning is still firmly entrenched.
card fld "Selection Perspectives 2"
Test-retest reliability of speech discrimination testing
If the Harvard report has been the nemesis to more refined product development for truly selective amplification,
then studies such as those by Shore, Bilger and Hirsh (1960) and Thornton and Raffin (1978) have exposed the Achilles
heel of the Carhart procedure: speech discrimination testing. These studies have demonstrated that speech discrimination
testing, as currently performed, cannot reliably distinguish between different hearing aids. Especially if those hearing
aids are closely matched in performance. Why? Because the typical speech discrimination test does not use enough
words. If it did use enough words to be reliable, it would not be practical.
The ability of a hearing aid to improve the understanding of speech is fundamental to the Carhart procedure
(and others similar to it). The method used to assess intelligibility involves presenting monosyllabic, 50-item word lists,
to the listener for each hearing aid tested in addition to the unaided condition. Each hearing aid is selected with the
expectation that it will produce good speech discrimination. So, ideally, differences between hearing aids will be fairly
small. Can a speech discrimination test pick the best aid? Generally, no, because the differences between aids are usually
equal to (or less) than the test-retest reliability of the speech discrimination tests. So, hearing aid "A" could be the best one
the first time around, but if the test were repeated aid "B" would be the winner, or "C" if the test was performed a third
time! In order for speech discrimination to become a reliable tool, capable of identifying fairly small differences between
hearing aids, a large number of words would have to be given. But, the clinician's time and the patient's energy are
limited in supply. So, usually what happens is that half (25) word lists are given, making the situation even worse. As
currently practiced, speech discrimination testing is not totally useless, but it must be remembered that fine distinctions
can not being made, but grossly inappropriate aids can be identified.
The 30-day trial
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Perhaps the 30-day trial is the enlightened approach to a hearing aid evaluation. Is it possible to simulate in the
clinic the variety of listening experiences unique to the individual? No. Can we, in a practical sense, reliably measure
with speech audiometry the small differences that exist between aids in all these different communication environments?
Again, no. So, what is to be done? Give the person a month, or so, to wear to aid and form a subjective opinion of how
well they are being benefitted. That's the 30-day trial. It is not a new idea.
The hearing aid industry has long been accused of abusive sales practices, and misrepresentations of their
product. Sentiments seemed to have come to a head in the mid 1970's when the Food and Drug Administration (FDA)
and the Federal Trade Commission (FTC) held hearings and issued intents to regulate. The FDA regulations were
finalized and are now in effect, but the FTC rulings were not. The FDA's major effort to combat unfair and inaccurate
claims (i.e., "truth in labeling") was the user instructional brochure. This brochure was supposed to give the user
important, truthful, and realistic information about their hearing aid. The FTC was effectively lobbied and its rulings
never became final regulation. Basically, the FTC's answer to "abusive and unfair sales practices" was the mandatory 30day trial period. The FTC recognized at that time there was no hearing aid selection/evaluation procedure with proven
predictive validity. Future hearing aid success could not be predicted. Unfortunately, today, this is still true.
Real-ear Measurement Devices
Real-ear measurement devices may be viewed as an alternative to the Carhart procedure for measuring the
performance of hearing aids. These devices function very well in two areas:
•determining if the hearing aid is providing the amount of gain that it should; and
•giving the dispenser immediate feedback about any modifications that are made to the hearing aid.
Background information
Recall from our previous discussion how differences exist between how the hearing aid performs in the hearing
aid test box versus on the average real ear? Also, remember how each individual ear is different, so that a hearing aid
performs differently on each person? Prior to the advent of real-ear measurement devices, sound field testing was the
only method the clinician could use to determine real-ear gain. The client, placed in the sound field, would have
thresholds determined under two conditions: with the aid and without. The difference between aided and unaided
(warble) pure-tone thresholds would represent the amount of real-ear gain the aid actually provided. Using this method
for determining real-ear gain, also called "functional gain," has some problems associated with it. Most of them relate to
practical problems of time and patient cooperation. It takes valuable clinical time to determine thresholds, and patient's
are not willing to sit for hours on end having their hearing tested. So, with functional gain it is not practical to finely
resolve the real-ear gain curve (say, by testing 50 to 100 frequencies), or to repeat all these threshold measurements every
time a tone control is tweaked. Of course, functional gain becomes even less practical with infants and very young
children.
The alternative is real-ear measurement devices. The client, wearing the hearing aid, is placed in front of a
loudspeaker. The signal emitted by the speaker is picked up by a reference microphone which is placed close to the
microphone of the hearing aid. So, the reference microphone "sees" the input to the hearing aid. A probe microphone is
attached to a short piece of very thin, soft, flexible silicone tubing. This tube fits into the earcanal, between the earmold
(or shell of the ITE) and the canal wall. It can be threaded to a distance of only about 5 mm from the tympanic membrane.
The probe microphone samples the SPL output from the hearing aid at the eardrum, which is the entrance to the rest of
the auditory system. In-situ gain is the difference between the SPL going into the aid versus that coming out (SPLout SPLin). Correcting the in-situ gain for the effect of the earcanal resonance produces what is called "insertion" gain.
Insertion gain is a true measure of the real-ear performance of the hearing aid. Under normal circumstances insertion
gain is the same as functional gain.
So, what advantages does measuring insertion gain have over functional gain, especially if both give the same
results? The answers are mainly speed and cooperation. An insertion gain device can finely resolve the real-ear
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frequency response curve in a very short time: in less than a second the real-ear gain for 100 (or so) frequencies can be
measured. You, the dispenser, could make 5 to 10 minor (or major) changes to the aid (tone control rotation, venting, etc.)
and obtain immediate feedback as to what happened. This would be totally impractical with repeated threshold
measurements, as with functional gain. Also, other than sitting still, these measurements require no cooperation from the
patient. So, it is more feasible that insertion gain could be measured on the very young.
Using real-ear devices as an alternative to Carhart's method
So, we have already established that real-ear measurement devices can rapidly, and practically, obtain
information about how the hearing aid is performing on the individual. How does this fit into the entire
selection/evaluation procedure? First, a prescriptive method is used to determine the amount of gain required. This
would be the "target" gain. The real-ear measurement device would give a display allowing us to determine how close
the output of the actual aid matches the target. So, this would validate the initial selection. But, what about the
evaluation?
An objective (Carhart-like) evaluation typically tries to assess the impact the hearing aid has on speech
intelligibility. But, is it necessary to actually deliver words to the individual in order to get information about speech
intelligibility? No, not really. There already exists an ANSI standard designed to do exactly this (ANSI S3.5-1969). It is
based upon an older, and very accurate method, for assessing the intelligibility of noisy communication systems that
transmit speech. It was developed in the 1940's, and was referred to as the articulation index. Ideas fundamental to the
articulation index have been adapted to the hearing aid evaluation (see Dugal, Braida and Durlach, 1980 for a review of
the articulation index). So now, with different computer programs, it is possible to get a very good idea of the impact a
hearing aid might have upon speech intelligibility, without having to present the speech. Which is good considering the
problems with test-retest reliability of speech discrimination procedures.
In summary, real-ear measurement devices offer the opportunity to obtain information about how the hearing aid
is affecting the intelligibility of speech for the person who is wearing the hearing aid.
Digital and Programmable Hearing Aids
The traditional hearing aid is an analog device. The term "analog" implies a continuously varying quantity; such
as the smoothly varying electrical output from the microphone which is a direct analog representation of the input sound
pressure. However, it is possible to convert the analog waveform into an equivalent digital representation. An analog-todigital converter (ADC) samples the waveform at discrete time intervals and assigns a digital value (a number) to
represent the instantaneous value of the amplitude. The reverse can happen also. A digital-to-analog converter (DAC)
can translate the stream of numbers back into a fairly close facsimile of the original analog waveform. This is the basic
process involved in producing the popular compact disks, which reproduce music with such stunning quality. Given a
fast enough sampling rate (say 40 KHz) and enough resolution (16 bit resolution gives a 96 dB dynamic range), extreme
hi-fidelity is possible. However, this is not the major advantage to digital hearing aids.
Digital hearing aids
The real promise of digital hearing aids lies in what happens between the ADC and the DAC. While the
(originally analog) signal is in a digital state, it can be manipulated by a computer. And what is theoretically possible
with a computer? Anything! Any sort of signal shaping that can be performed with regular analog hearing aids, can be
simulated mathematically. Sophisticated algorithms, i.e., computer programs, could be developed to process the signal in
such a way that may result in dramatic benefits to the user. Given a sufficient amount of computing power, time, and a
fundamental human understanding of what needs to be done, anything is possible. And here lies the problem. A useable
(wearable) hearing aid has practical limitations in power consumption, and this effectively limits computing resources.
Also lacking is our own understanding of the speech recognition process, and exactly how it is influenced by hearing loss.
Currently, digital hearing aids are larger, perhaps less cosmetically appealing, and certainly more expensive than analog
hearing aids. Never-the-less, the true digital hearing aid holds extreme promise for the future.
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Programmable hearing aids
Of course the entirely digital hearing aid is programmable, but the term "programmable hearing aid" usually
refers to hybrid analog-digital devices. These hearing aids are basically regular hearing aids (analog devices) that have
digital circuitry that controls functions traditionally served by trim pots. These miniature controls, manually adjusted by
the dispenser, are used to fine-tune the fitting. The tone controls, output and compression controls, etc. all allow the
dispenser added flexibility at the time the hearing aid is finally fit. Considering individual listening preferences, ear
differences, and so on, it is not a bad idea to have this flexibility. Unfortunately, with size limitations, only a few controls
with limited flexibility can realistically be implemented. Digital controls have no such limitations, and have some other
surprising benefits.
Imagine the following situation. A manufacturer sells only one "circuit" for its ITE or BTE aids. However,
effectively over a million different combinations of frequency responses, output levels, compression ratios, and so on, can
be derived from this one circuit. All the different possible hearing aid specifications the manufacturer has to offer are
available from that one instrument. During the hearing aid evaluation procedure, one of these aids is being worn by the
client. The earmold (or shell, as apropriate) is custom fitted to their ear. A connecting cable interfaces the hearing aid to a
control panel operated by the dispenser. Using this control panel, the dispenser can adjust the digital controls of the
hearing aid to select whatever frequency response is called for by the selection protocol. The dispenser tries a number of
possible configurations and, finally, the patient confirms one of them as "best." The dispenser presses a button and those
exact settings of the digital controls are stored within the hearing aid. The cable is disconnected, and the client will leave
with the same "listening experience" they purchased. This is the appeal of the programmable hearing aid.
Perhaps later the client might decide that this first arrangement is not as appealing as originally thought. Or,
maybe the audiogram has changed and a "new" hearing aid is needed. Instead of suffering the cost and inconvenience of
purchasing a new aid, the old one can be changed simply by reprogramming. Instead of the dispenser realizing profit
from a new instrument sale, could they be besieged by repeat visit customers in search of the holy Grail of hearing aids?
Maybe mass production of these aids could improve reliability, reduce cost, and increase the number of people willing to
try a hearing aid.
There are many potential benefits, as well as possible drawbacks. But isn't it curious, what with all the changes
over the years, that the hearing aid selection and evaluation process has now come full circle? Maybe it is just that
"everything old is new again," or that we are "back to the future," but are we now seeing the rebirth of the master hearing
aid? Not as hype, nor as a sales tool, but as a legitimate alternative, and improvement, to the traditional method of
dispensing an aid.
Summary
So, is the digital hearing aid the "wave of the future"? Will subsequent developments allow the digital aid to
realize its full potential? Surely, technology will make it feasible and practical. With computer hardware, quantum leaps
have been made in the last 10 years. But is our understanding of human communication sufficient to take advantage of
this raw computing power? Will the digital aid be substantially superior to the hearing aids currently on the market?
Perhaps the programmable hearing aids will be a more viable, cost effective, compromise approach. Who can know
exactly? But, one thing is for certain. As always, we live in exciting times.
References
American National Standards Institute. Methods for the Calculation of the Articulation Index, ANSI S3.5-1969. New
York: ANSI.
American National Standards Institute/Acoustical Society of America. American National Standard Specification of
Hearing Aid Characteristics, ANSI S3.22-1982. New York: ANSI.
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Carhart R. Tests for selection of hearing aids. Laryngoscope 1946; 56:780-94.
Davis H, Hudgins CV, Marquis RJ, Nichols RH, Peterson GD, Ross DA, and Stevens SS. The selection of hearing aids.
Laryngoscope 1946; 56:85-115,135-63.
Dugal RL, Braida LD, and Durlach NI. Implications of previous research for the selection of frequency-gain
characteristics. In Studebaker GA and Hochberg I, Eds. Acoustical Factors Affecting Hearing Aid Performance. Baltimore:
University Park Press, 1980.
Hawkins DB and Haskell GB. A comparison of functional gain and 2 cm3 coupler gain. J Speech Hear Dis 1982; 47:71-76.
Pascoe DP. Hearing aid evaluation. In Katz J, Ed. Handbook of Clinical Audiology. Baltimore: Williams & Wilkins,
1985.
Shore I, Bilger RC, and Hirsh IJ. Hearing aid evaluation: reliability of repeated measures. J Speech Hear Dis 1960;25:152170.
Skinner MW. Hearing Aid Evaluation. Englewood Cliffs: Prentice-Hall, 1988.
Thornton AR and Raffin MJ. Speech-discrimination scores modeled as a binomial variable. J Speech Hear Res
1978;21:507-18.
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