Download Card number_____ 1

Hearing Aid Lectures Handout
de Jonge
Page 1 of 55
Card number_____ 1
card fld "card field id 1"
Some Issues in Hearing Aids
•Preparation (academic and practical) of hearing aid dispensers
◊What's typical in the industry? Apprenticeship.
◊Should audiologists apologize for only one course? Like Tom Coffman suggested (Phonak rep).
◊Much of (non-audiologist) coursework is included in our basic courses. An example is Ernie Zelnick's courses at
New York City College in Brooklyn.
•How do you sell a hearing aid?
◊Advertise, test hearing, sales pitch, down payment (50%?),
◊Ear impression, fill out order form, mail to manufacturer, deliver aid, do some AR, collect balance, [disappear, go out
of business — not,] pay account monthly, returns for credit
◊Doing the right thing is an economic imperative
•Hearing aid controls and functions, the "anatomy and physiology" of a hearing aid.
◊See stack "Phonak 805"
•Introduction to Hearing Aid Selection stack, choosing the prescription.
•History of hearing aids
◊See scans (next card), transistor, integrated circuits
•Hearing aids… what are they?
◊ITE, ITC, CIC, BTE, programmables, body, eyeglass, CROS versions
◊All have the same basic components:
mic, amp, receiver, battery, VCW, etc.
Major specifications:
◊Gain, frequency response, MPO or SSPL
◊ Input + Gain = Output
◊Feedback is a major problem; it limits gain
Generally, what's good?
◊High frequency emphasis for good consonant intelligibility
◊The AI (articulation index) predicts intelligibility
◊Lots of lows for good quality judgments
◊Good signal-to-noise ratio
◊Ideal Characteristics
•Measuring hearing aid performance
◊ANSI standards in 2-cc coupler
◊Functional gain…the lower the better?
◊Insertion (etymotic) gain…in situ, too
REUR, REOR, REAR, REIG (REAR - REUR), RESR
REUG, REOG, REAG, REIG (REAG - REUG), RESR
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•Hearing aid coupling… can significantly change frequency response of hearing aid… as much as electronics
◊User satisfaction influenced by comfort
◊Venting…voice, stuffy, high frequency emphasis, feedback
◊Diameter of sound bore…Mead Killion's designs increase diameter for better highs & vice-versa
◊ Dampers to smooth (no sweat) response
•Hearing aid selection… choosing the specifications
Main considerations:
◊Converting the audiogram into a frequency response curve
◊Converting the UCL contour into a SSPL90 curve
◊Mirroring the audiogram, earliest procedure
◊Victoreen, Wallenfels, Berger, Pascoe, Lybarger etc.
◊POGO, NAL…common methods of hearing aid selection
Harvard Report: selection procedures are irrelevant, one aid (frequency response) fits all
•Hearing aid evaluation (verification)… validating the selection
◊Carhart procedure for comparisons between aids
Shore, Bilger, Hirsh article… Schwartz article
◊What audiologists were doing
◊What dealers were doing
◊Master hearing aids
What you hear, is what you get… or is it?
Digital hearing aids…future master?
◊Real-ear measurements using probe-mic systems (probe measures)
◊No universally accepted procedure with proven predictive validity for selection or validation (30-day
trial)
•Binaural issues
◊Lots of theoretical evidence to support two better than one
√Elimination of head shadow effect
√Binaural squelch
√Binaural summation
√Localization
◊Better hearing in noise
◊Most prefer binaural…monetary motives, cosmetic, wind noise not withstanding
◊Profit motive…audiologist accusations
◊Difficulty in objectively measuring improvement… especially if testing environment inadequate
•CROS hearing aids…waning of initial enthusiasm
◊Power CROS, cris CROS, BICROS, FROS, IROS (open mold)
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•The legal environment
◊You can be sued…genetic hearing loss, batteries, binaural
◊inform people of their options
◊FDA & FTC guidelines
◊State Licensure
•Taking an ear impression
•Hearing aid candidacy
•Performing ANSI measurements
•Using "Hearing Aid Selection" stack
◊Selecting target REIG, RESR, 2-cc gain, 2-cc SSPL
•Ideal hearing aid characteristics, Leijon's model
•Real-ear probe mic measures
•Effects of ear canal/middle ear load impedance on the frequency response
•Visual analogies to hearing loss and hearing aid processing using Adobe Photoshop
(slides required)
•Spectrograms of hearing aid processed speech
(slides required)
card fld "Harvard"
Davis, et al., "The selection of hearing aids," Laryngoscope, 1946.
•This was a study investigating appropriate design parameters for hearing aids
•A variety of subjects with different types of losses and configurations were tested
•Most had best speech recognition with one type of response: mild high frequency emphasis
•Does this mean, however, that they couldn't benefit from a more careful adjustment of the response contour?
card fld "Performance"
Why measure hearing aid performance?
•to match a hearing loss to target response curves
•to determine if a hearing aid is working properly
◊when initially ordered from the manufacturer
◊following repairs
◊to rule-out mechanical failure as a cause of
customer's complaints
•to determine if the aid matches ANSI specifications. ANSI was originally intended for quality control, not patient
fitting.
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card fld "FTC"
Proposed FTC regulations
Title 16 Part 440.1: June, 1975
Summary
The problem is to deal with abuses:
•numerous reports of experiences users have with unusuable hearing aids
•purchased at great financial sacrifice
•abusive sales transactions/tactics
The remedy:
•primary remedy is buyer's right to cancel. After 30 day trial the buyer can get most of their money back
•Appointments are required for home visits. Unannounced home visits are prohibited.
•Sellers must disclose their status as sellers and avoid using deceptive representations
card fld "FDA"
Summary of FDA Regulations: Hearing Aid Devices
Title 21 Part 801.420 and 801.421: February, 1977
Highlights of the regulations…
801.420 Professional & patient labeling
•a hearing aid is any wearable device (auditory trainers, other assistive devices are not considered hearing aids)
•sale or purchase of hearing aid includes any lease, or rental. Virtually any transaction is construed as a sale
•a hearing aid is used if it has been worn for any length of time, excluding an evaluation. It must be labeled as used
•hearing aids must have make/model/manufacturer and year of manufacture displayed
•user instruction brochure must accompany hearing aid, and contain information about…
◊hearing aid controls & their function, accessories
◊use & maintenance, repair service
◊things to avoid (dropping, immersion in water)
◊side effects (wax, irritation), medical attention
◊aid will not restore normal hearing
◊aid should be used as part of an AR program
•warning statement if output exceeds 132 dB SPL
Hearing Aid Lectures Handout
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•criteria for medical referral
◊visible congenital, traumatic deformity
◊history active drainage (90 days)
◊sudden, rapidly progressive loss (90 days)
◊acute/chronic dizziness
◊unilateral loss of sudden, recent onset (90 days)
◊ABG ≥ 15 dB @ 500, 1000, 2000 Hz
◊significant cerumen accumulation, foreign body
◊pain or discomfort
•technical data (ANSI specs)
•children with hearing loss should be referred to an audiologist
802.421 Conditions for sale
•medical evaluation required. Written statement by physician within last 6 months
•informed adult (18 years) must sign a waiver
•before signing any statements, user must have opportunity to review brochure
•records must be kept for 3 years
card fld "Licensure"
Licensure
Highlights…
Chapter 345 RSMo, Speech pathologists & clinical audiologists
•basic requirement is the CCC
•physicians exempt
Chapter 346 RSMo, Fitters and dealers, hearing Aids
•physicians exempt
•21 years of age, good moral character, high school education (or equivalent, GED)
•obtain temporary permit, begin selling hearing aids. A sponsor is required, sponsor must…
◊hold a license
◊provide all training, theoretical & practical
◊supervise first 5 evaluations
◊review all receipts
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•after 1 year, take an examination (written & practical) to obtain license
•renewal of license requires…
◊10 CEUs
◊calibration statement
•a license can be revoked, or denied for reasons of misconduct
card fld "Advertise"
•Advertisement in newspaper, magazine (Reader's Digest) for
◊Free booklet
◊Free non-working hearing aid
◊Videotape about hearing loss
◊This generates sales leads, and often unannounced home visit with high pressure sales tactics
◊Most abuses occur in such home visits
•Basic newspaper ad: business name, address, license number
◊free electronic hearing test
•Direct mailing
◊e.g., targeting all >65 yrs within a zip code
◊e.g., your customer database, fittings > 4 or 5 yrs old
•Word of mouth… very important
◊customers spread the word when they are happy, and
◊tell even more people when they are not
•Open house with factory trained representative
◊the "trustworthy, mysterious expert from afar"
•Celebrity endorsement (eg., Art Linkletter, Loren Greene)
•Television ads (Miracle Ear). People respond, are put into a database which is sold to dealers
◊significantly adds to cost of aid (≈$200 or more)
card fld "HAS Test"
Self assessment for using the Hearing Aid Selection (HAS) stack to assist with judgments in hearing aid fitting.
Go through the following steps. At each step document very carefully what you have done, so that another person could
replicate your findings. You lose points if I cannot verify your results. You can document your work in whatever way is
most efficient: writing it down, printing an HAS card, printing from the Fonix 6500, etc.
Begin with HAS in its default state (normal hearing, speech at ACL, normal REUG, vents closed, etc.).
1. You need to begin with a hearing loss. Also, write a brief history for your patient. Choose a typical sensorineural
hearing loss and enter it into HAS.
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Questions:
•Are you testing with supra-aural headphones or insert earphones?
•What is the REDD? Change the REDD for a couple of frequencies and observe/document the effect on the audiogram.
When you are finished, reset the REDD to normal.
•What is the effect, in percent correct for words and sentences, of this hearing loss on the patient's ability to hear ACL
speech? Speech that is faint (55 dB SPL)?
•Does this patient need a hearing aid?
2. Determine real-ear targets for gain and output.
Questions:
•What procedure did you use for selecting a target gain curve? What effect does this have on improving speech
intelligibility? Your answer should be in percent correct. What is your target REIG?
•What method did you use to estimate the LDL? What values did you obtain for LDL? What is the RESR?
3. Convert the real-ear targets to 2-cc coupler targets. You need to select a type of hearing aid (i.e., BTE, ITE, ITC, etc.).
Questions:
•What is the target 2-cc coupler use-gain and FOG? Use 15 dB of reserve gain for a BTE, otherwise use 10 dB.
•Is the coupler use-gain greater or less than the REIG? What are the four major reasons why real-ear gain is different
from coupler gain?
•What happens to the target REIG as you change the type of hearing aid? What happens to the 2-cc coupler gain as you
change the type of hearing aid. Document what the effect is from changing from an ITE to a CIC. Why is there a
difference?
•What is the target OSPL90? Does this change with type of hearing aid? What is the change?
•Do you want the target OSPL90 to equal the OSPL90 predicted from the LDLs? Or, do you want the OSPL90 to be less?
If so, how much less, and why?
4. Measure the REUG; enter it into HAS to customize your targets. You can use the Fonix 6500 to measure an REUG,
either your own or a friend's.
Questions:
•How does the measured REUG differ from average?
•Does changing REUG change the target REIG? Does it change the target 2-cc coupler use gain or FOG? What is the
difference?
5. Measure the RECD; enter it into HAS to customize your targets again. You can use the Fonix 6500 to measure an
RECD, either your own or a friend's.
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Questions:
•How does the measured RECD differ from average?
•Does changing RECD change the target REIG? Does it change the target 2-cc coupler use-gain or FOG? What is the
difference?
•How does changing the RECD affect the relationship between RESR and OSPL90?
6. Select a hearing aid for the purpose of matching the target 2-cc use-gain and OSPL90. Adjust the VCW and trim pots
until you obtain a reasonable approximation to the targets.
Questions:
•How close were you able to come to the target 2-cc use-gain and OSPL90 curves?
•Compare the peak use-gain with the peak FOG to determine the reserve gain. What was the reserve gain?
7. Determine the REIG and RESR of the hearing aid. Keep the hearing aid VCW in the use-gain position you selected to
match the coupler target. You can measure the hearing aid in your own ear, or you can recruit a friend. Use a custom
earmold. If you don't have a custom earmold, try an alternative: a Comply earmold, Doc's Promold, or you can fabricate
one from a silicone ear impression (or use Insta-Mold).
Questions:
•How closely did your predicted REIG and RESR match you measured REIG and RESR?
•Were you able to modify the hearing aid/earmold to achieve a closer match? How closely were you able to come to
matching the target REIG and RESR?
8. Compression. Use HAS to determine target compression ratios and input-output functions.
Questions:
•Select the "Compression" menu item from the "Other…" menu to observe calculated compression ratios. Describe the
logic behind how these compression ratios were calculated.
•Select the "Input-Output Function" menu item from the "Other…" menu. Use the capabilities of this card to match the
target input-output function for a frequency. How was the target you are trying to match obtained? Assume that you are
trying to match the target in the test box. What settings did you choose for the compression amplifiers?
•Place the hearing aid in the test box and run an input-output function. Compare your measured input-output function
with the ideal input-output function. Its likely that there are differences. How will these differences affect the patient's
perception of loudness at different input levels?
9. Earmold venting. Use HAS to predict the effects of venting. Select different vent lengths and diameters.
Questions:
•How do the venting changes affect the REIG? The 2-cc coupler use-gain?
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•Choose one particular vent and describe how (compared to the closed mold) the vent affects the audibility of the speech
spectrum. What is the difference in intelligibility between the vented vs. unvented condition.
10. Special earmolds. Use HAS to predict the effects of earmold modification. Select different earmolds and observe the
effect of each on the REIG and RESR.
Questions:
•Choose an earmold that will increase the high frequency response for the listener. What is the effect upon the REIG and
RESR? The 2-cc use-gain?
•Identify an earmold that might help with reducing feedback. Hint: Feedback is related to high frequency gain.
Card number_____ 2
card fld "History"
Historic Devices for Hearing: The CID-Goldstein Collection, ML Koelkebeck, C Detjen, D Calvert, 1984.
The Hearing Aid: Its Operation and Development, Kenneth Berger, 1970.
The pre-electric era (acoustic) 1700s, 1800s
•Speaking tubes generally offered very little gain. The speaker probably used greater vocal effort. The S/N ratio was
probably improved.
Speaking Tubes
Floral Tube
Canteen Receptor
•Ear trumpets, depending upon their design often gave appreciable amounts of gain, up to 20 dB in the 1000 Hz
frequency region. The London dome resonator was impressive with about 30 dB of gain.
Ear Trumpets
Ear Trumpets Mini
London Dome
KEMAR Acoustic Aids
KEMAR Smiles
KEMAR, Front
KEMAR, Inside
London Dome REIR
Flower Vase Receptor
Hearing Urn
Acoustic Chair
•Ear scoops were an early attempt at miniaturization. Ear inserts were like an "acoustic ITE" and were not effective,
despite the advertising claims. Misleading advertising has a long history, including the more recent claims by Miracle Ear
and Beltone that their circuits can eliminate background noise.
Aurolese Phones
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Ear Scoops
Ear Inserts - Artificial Drum
Fensky Advertising
Artificial Eardrum Ad
•In response to his own conductive loss, and noticing that he could hear his watch ticking when placed between his teeth,
Rhodes developed a bone conduction device.
Rhode's Audiphone
Rhode's Audiphone Response
Acoustic Fan
Electric hearing aids, late 1800s thru 1920s, 1930s
•Alexander Graham Bell invented the telephone in 1876 which is associated with attempts to aid his mother and wife
(sic), Mabel Hubbard.
•In 1944 it was estimated that 50,000 carbon mic aids were still in service.
•The carbon microphones were an improvement over the acoustic devices, providing more gain. Frequency responses
were "peaky."
Carbon Mic Aid 1
Carbon Mic Diagram
Carbon Aid Response
Carbon Mic Aid 2
Detecta-Phone
Selex-A-Phone
Aid 1900s
Aids 1920s
Aids 1930s
The electronic era (vacuum tube, 1930s, 1940s; transistor and ICs 1950s+)
•Vacuum tube was invented by Lee DeForest in 1906. The vacuum tube amplifier produced good quality sound with
increased gain. The device required a great amount of current, and generated a lot of heat.
•Vacuum tubes operated on the principle of a small current (from the microphone) controlling a large working current.
Transistors operate on a similar principle.
•The original Radioear aid was manufactured in 1924, sold for $300 to $600, and it weighed 220 lbs.
Radioear Vacuum Tube
Vactuphone
Vactuphone Diagram
Triode
Triode Details
Aids 1940s
•The transistor was developed in 1947 at UI by John Bardeen and coworkers. The first transistor hearing aid was
manufactured in 1953. The integrated circuit was developed in 1960 and hearing aids using them emerged in 1964.
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•The transistor produced high fidelity amplification in a very small package, required very little current to operate, and
generated little heat. ICs are simply a large number of transistors (resistors, capacitors) placed on a single silicon wafer
via a photographic process.
Transistor Size
Vacuum Tube - Transistor
Integrated Circuit
Aids 1950s
Aids 1960s
More recent hearing aids, test box
•The following is a dissection of a Siemens low profile ITE hearing aid
ITC Opened 3
ITC Opened 4
ITC Opened 1
ITC Opened 2
ITC Opened 5
•The "RejuvenAIDer" from MedRx
◊ Drying Chamber - Expedites evaporation of internal moisture vapor
◊ Vacuum Wand - Suctions debris and ear wax from within receiver and microphone tubes
◊ Circuit Drain Test Meter - Check integrity of circuitry and predict battery life
◊ Battery Tester - Verify condition of battery
◊ Volt Meter - Used in troubleshooting
RejuvenAIDer.jpg
•The OMNI hearing aid fits my left ear. The hearing aid stethoscope is attached to the same aid and show the SAVs for
adjusting vent size and the wire loop for removing wax from the receiver tube.
OMNI ITE 1
OMNI ITE 2
Stethoscope ITE SAVs Loop
AdHear Wax Guard.JPG
•Typical form factors for hearing aids, scans of advertisements for OMNI, Starkey CICs, ITCs, ITEs
Magnatone BTE to CIC
Starkey ITE, ITCs
Starkey ITEs In-Situ
OMNI Tymp 2000 CIC
OMNI CIC In-Situ
CICs and OMNI ITE
CIC Power Flextip
CICs Flextip Extended Rec Tube
Shells ITE No Faceplate
Canal Aids
Starkey Programmable
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•Phonak BTEs, a Widex aid with a transparent case, and 3M, ReSound, and Phonak programmable hearing aids.
The programmables vary according to:
◊whether the signal path is analog or digital
◊whether a remote control is required,
◊how many bands split the spectrum,
◊how many channels split the spectrum,
◊how many memories are available, and
◊other details/special features of the hardware and fitting software:
- type of compression (linear, curvilinear, fast acting syllabic, slow),
- use of directional microphones,
- automatic changes in directionality pattern to direct null toward the source (like Phonak's dAZ which changes from
cardoid to supercardiod to hypercardoid),
- automatically changing the memory to fit the listening environment,
- voice activity detection algorithms that reduce gain when no speech is detected,
- feedback suppression (by generating 180° signal to cancel the phase, or reducing gain in a particular band),
- expert software for troubleshooting user complaints and changing fitting parameters,
- CDs with environmental sounds to simulate realistic listening environments,
- subjective self-report questionnaires (like COSI and APHAB),
- patient training (AR) software like Oticon's Clear Speech and HearLink, etc.
Phonak Pico CSHT
Boot Jon P With Cable ATU
Boot Phonak Audio Input
Phonak BTEs Color
Widex 6x
3M MultiPro BTE
3M Programming Cable
ReSound NOAH
ReSound P3
LGOB Loudness Sones
ReSound Remote ITCs
ReSound Sculpture
Phonak PICS BTE Remote
Phonak Remote Case Off
Phonak Remote Case On
Piconet 2P2 AZ Remote
Claro Aids Remote.JPG
Directional Mic Patterns.jpg
Claro dAZ.JPG
Performing ANSI measurements
•Details of the Fonix test box, 2-cc couplers, the reference and probe mics used for real-ear measures. To run the ANSI
test (make 2-cc coupler measures called for by the ANSI standard)…
◊Turn the monitor and Fonix 6500 on
Fonix 6500
Probe Microphone
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◊Place the microphone in the center of the test box
Test Box Inside
◊Press "Level" button to level the system (equal output for all frequencies). Leveling is more of an issue for absolute
measurements (as opposed to relative measures, like gain) and for WDRC hearing aids (where gain changes with input
level). The software stores the old leveling data. Check to make sure leveling is accurate before proceeding.
◊Connect the battery pill, select correct battery type. You can also use a regular battery.
HA-1 ITE In-Situ
Battery Pills 675 13
◊Adjust trim pots to give greatest gain, output, compression, and widest frequency response (or adjust aid to how the
patient is going to wear it — depending on your reason for making the measurements)
◊Rotate VCW to FOG
◊Attach the aid to the HA-1 or HA-2 coupler
◊Remember to plug vent from inside on ITEs
HA1-Coupler OMNI
HA-1 Coupler Inside
HA-2 Coupler
HA-2 Coupler Apart
HA-2 Coupler Phonak
◊Via Fonix 6500 menu, select the ANSI standard (S3.22-1987, S3.22-1996, S3.42-1992) type of aid (linear, compression,
etc.)
◊Press the ANSI button
◊The test will run automatically, then stop with all information displayed on one screen (SSPL90, FOG curve, harmonic
distortion, etc.)
ANSI S3.22-1987: movie
go card ANSI Example
◊Before the test is completed, it may halt and ask you to adjust the VCW to the reference test gain control position. Just
reduce the gain until it matches the target number on the screen.
•Three views of a good ear impression
Ear Impression 1
Ear Impression 2
Ear Impression 3
•The OMNI order
OMNI Matrix
OMNI Order Form
OMNI Data Sheet
OMNI Order 1
OMNI Order 2
OMNI User Instruct Brochure
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card fld "Silicon"
•Transistors are semiconductor devices, usually made from silicon.
•Silicon has an atomic number of 14, and it has 4 electrons in its outer orbit. It is electrically neutral (14 protons, 14
electrons) but would be more stable with 8 electrons in its outer shell. To satisfy this need silicon forms a lattice structure
where each molecule shares an electron from 4 neighbors.
•"N" type material is silicon doped with phosphorus which has 5 electrons in its outer shell. The phosphorus molecule
fits into the lattice like a silicon molecule, but it has an extra electron. This permits N to conduct electricity.
•"P" type material is silicon doped with boron. Boron fits in like phosphorus but it has only 3 electrons in its outer shell.
P-type material can be thought of as containing "holes" which would like to be filled with electrons.
•Current will flow in an NPN transistor for a brief time until the holes are filled with electrons. The accumulation of
negative charge repels further electron flow and current flow ceases.
•If electrons are pumped out of the base P region (i.e., by a small positive voltage from the microphone), then current will
flow from one N region to another (the collector to emitter).
•For every electron taken out of the base, about 50 electrons can flow from the collector to emitter.
•A PNP transistor works similarly, but current flows in the opposite direction
Card number_____ 3
card fld "The 7 Habits…"
The seven habits of highly effective people, Steven Covey.
1. Be proactive.
◊Plan ahead; attention to detail helps save time, ensures a good outcome.
2. Begin with the end in mind.
◊Have a vision of what you want to accomplish.
3. Put first things first.
◊Prioritize.
4. Think win-win
5. Seek first to understand, then to be understood.
6. Synergize.
7. Sharpen the saw.
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card fld "Info…"
From Mueller, Hawkins & Northern, "Probe-Microphone Measurements" (1992), Singular Publishing Group, Inc.
"Probe-microphone measurements should be made with a pre-determined purpose and a goal. Simply observing the
amount of amplification provided by a hearing aid can be enlightening, but adjustment of the hearing aid to some desired
performance is where the benefits of probe measurements are realized."
Card number_____ 4
Card number_____ 5
Card number_____ 6
Both Gain and SSPL90 vary as a function of frequency
•Frequency response
A plot of Gain vs. frequency
•SSPL90 curve
A plot of SSPL90 vs. frequency
Illustrate how the I/O function changes…
•For linear circuits when gain and SSPL90 changes,
•For input-level-dependent and output-level-dependent compression amplifiers as knee point and compression ratio
change.
•Most typical compression circuits act as output limiters. With input compression, the gain control also controls the
saturation level.
Card number_____ 7
Card number_____ 8
Card number_____ 9
Card number_____ 10
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Card number_____ 11
Card number_____ 12
Card number_____ 13
card fld "Shore"
Shore, et al. JSHD, 1960
•15 adults with conductive, mixed, SN hearing loss (5 in each group)
•each tested with 4 body aids set to "good" & "bad" settings
•evaluation repeated on 4 separate occasions
•tests: Gain, WRS in quiet, noise
Conclusion
•tests could not reliably distinguish…
-between different aids, or
-settings of the aids
card fld "Schwartz"
Daniel Schwartz, in a study similar to Shore, et al.,
•Subjects were repeatedly evaluated with the same aid
•they most often liked #2!
card fld "Thornton"
Thornton & Raffin, JSHR, 1978, "Speech discrimination scores modeled as a binomial variable"
•in order to detect small differences in performance between hearing aids, you need a fairly large number of words
•with only 50 words, there must be about a 14% difference for significance
bkgnd fld "bkgnd field id 1"
Carhart Hearing Aid Evaluation Procedure (1946)
•Deshon army hospital in Butler Pennsylvania
•based upon the results of the Harvard Study
(one aid fits all)
•a tremendous influence on how hearing aids evaluated
•Areas evaluated
-improved hearing and understanding for speech
-tolerance for loud speech
-efficiency in noise
Procedural Steps (7 in all):
Unaided (steps 1-3) & Aided (steps 4-7)
Hearing Aid Lectures Handout
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1. SRT
2. UCL
3. Word recognition (@25 dB SL re: SRT)
Some aspects of the procedures, as they were popularly used, were eventually modified
Aided procedures:
4. SRT
-VCW to comfort setting, 40 dB HL connected discourse
-VCW @ max
•differences between aid ≤ 6 dB, insignificant
•15 dB HL residual loss is adequate
•importance of this criterion increases as SRT exceeds 15 dB
5. UCL
-same VCW settings
-connected discourse increased in 5 to 10 dB increments (until patient signals)
•80 dB HL considered ample
6. Efficiency in noise
-two types of noise: BBN & complex (fo = 120 Hz)
-VCW to comfort for 50 dB HL speech
-noise level increased until spondees masked
•S/N ratio an index of efficiency
7. Word recognition in quiet
-VCW to comfort for 40 dB HL speech
-PAL PB-50s presented at 25 dB SL (re: aided SRT)
Problems with the Carhart procedure:
•Shore, Bilger & Hirsch article
•summary by Schwartz (number 2)
•Thornton & Raffin
Conclusions:
•The procedure cannot reliably pick a winner
•Differences between aids are relatively small, on the order of test-retest reliability
-number of words vs magnitude of % difference needed for significance is excessive
-time constraints for testing (patience of patients)
-half word lists make things worse
Other problems:
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•The hearing aid evaluated was usually not the one fitted
◊There could be differences between different instances of the same make/model
•The aid was evaluated with a stock earmold with different acoustical characteristics than the patient's custom mold
would have
•The Carhart procedure is impractical with custom ITEs (80% of the current market)
◊The buyer is forced to purchase a "pig in a poke"
Master hearing aids: the solution, or a sales pitch?
•Ideally, a master hearing aid would be a device allowing a perfect simulation of the manufacturer's entire inventory
◊In reality, the master hearing aids produced much higher fidelity sound than the real hearing aid
•Digital and programmable hearing aids, the future master hearing aids?
◊Nicolet's Phoenix
◊Siemen's Triton
◊Ensoniq
◊Widex Quattro
◊3M Memory Mate
◊Maico PHOX
◊Argosy 3 channel clock
◊Resound, developed by AT&T
•The programmables use digital circuitry (which functions like trim pots)to control the analog functions of the hearing
aids, often allowing for thousands of unique combinations
◊usually the aids are quite expensive (Resound costs about $800-900 wholesale, $2000 retail, $3500 for binaural
◊They may require expensive programming units to "twist the digital screwdriver"
◊Some aids require remote controls to turn the aid on, adjust gain
•But the programmables offer advantages…
◊Users can listen to exactly what they buy
◊A new frequency response can be programmed (say, with fluctuating hearing loss, new preference)
◊Some programmables offer multiple memories: the user can functionally have more than one hearing aid… for different
situations (speech in quiet, high level noise, music)
◊Programmables offer the promise of more sophisticated algorithms to produce better sound quality, intelligibility
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Card number_____ 14
card fld "card field id 1"
Digital Sound
1. The analog signal is transduced to a voltage by the microphone
•analog means continuous, smoothly varying
•all sound in the atmosphere is analog
•the output of the microphone is analog
2. The analog signal is sampled by the ADC (analog-to-digital converter) at a particular rate
•usually the signal is sampled at a constant rate, say 20,000 Hz
•to accurately capture a frequency, you need at least 2 data points per cycle
•a 20 kHz rate would only allow for a bandwidth of 10 kHz (as shown by Nyquist)
•a sample would be taken at every .05 msec interval
3. At each interval, the instantaneous amplitude of the waveform is measured (by the ADC)
•the analog signal is digitized, the continuous signal is quantized to discrete values
•16 bit resolution, 0 to 65535, 96 dB
•8 bits, 0 to 255, 48 dB
•the analog signal is reduced to a stream of numbers
4. While in the digital state, the numbers can be manipulated mathematically
•DSP (digital signal processing)
•amplified, filtered, any form of spectral shaping or other kind of processing (e.g., time delays to simulate reverberation
and listening in a concert hall)
5. Via the DAC (digital-to-analog converter), the numbers can be translated back to an analog voltage waveform
•the reconstructed signal can be amplified,
•sent to a receiver (loudspeaker)
Card number_____ 15
Card number_____ 16
Card number_____ 17
Card number_____ 18
card fld "Info…"
ReSound Hearing Aid…
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•Gain at 50 dB SPL input, and at 80 dB input can be varied over a range of approximately 50 dB, or less depending upon
the band
◊This controls the compression ratio, which can vary over a range of 1:1 (linear) to 3:1.
•Gain can be set independently for two frequency bands, low and hi frequency
•The goal is to have frequency response vary with input level, to keep sounds audible, but not too loud
•The crossover frequency can be adjusted over a range of 400 to 3700 Hz in 1/2-octave steps
•Two programs can be entered
Other Specifications…
•filter skirt for hi band +18 dB/oct, low band -12 dB/oct
•Knee point is 45 dB SPL, AGC release time is 40 msec. Overall input AGC threshold is 85 dB SPL.
Gain at
500 Hz 50 dB
2 - 37 dB
2000 Hz
2 - 44 dB
80 dB input
2 - 21 dB
2 - 28 dB
Card number_____ 19
Card number_____ 20
card fld "Caption"
Figure 2. Locations of sound sources are typically expressed as azimuths in the horizontal plane.
Card number_____ 21
card fld "Caption"
Figure 3. Interaural intensity differences, measured in dB, as a function of azimuth. Differences in level are displayed for
sound sources placed symmetrically about the midline. Results are shown for different frequencies. For example, at ±90°
(a total angle of 180°) one source is placed directly in front of the right ear, the other in front of the left. At 200 Hz the ear
difference would be about 3 dB. Data are from Shaw, 1974.
Card number_____ 22
card fld "Caption"
Figure 4. Spondee thresholds as a function of azimuth. Positive azimuths represent monaural listening with the open ear
in the far ear condition. Negative azimuths are for the open ear when it is the near ear. Thresholds for both monaural
conditions are compared to binaural thresholds (when both ears are open). Near ear and far ear thresholds were poorer
than binaural thresholds. Compared to binaural performance, the head shadow was about 10 dB at +90°. Data was
taken from Dirks and Wilson, 1980.
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Card number_____ 23
card fld "Caption"
Figure 5. Poorer monaural spondee thresholds (as compared to binaural) when listening in noise. For condition B, where
the unoccluded ear was "completely shadowed," the 50% recognition threshold was 11.5 dB poorer than the binaural
condition. Data was taken from Dirks and Wilson, 1980.
Card number_____ 24
Card 24
Card number_____ 25
Card number_____ 26
Card number_____ 27
Card number_____ 28
Card number_____ 29
Card number_____ 30
Card number_____ 31
Card number_____ 32
card fld "Impression"
Taking the ear impression
◊ethylmethacrylate
◊silicone
card fld "Goals Info"
Goals
•comfortable, yet tight enough to avoid feedback
•avoid alterations that impede sound clarity (i.e., reverse horn)
•enable acoustic tuning
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•cosmetically appealing
card fld "Impression Info"
Ear Impression
1. Get all material, tools you will need out and ready
◊otoscope, earlite, scissors
◊block
◊impression material
◊spatula, mixing cup
◊syringe
◊order form, mailing box
2. As you go along, explain the procedure
◊to allay fears
◊gain trust, confidence
◊generate interest
3. Inspect ear with otoscope, look for contraindications to taking the impression:
◊impacted wax
◊foreign objects
◊active inflammatory disease
4. Insert block (using earlite) just past the second bend
•trim excess hair, if needed
•be careful with surgically modified ears!
5. Mix impression material, and place in syringe
Ethylmethacrylate
•1 teaspoon liquid to 1 tablespoon powder
• a 3:1 ratio
•work quickly with the material, while it is still soupy
Silicone
•mix equal parts thoroughly together
•material sets up more slowly
6. Place tip of syringe into ear (at block) and begin to shoot material.
•slowly withdraw syringe
•allow material to mushroom
7. Completely fill the concha and external ear
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•no air pockets
•no lines, creases
8. Have patient make normal jaw movements…
◊unless it is mandatory to have a very tight seal
9. After material sets up (10 minutes), remove
•gently peel away
•do not distort impression
10. Check to make certain all material, block is out of ear
•if necessary, take client to emergency room
11. Fill order form, package, and mail box
card fld "Material"
Choose material, then style
◊hard
◊soft
card fld "Material Info"
Earmold Material
Tints:
◊clear, pink, flesh, brown
Material:
◊lucite (hard)
◊PVC, silicone, polyethylene are soft
◊Some soft materials are softer than others
◊Some are more durable, others tear more easily
◊Some harden and discolor more easily than others
◊Ask earmold lab for their recommendations
Factors to consider…
•texture of ear
◊hard pinna, soft works better
•pinna mobility
◊large range of pinna motion, soft works better
•feedback
◊soft is better, better seal, better damping
•heat
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◊lucite is a better conductor
•lucite is generally preferred, especially for mild, moderate losses
•hybrid molds are available, lucite with soft canal
card fld "Style"
Style
◊shell
◊half shell
◊skeleton
◊canal
◊canal lock (partial concha
rim)
card fld "Style Info"
Selecting style
Consider…
•client's dexterity
◊bigger the better (shell)
•cosmetic appeal
◊smaller the better (canal)
•pinna mobility
◊canal/canal lock
•retention
◊more mold the better (shell)
•degree of hearing loss
◊more mold the better (shell)
Earmold types…
•skeleton
◊most popular, easily modified
◊light, inconspicuous
◊good for mild, moderate losses
•canal/canal lock
◊least conspicuous
◊good for large pinna mobility
•half shell
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◊better retention than canal
◊better with dexterity problems
•shell
◊best retention
◊fewest feedback problems
◊good for severe losses
card fld "Goals"
General Goals for
◊an earmold
◊ITE shell
Card number_____ 33
card fld "Who needs"
Who needs an aid?
◊Psychological factors
◊Amount of hearing loss
◊Word recognition ability
card fld "Which ear"
Which ear(s) to fit?
◊Monaural vs. binaural
◊CROS, BICROS
card fld "What type"
What type of aid?
◊ITC
◊ITE
◊BTE
◊Body & Eyeglass
card fld "Options"
What options?
◊trim pots
◊telecoil
◊direct audio input
◊use with assistive device
card fld "Earmold"
What type of earmold?
◊vented
◊horn
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◊other
card fld "Who needs info"
Conventional contraindications…
•Low cognitive skills
•Debilitating illness, incipient death, coma, etc.
•No motivation
•Negligible benefit to communication, no need to communicate, not a priority
•Relatively mild hearing loss, but there is a rationale for fitting normal hearing
•Profound hearing loss, but the aid could serve as a tactile device
•Resistance to amplification diminishes as the ability to follow conversational speech (sentence material)
◊falls below 70%
◊for ACL speech this occurs with an AI value ≈20%
◊the PTA approximates 45 dB HL
•Normal hearing for low frequencies, hearing loss begins at 2 KHz or above, but…
◊It is a challenge to increase bandwidth without adding noise to the low frequency portion of the spectrum
◊These individuals can experience as much benefit from amplification as other, more conventional losses
•Poor speech recognition, but…
◊speech recognition may improve with better frequency response (i.e., flat speech audiometer response probably not
optimal)
◊even poor hearing when combined with vision may show material improvement
•Do not dispense an aid if the only benefit is the financial gain of the dispenser
card fld "Which ear info"
•Binaural is the logical choice for those with symmetrical and asymmetrical losses
•Don't fit binaural in special cases of CANS pathology
•Fit binaural as if two monaural fittings
•Fit better ear monaurally if the poorer ear cannot contribute unaided
•Fit poorer ear monaurally if the better ear can contribute unaided
•Avoid fitting the draining, infected ear
•Fit the ear needed for special circumstances (i.e., taxi driver fit right ear, ear used or not used with telephone)
•Avoid CROS fitting unless there is some hearing loss in the good ear
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•Use BICROS when there is a dead ear, but the other is aidable
card fld "What type info"
BTE aids
•Are considered bigger, bulkier, less cosmetically appealing. Users may view then as "old fashioned."
•May be uncomfortable when used with eyeglasses, close fitting pinna
•Usually have a full complememt of trim pots (output & tone control)
•Usually have a telecoil
•Usually have a directional microphone
•Usually have compression circuits
•Are considered of better quality, more reliable, rugged since they are not custom made
•More appropriate for severe to profound losses
•More flexibility with earmold modifications
•Can use larger batteries
•Typical choice for fitting children with growing ears than might require new mold
ITE
•Smaller size considered more cosmetically appealing, most popular aid
•Perhaps easier to insert than BTE (one molded piece)
•Appropriate for all but most severe losses
•Custom, human intervention in the manufacture believed to reduce quality compared to BTE
•Extra trim pots add to cost, open shell, and may reduce reliability
•Larger case (than ITC) usually accomodates all electronics, including advanced circuit options like K-AMP
•Can usually achieve fairly large diameter vent (3 or 4 mm), with SAV
•Usually do not have telecoil or directional microphone, but these options are often feasible
•Usually not considered a good choice for younger children
ITC
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•Considered most cosmetically appealing (least visible), and may be demanded by user
•Considered least reliable of all types, most prone to failure
•Size is at a premium, the individual's canal may be too small to fit all components
•Extended canal portion may provide uncomfortable fit
•Extended canal reduces residual canal volume, increasing output level, especially for the higher frequencies
•Large vents are not feasible, often causing problems with the occlusion effect, fittings where hearing in the low
frequencies is normal
•Deeper location of mic takes partial advantage of canal resonance, giving greater output with less gain, less battery drain
•Close proximity of mic & receiver may cause feedback problems
•Batteries are smaller, shorter life, more expensive
•Not usually considered optimal choice for more severe losses
Body
•Used to be used with profound losses, but BTEs provide adequate gain
•Least cosmetically appealing
•Body location does not take advantage of head diffraction effects
•Seldom used
Eyeglass
•Were a popular choice, avoided paddles of glasses interacting with BTE for individuals with close fitting pinnae
•Compatible with CROS arrangements
•Few frame styles
•Not easily removable for brief periods
•Lose glasses if hearing aid malfunctions, vice-versa
•Complicates fitting
•No longer popular
card fld "Options info"
Trim pots
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•Add to the cost of the aid, may reduce reliability by breaching integrity of the shell
•Can be useful for fine tuning the fit; i.e., accommodating ears with a high level of individual difference
•An output control may reduce problems with LDL being exceeded
•A tone control can adjust the low frequency portion of the spectrum
Telecoil and Direct Audio Input
•Can be useful for coupling the aid to external devices (FM auditory trainers, TV devices, tape recorders)
•Important consideration for children functioning in the classroom
card fld "Earmold info"
•Venting earmold can alter frequency response
◊reduce lows
◊allow direct sound to enter canal
•Venting can improve the quality of the user's voice by reducing occlusion effect
•Horn, or belled molds can improve the impedance match between the earmold plumbing and the ear canal, resulting in
◊better high frequency response
◊Libby horn, 3 mm horn
•Killion's special molds can be used to fit a variety of losses…
◊high frequency
◊reverse slope
•Dampers can be used to smooth frequency response
◊act as an output limiter
◊reduce feedback
•Dampers have an unfortunate tendency to clog with moisture
Card number_____ 34
card fld "card field id 1"
Major Functions Provided:
•Simulated anechoic chamber
•2-cc coupler SPL measurements
•A calibrated (level) input SPL across frequency
√input SPL control method
(not implemented)
√equivalent substitution method
√substitution method
•Spectral analysis
•All ANSI specifications
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card fld "card field id 2"
A Typical Test Box
card fld "Leveling Methods"
Controlling input SPL across frequency (referred to as leveling).
•Input SPL control method uses a reference microphone to continuously monitor (and adjust) SPL at the hearing aid mic.
The equivalent substitution method produces similar results.
•With the substitution method, the input SPL is measured (and stored, for later adjustment), with the hearing aid absent,
at the position the hearing aid mic will later occupy.
•The equivalent substitution method uses a dummy control mic, physically similar to the real mic. The input SPL is
measured and stored at the position the dummy mic will occupy during the actual hearing aid measurements. This
method is more accurate than the substitution method for the higher frequencies, where diffraction and reflection from
the aid affect the input SPL at the hearing aid microphone.
card fld "Why…"
Reasons for doing ANSI S3.22 (coupler in the test box) measurements:
•To find out whether a particular hearing aid meets manufacturer's specifications published for that model (this is the
focus of the ANSI standard);
•To determine if the hearing aid is working properly, to troubleshoot user's complaints;
•To verify that the hearing aid is appropriate for the user's hearing loss (the correct prescription);
•To adjust the hearing aid to a prescription, fitting the hearing aid in the test box to expedite the real-ear fitting and
verification process (especially for children and the difficult to test).
Card number_____ 35
Card number_____ 36
Card number_____ 37
Card number_____ 38
Card number_____ 39
card fld "card field id 1"
3.3 Acoustic Gain
•referenced to an output SPL (re: 20 µPa) in a 2 cc coupler (HA-1 or HA-2)
•not real ear!
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3.4 Gain Control
•reserve gain ( about 10 dB?)
•taper usually not linear
•often affects SSPL, too
3.5 Saturation Sound Pressure Level for a 90 dB input (SSPL90)
•input 90 dB
•gain --> full on
•saturation may occur at other levels
3.6 High-Frequency Average SSPL (HFA-SSPL90)
•average for 1000, 1600, and 2500 Hz (HF-PTA, let's call it)
3.7 Full-On Gain (often called FOG)
•common manufacturer supplied specification
3.8 High-Frequency Average FOG
•average for 1000, 1600, and 2500 Hz
3.9 Reference Test Gain Control Position (RTGCP, let's call it)
•HF-PTA output is 17 dB less than the HFA-SSPL90
√65 dB + 12 = 60 dB + 17
√RTGCP is maximum VCW rotation that will not distort speech peaks
•input level is 60 dB
•if desired output level cannot be reached, then RTGCP is full-on gain
•AGC aids set to full-on (AGC aids are treated no differently from linear aids in S3.22-1996)
3.10 Reference Test Gain (RTG, let's call it)
•RTG = HFA-SSPL90 - 77 dB
•RTG may be FOG
•RTG may be less than HFA-SSPL90 - 77 dB, especially for mild gain aids
3.11 AGC Aid
•gain varies as a function of input level (excluding peak clipping)
3.12 AGC Knee Point
•where the input-output curve departs from linearity by 2 dB (either 3 or 4 dB in S3.22-1996) when the input level
changes from 55 to 80 to 55 dB (55 to 90 to 55 dB in S3.22-1996)
3.13 AGC Control
•any control affecting AGC function (except for tone-, gain-, or peak-clipping control)
3.14 Directional Hearing Aid
•microphone output (sensitivity) varies as a function of sound incidence (azimuth)
4.4 Sound source
•200 - 5000 Hz
•50 - 90 dB
•accuracy
ñ1.5 dB (200 - 2000 Hz)
ñ2.5 dB (2000 - 5000 Hz)
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4.7 Earphone Coupler
•HA-1 (ITE) or HA-2 (BTE, or button receiver)
4.10 Battery Current Drain
•uses a battery pill (battery simulator)
•measures current drawn by hearing aid
•estimates battery life
•hearing aids drawing too much current are defective
5.2.3 Acoustic Connection to Coupler
•vents should be plugged
•user's earmold should not be used (ANSI specs are not appropriate for measuring earmold characteristics)
5.2.4 Accessories
•any special earhook, filter, etc. should be described
5.2.5 Basic Setting of Controls
•tone control should be set to give widest response
•all other controls set to give greatest HFA-SSPL90 and HFA-FOG, but
•AGC aids should be set to give the greatest amount of compression
•Of course, you can adjust controls to other positions to make measurements that may be more relevant…just note what
you did.
6 Recommended Measurements, Specifications, and Tolerances
•measurements are highly reproducible
•Many conditions are not measured
√canal resonance
√middle ear impedance
√head diffraction & torso effects
√complex (frequency) inputs
√interaction between the above
6.1 Curves
•One decade equals 50 dB
6.2 SSPL90
•90 dB input saturates the aid
6.3 HFA-SSPL90
•±4 dB of manufacturer stated
6.4 Full-on Gain
•VCW to full on
•input level 60 dB, choose 50 dB instead if:
√output curve approaches SSPL90 curve
(within 4 dB)
√AGC aid
6.5 HFA-FOG
•±5 dB of manufacturer stated
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6.6 RTGCP
•use 60 dB input unless AGC
•AGC use 50 dB input
6.7 Reference Test Gain
•previously defined
6.8 Frequency Response Curve
•VCW in RTGCP
•input 50 or 60 dB
6.11 Harmonic Distortion
•high values may indicate a defective aid
•VCW in RTGCP
•input 70 dB @ 500 Hz & 800 Hz, or 65 dB @ 1600 Hz
•if the frequency response rises more than +12 dB, then ignore distortion measures
6.12 Equivalent Input Noise Level (Ln)
•high values may indicate a defective aid
•VCW in RTGCP
Ln = L2 - (Lav - 60)
•Lav is the HF-PTA output
•L2 is the coupler output due to noise (i.e., no input signal)
Notes:
•Results for AGC aids can be too high, especially if the knee-point is below the input level
√gain is less with the pure-tone input
√gain is greater with the noise (no input)
•for AGC aids set the input to 50 dB (hopefully below the knee point)
•ultra high frequency emphasis aids (low gain at 1000 Hz) may also show high Ln
√VCW set higher to achieve RTGCP
•wideband aid may show high Ln, noise level is a function of signal bandwidth
6.12 Battery Current
•high values may indicate a defective aid
•VCW in RTGCP
•current drain measured (using a battery pill) to a 1000 Hz tone 65 dB SPL
6.13 Coupler Sound Pressure Level with Induction Coil
•VCW set to full-on
•magnetic signal of 1000 Hz
•magnetic field strength of 10 mA/m
•measure coupler output
Notes:
•orient the aid to get greatest output
•also, you can run a response curve
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•S3.22-1996 includes a telephone magnetic field simulator – it looks like a telephone handset - and other measurements
designed to provide information about the aid's frequency response when using telephone
card fld "card field id 2"
Selections from ANSI S3.22-1982, 1987, 1996, S3.42-1992
card fld "S3.22-1987"
In 1987 the standard was modified to include…
3.15 Special Purpose Hearing Aid
…an instrument whose full-on gain at 1000, 1600, or 2500 Hz is less than the maximum full-on gain minus 15 dB.
…the manufacturer may choose to substitute three frequencies (1/3 octave, separated by 2/3 octave), the special purpose
average (SPA), for the standard frequencies used for determining the HFA.
card fld "S3.22-1996"
The ANSI 1996 update…
•Changes the AGC test. Settling level is 3 dB for attack time, 4 dB for release time (it was 2 dB). The level changes from 55
to 90 (not 80) to 55 dB SPL. The I/O test can be done for 250 - 4 kHz (not just 2 kHz).
•The reference test gain control position is the same for AGC and linear aids (previously VCW was set to full-on for AGC
aids).
•OSPL90 replaces SSPL90, since aids may not saturate at 90 dB.
•The induction coil test method has been updated to more accurately simulate the hearing aid's behavior with a
telephone.
•Several optional tests are included in annexes: gain contol characteristics, effect of tone control on frequency response,
etc.
card fld "S3.42-1992"
ANSI S3.42-1992. Testing hearing aids with a broad-band noise signal.
•This standard specifies methods for measuring the frequency response of a hearing aid using a broad-band noise signal
similar to the composite signal used in the Fonix 6500. The noise signal is shaped to simulate the speech spectrum with a
crest factor of 12 dB.
•For linear hearing aids or AGC aids operating at levels below the knee-point, the frequency response should be similar,
whether a pure-tone sweep or composite signal is used. For hearing aids where gain is dependent on the input level, there
can be differences. A frequency response obtained from a speech-shaped composite signal may be more representative
(valid) of how the hearing aid responds to real-world signals.
Card number_____ 40
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card fld "S3.22-1996"
The ANSI 1996 update…
•Changes the AGC test. Settling level is 3 dB for attack time, 4 dB for release time (it was 2 dB). The level changes from 55
to 90 (not 80) to 55 dB SPL.
Card number_____ 41
Card number_____ 42
Card number_____ 43
Card number_____ 44
Card number_____ 45
card fld "Design Objectives…"
Design Objectives…
For example, if an individual with a unilateral loss listens to sound, first through the normal ear, and then to hearing aid
processed speech in the poorer ear…
It would sound the same! Generally, the person would be unaware of the hearing aid. Wouldn't feel it, hear any noise or
distortion, etc.
•Speech is amplified to a level that is clearly audible, and within the user's dynamic range. The acoustic cues used to
recognize speech should be preserved.
◊Above threshold, but below LDL
◊Would above LDL be Ok, if it were above LDL for normals?
◊The low intensity components of speech should be audible (i.e., voiceless frication noise)
◊The more intense components (like vowel peaks) should not exceed LDL
◊For a broad bandwidth, all segments of the spectrum from 200 to 6000 Hz, over a 30 dB range should be within the
dynamic range
◊Contrast between peaks and valleys within the syllable should be preserved, as normal as possible, or even enhanced
-i.e., increase CV ratio
◊Suprasegmental features maintained (stress, intonation, juncture)
-Amplitude differences between syllables should be maintained, as normal as possible
-Temporal differences should be maintained
-Compression changes normal amplitude and temporal differences
-In multichannel aids, compression can reduce the contrast between different portions of the spectrum (i.e., the peaks of
F1 and F2 formants relative to the valley in between)
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◊Fish & Chips
◊Howdy Doody
•Noise should be minimized
◊Inherent circuit noise
◊Amplified environmental noise
√currently, technology does not permit meaningful noise cancellation (except for directional microphones,
multimicrophone technology), noise cancellation that improves speech intelligibility
√beamforming, with multimicrophone arrays, can be highly effective in eliminating noise
√Zeta noise blocker, Manhattan II circuit are older examples of attempts to cancel noise
√with the fully digital HAs, maybe algorithms will be developed that will be effective in reducing noise
◊S/N ratio should be high (i.e., +30 dB)
•Distortion should be minimized
◊Harmonic distortion, peak clipping
◊Intermodulation distortion
◊Headroom should be maximized
•Speech should be intelligible
◊As predicted by Articulation Index (AI)
◊Strongly related to audibility of spectral information from 1000 - 6000 Hz
√Intelligibility increases as 0 ≤ SL ≤ 30 dB
•Speech should sound natural, have a pleasant quality
◊Extended high frequency response (treble), balanced by…
◊good low frequency response (bass)
◊smooth response (no peaks), no ringing or transient distortion, intermodulation distortion
◊"crinkling" of the REIG, REAG, or 2-cc coupler gain curves is an indication of intermodulation distortion
•The person's own voice should sound natural
◊sounds created by chewing should be natural, especially crispy food
◊no denture clicking
◊jaw movements shouldn't unseat the earmold, causing feedback
•Balanced, binaural fit
◊4 to 6 dB less gain for binaural (as compared to monaural)
All of the above also applies to…
•Other sounds (music, doorbells, microwave beeps, bird whistles, chain saws, telephone ringers, etc.)
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•Different types of hearing losses
◊flat, falling, rising, notched sensorineural, etc.
◊conductive
All design objectives, ideally, would be achieved independent of the listening environment — the hearing aid would
recognize where it is and adjust appropriately; i.e., the patient, using a programmable, would not have to intervene to
change to a different memory.
◊Quiet listening, living room
◊Noisy environments, high input levels
◊Anechoic or reverberant sound fields
◊Telephone listening
◊Music (wide dynamic range signal)
card fld "Prescriptions…"
Prescriptions…
•The goal of the NAL procedure is to take ACL speech and…
◊Amplify it to a normal loudness level
◊A 60 phon loudness contour
•The loudness density normalization procedure (Leijon, 1991)…
◊Attempts to restore normal loudness to the entire speech spectrum for the hearing impaired listener…
The procedure…
1. Pick a critical band, CB, to begin with (i.e., choose one, then repeat the process for all the others). Begin with CB at
frequency, F.
2. Measure the SPL of the speech contained within that CB, Lspeech1, at the eardrum of the hearing-impaired listener.
3. Determine the loudness of Lspeech1 in sones for a normal hearing listener, Snormal.
4. Determine the amount of hearing loss for the hearing impaired listener at F (referenced to SPL at the drum), HTLdrum.
5. Calculate the widening of the impaired listener's CB, ∆CB, based upon the amount of hearing loss. ∆CB = 1 + HTL/25.
6. Use ∆CB to calculate the change in the loudness growth exponent, LG = 0.23 for normals (LG = 0.262 for a 50 dB HL
loss).
7. Use…
◊Lspeech1
◊HTLdrum
◊∆CB
◊LG
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to calculate Lspeech2 producing a loudness equal to Snormal.
8. Lspeech2 for the impaired ear is equal to the loudness of Lspeech1 for the normal ear.
9. Determine insertion gain, IG…
IG = Lspeech2 - Lspeech1
One very useful aspect of the LDN procedure is that it predicts frequency responses at levels other than ACL.
•Fig6 and DSL (Seewald's Desired Sensation Level procedure) were developed with similar goals, normalizing loudness.
card fld "Opposing Factors…"
Opposing factors…
•Reduced headroom is a bad thing…
◊distortion occurs at lower input levels
◊distortion products increase the loudness of the signal
◊LDLs occur at a lower SPL
but,
◊increased headroom increases hearing aid output levels, and
◊may cause undistorted output exceeding the user's LDL
•Exceeding the user's LDL is a bad thing, but…
◊Reducing the SSPL by adjusting a trim pot, reduces headroom and lowers the threshold at which distortion will occur,
and…
◊reduces quality judgements
•Compression limiting reduces harmonic and intermodulation distortion, but…
◊it produces its own form of temporal distortion, depending upon the time constants employed
•A broad band hearing aid is a good thing, but
◊as the bandwidth increases, noise levels (inherent & amplified room noise) increase
◊So, don't use a broad band response unless its necessary (i.e., don't use it with ski slope 3 KHz losses, or for telephone)
•Class A amplifiers can have sufficient headroom (and therefore low distortion levels) if the bias current is high enough
◊When bias current is high enough, battery current drain is unacceptably high
•Class B amplifiers can have good battery drain, distortion, and good headroom
◊but they are too large to fit into small canal instruments
•Class D amplifiers have good size, battery drain, headroom and distortion
•Harmonic distortion (created by clipping) is a "bad" thing…
◊but studies have shown that it does not severely affect intelligibility
•Clipping (say, of intense vowel components) generates high frequency distortion components that are supposed to mask
weak consonants, but…
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◊the consonant is actually part of the structure of the vowel
◊the less intense, high frequency portion of the consonant (i.e., aspiration of a /t/, frication noise of an /s, ø/ is
temporally separated from the vowel
card fld "Yesterday vs…"
Differences between yesterday's and today's hearing aids…
Considering programmable (analog) hearing aids, probably the major features are…
•Better flexibility in hitting the target, greater precision
•Multiple channels (low/hi, low/mid/hi). The ability to adjust a hearing aid's characteristics separately for different
portions of the spectrum.
◊Separate gain for each channel
◊Adjustment of compression…
-threshold(s),
-output level, and
-compression ratio
•Multiple memories. The ability to have different hearing aids (i.e., all the features of the above) available at the touch of
a button. For different listening environments…
◊Quiet (NAL) response,
◊Telephone (reduced high frequency response, beyond 3 KHz)
◊High level noise (reduced gain, flatter response)
From David Fabry, 1991 (Vanderbilt Hearing Aid Report II)…
Resolved issue…
•Noise reduction hearing aids that use a single microphone do not improve speech intelligibility, except under
constrained conditions.**
Unresolved Issues…
•With hearing aids that offer level dependent amplification, what should the target frequency responses (REIG) be at
each level?
•For hearing aids with multiple memories, how should the targets be calculated for the different listening environments
**The Manhattan II (BILL) circuit actually reduces the high frequency response somewhat in response to low frequency
noise
◊With a vented mold, the lows enter
◊The circuit would actually function more like a TILL
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card fld "Odds & Ends…"
•Symmetrical Peak Clipping
◊generates odd harmonics of input waveform
◊created by diode clipping
◊typical of class D amplifiers
◊called soft peak clipping
•Asymmetrical Peak Clipping
◊generates odd and even harmonics
◊created by placing resistors in series with transistor output stage
◊used with class A amplifiers
◊thought to produce a poorer sound quality (compared to symmetrical)
•Can use SoundEdit to illustrate changing CV ratios, peak clipping, high-pass filtering, graphic equalization
•Cranmer (1990)
◊20% of the respondents to a survey indicated that no form of testing was used after fitting a hearing aid to an ear
•Bratt and Sammath (1991) indicated that only 27% of VA audiologists always used probe-tube measurements. Only 25%
always performed an electroacoustic check. (I paid Dillard's $$ for a pair of slacks, after driving 62 miles to pick them up,
the right rear side had two creases with a crinkle inbetween. I drove back a week later, and the pants came back with the
same problem.)
•From Mead Killion, the old adage…
◊"Wear it awhile and you'll get used to it."
◊But please, give 'em something good to listen to
◊Auditory deprivation (Silman & Silverman)
-The unaided ear of bilaterally symmetrical hearing losses shows a loss of word recognition ability— which is
reversible.
◊Auditory acclimatization (Gatehouse)
-The unaided ear of bilaterally symmetrical hearing losses is better at listening to faint speech, the aided ear is better at
higher level speech
-Reorganization of the CANS takes place in response to changes in sensory input (i.e., changing frequency response).
After reorganization, word recognition ability improves.
-The improvement takes time: 4 to 6 weeks is typical, but may take 8 to 12
◊Implications:
-At the initial evaluation the patient might not be able to judge what is the best frequency response.
-At the initial evaluation speech recogniton testing may not be a valid indicator of performance 2 to 3 months hence.
Will MCLs and LDLs for tones change too?
-Give users what they need, not what they want? Counsel them to accept it?
•Nobody needs an output control…
◊During the course of a typical day, I am never driven to LDL
◊Hearing-impaired people have an LDL greater than or equal to me
◊Therefore, they do not need an output control
◊What they need is a circuit that provides less gain for high input levels (i.e., unity gain (0 dB) for more intense sound)
◊Everybody needs the K-Amp, or similar processing
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card fld "Caption"
Figure 5. The audiogram used to determine frequency responses in figures 7 through 12. This is the mean audiogram of
subjects used in a validation study of intelligibility and quality judgments of the LDN-C and LDN-P procedures (Leijon et
al., 1991).
card fld "Info…"
Loudness-Density Normalization (LDN)
To further explore issues relating to possible need for modification of the frequency response in binaural fittings,
a model based upon Leijon's loudness-density normalization procedure was developed. The model was developed
specifically to explore effects related to binaural summation, and the changes in input spectrum to an ear produced by the
head shadow effect. Use of Leijon's method was appealing for several reasons. First, hearing aid prescriptive procedures,
such as the revised NAL, are typically intended to select gain based upon a loudness criterion, to amplify all speech bands
to an equally comfortable loudness level. Second, recommendations for modifying the frequency response (i.e., 3 to 6 dB
reductions) are based upon listener's perceptions of binaural loudness. Third, Leijon's calculations are based upon an
accepted model of loudness summation. Fourth, Leijon's procedure has been validated upon a group of hearing-impaired
listeners.
Leijon's calculations produce a real-ear insertion gain (REIG) that purport to restore normal loudness
contributions for amplified speech. The listener is assumed to have cochlear pathology with recruitment. Recruitment is
either complete at 110 dB HL, or partial. The gain required to restore normal loudness for the speech peaks contained
within each critical band is determined by an algorithm expressed by Leijon as a short computer program. His model
was implemented here using a HyperCard stack developed by the author. The interested reader is directed to Leijon for
details of the calculations. The method was validated using a group of 26 elderly hearing aid users with the mean
audiogram shown in Figure 9-5. All users listened through their own hearing aids to speech presented in a noise
background via loudspeaker. The input spectrum was shaped to give an effective frequency response corresponding to
the revised NAL procedure, or Leijon's LDN procedure (other frequency responses were also simulated, but are not
presented here). Recordings from an FM radio transmission were used for subjective quality ratings, and sentence
material was used for intelligibility testing. Results indicated that intelligibility was similar for the two frequency
responses, both were acceptable. But, the quality judgments for the LDN response were slightly superior to the NAL
response (statistically significant at the 0.2% level).
Card number_____ 48
card fld "Caption"
Figure 9-6. A comparison of the gain recommended by the NAL procedure versus the loudness-density normalization
procedure for the audiogram in figure 5. LDN-C assumes recruitment is complete at 110 dB HL, whereas LDN-P assumes
partial recruitment. The input spectrum is average conversational level speech at an overall level of 65 dB SPL.
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card fld "Caption"
Figure 9-7. REIG varies depending upon the input level to the hearing aid, and upon the assumptions concerning how the
ear recruits, either complete (LDN-C) or partial (LDN-P) recruitment. Gain is greatest for low input signals (speech at an
overall level of 35 dB SPL) versus average conversation level speech (65 dB SPL).
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card fld "Caption"
Figure 9-8. Differences in REIG (LDN-C) based upon the SPL of speech input to the hearing aid. At low levels, more gain
with a pronounced high frequency emphasis is required to normalize loudness. At higher levels, less gain and a flatter
response is needed.
Card number_____ 51
card fld "Caption"
Figure 9-9. Differences in REIG required to normalize loudness for the near ear and far ear for a sound source located at
±60°. The speech spectrum (input levels of 35 and 65 dB SPL) has been modified to reflect the attenuations produced by
the head shadow effect shown in figure 2.
Card number_____ 52
card fld "Caption"
Figure 11. Differences in the amount of gain needed to normalize loudness when the input spectrum changes. The curve
labeled "treble emphasis" was created by subtracting 30 dB from the normal speech levels from 250 to 1000 Hz. The curve
labeled "bass emphasis" was created by subtracting 30 dB from the normal speech levels from 2000 to 4000 Hz. In both
cases 15 dB was subtracted from the normal level at 1500 Hz.
Card number_____ 53
card fld "Caption"
Figure 9-10. Differences in the REIG predicted by the LDN procedure for listening to a speaker located at a 0° azimuth (68
dB SPL input) versus a child monitoring his or her own speech at ear level (73 dB SPL). The speech spectrum was
modified to include spectral changes according to measurements made by Cornelisse et al. (1991).
Card number_____ 54
card fld "Caption"
Figure 9-11. Small differences in the REIG needed to normalize loudness for monaural vs. binaural fittings. The
assumption is that the binaural fitting increases loudness (relative to monaural) equivalent to a 3 to 9 dB change in input
level. The 3 dB change was for a softer speech input level of 35 dB, the 9 dB change was for an input of 80 dB SPL.
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card fld "Caption"
Figure 10. Differences in gain required to normalize loudness as hearing loss increases. The configuration of the
audiogram does not change, but a constant amount is added or subtracted from each threshold. For example, the
audiogram with a PTA of 83 dB was created by adding 40 dB to each threshold of the audiogram with the 43 dB PTA.
The pure-tone average (PTA) is based upon 500, 1000, and 2000 Hz. The speech is presented at 65 dB SPL.
Card number_____ 56
card fld "Why…"
You cannot learn how to do anything without being able to (see, hear, feel, taste, smell) the consequences of your actions.
We need feedback.
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Probe-mic systems give us objective data, allow us to visualize the consequences of the decisions we make.
Patient comments are important for customer satisfaction, but our inferences may be wrong — we may "jump to the
wrong conclusion."
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card fld "Info…"
Composite tone…
•80 frequencies, from 100 Hz to 8000 Hz spaced at 100 Hz intervals, fc= 900 Hz, -6 dB/octave. Only 200 - 8000 Hz are
displayed.
•Crest factor is 12 dB, which is similar to the long term speech spectrum (crest factor is the ratio of the peak to rms, 3 dB
for a pure tone)
•Represents a "more natural" signal for a hearing aid to process
•The spectral display gives the overall level in a 100 Hz bandwidth, calculated via a FFT
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card fld "Methods"
The method described is called the "modified pressure method of equalization." The reference mic is located near the
surface of the head. Alternatively, the "substitution method of equalization" places the reference mic at the subject
reference point (center of the head) and stores the values.
The "differential comparison method" (using the modified pressure method) allows the SPL at the reference mic to be
subtracted from SPL at the measurement point (probe mic).
ANSI S3.79-1997 Methods of Measurement of Real-Ear Performance Characteristics of Hearing Aids.
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card fld "Info…"
Customized CORFIGs…
(Coupler Response for Flat Insertion Gain)
1. Measure REIG = REAG - REUG.
2. Without changing the VCW position of the hearing aid, measure 2-cc coupler gain.
CORFIG = (2-cc gain) - REIG
•Using the custom CORFIG may result in a more precise fitting than using average values, providing…
◊Attention is paid to detail, and measurement error is less than prediction error
•For the CORFIG to be accurate, the hearing aid used in the measurement should be identical to the one that will be
ordered
◊Same mic location
◊Same earmold plumbing
◊Same vent
◊Same residual canal volume (tip to drum)
•GIFROC = -(CORFIG)
REIG = (2-cc gain) - CORFIG
REIG = (2-cc gain) + GIFROC
(2-cc gain) = REIG + CORFIG
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card fld "card field id 2"
A comparison between the RESR and the OSPL90. The difference between the RESR and the OSPL90 is the RECD (The
real-ear coupler difference).
Card number_____ 78
card fld "Info…"
•The RECD promises a more accurate output selection. You don't have to rely on a median value.
•The RECD can be used to convert RESR to 2-cc coupler OSPL90, and vice-versa.
•For children, and others who may not be able to tolerate/cooperate with the loudness associated with a RESR, the RECD
can be used to predict the RESR
RESR = (2-cc OSPL90) + RECD
Procedure (Assuming you have the aid and the custom earmold)
1. Measure an REAR response with aid not in saturation, typical use-gain VCW position.
2. Without changing VCW, measure aid output on 2-cc coupler.
3. REAR - (Coupler output) = RECD
Caveats…
•Unless you fit the same earmold used in the measurement, you cannot control for vent effects, since you cannot obtain a
vented 2-cc coupler response
•The residual canal volume is a critical variable
Procedure (Assuming you do not have the aid and the custom earmold/ITE shell). For the Fonix 6500:
1. Place the Fonix in probe mode. Unplug the loudspeaker and, using a phonoplug to miniplug adapter, substitute an
ER3-A insert earphone for the loudspeaker.
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2. Set the output to 75 dB SPL (caution, too high an output may fry the Fonix!). The Fonix should be in composite mode,
manual mode.
3a. Determining an HA-1 RECD for ITE aid:
Place the calibration adapter into the HA-1 coupler. Thread the probe tube into the adapter until it protrudes into the
coupler by 3 mm. Hold the ER3-A foam eartip onto the coupler. Press Start/Stop to run a response. Save this as you
would an "Unaided" response.
Note: You need three hands to do this, unless you make clever use of velcro.
3b. Determining an HA-2 RECD for BTE aid:
(Note: The main difference between the HA-1 and HA-2 coupler response is the 18 mm long, 3 mm ID earmold bore.
You can use the procedure for measuring an HA-1 RECD and apply the earmold bore correction using the Hearing Aid
Selection program.)
Place the calibration adapter into the HA-2 coupler. Thread the probe tube into the adapter until it protrudes into the
coupler by 3 mm. Remove the foam eartip from the ER3-A. Insert the ER3-A onto the #13 tubing of the HA-2 coupler.
Press Start/Stop to run a response. Save this as you would an "Unaided" response.
4. Place the probe tube in the ear canal (about 6 mm from eardrum). Insert the foam eartip into the ear. Insertion depth
should place the tip of the foam plug at the same distance from the eardrum as the shell/earmold will be. Run
Start/Stop. Save the response as "Aided" response. The REIG now displays the custom RECD.
Card number_____ 79
card fld "card field id 1"
The real-ear to coupler difference (RECD) for a 65 dB composite input. OMNI ITE RESPL minus HA-1, 2-cc coupler SPL.
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card fld "card field id 1"
The real-ear to coupler difference (RECD) for a 90 dB pure-tone inputs. OMNI ITE RESPL minus HA-1, 2-cc coupler SPL.
Card number_____ 81
card fld "card field id 1"
RECD obtained using an ER-3A earphone (foam tip) in Bob's left ear versus an HA-1 coupler (composite). The two curves
were obtained 6 months apart.
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card fld "Info…"
Obtaining LDLs…
•Ideally, the RESR should not exceed the LDL contour
A procedure…
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1. Using Etymotic ER-3A earphones (with the probe tube threaded through the foam insert, or place the probe tube
between the ear canal and the foam insert) obtain LDLs at selected pure tones.
2. Record real-ear SPLs at LDL. Later, when the hearing aid arrives, these will be target RESR values.
3. Convert dB HLs for LDL measurements to 2-cc coupler values. This is easy since the ER-3A is calibrated in a 2-cc
coupler. The 2-cc coupler values become target 2-cc SSPL90 values for the aid.
Alternatively, use standard headphones to measure LDLs. This can be done with the probe in the canal to measure realear SPLs. Or, convert dB HLs to RESPLs using standard or custom REDDs (real-ear dial difference). Convert real-ear SPLs
to 2-cc coupler SPLs using custom, or average RECDs.
Caveats…
•The residual canal volume with ER-3A inserts is not necessarily the same as with the custom ITE (or earmold), hence the
target 2-cc SSPL90 values may be in error.
•The ER-3A does not take into account effects of earmold plumbing, or vent effect of the aid actually ordered.
Card number_____ 83
card fld "card field id 1"
Occlusion
Effect
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card fld "card field id 1"
Circuit Noise
Ambient Noise
Amplified Environmental
Noise
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card fld "Loudness"
It has been recommended (e.g., Mueller & Hawkins) that LDLs be measured with tonal stimuli, rather than noise-type
stimuli…
From Stelmachowicz et al. (1990)…
"When a complex signal is processed… the power handling capabilities of the system must be shared by all of the
frequency components simultaneously… The maximum output at each frequency will be less than that observed when
only a single tone is processed independently."
•OLn: Overall level of the noise
LPC: Level per cycle, unit BW (i.e., pure tone)
BW: Bandwidth of the noise (i.e. the number of tones)
OLn = LPC + 10 LOG(BW)
•The Fonix composite signal has 80 tones so…
◊10 LOG(BW) = 19 dB
◊if OLn = 90 dB SPL, LPC = 90 - 19 = 71 dB
•The pure tone input (2000 Hz) at 90 dB is amplified to 107.9 dB SPL (17.9 dB gain)
•The composite component at 2000 Hz enters at 71 dB and is amplified to 86.7 dB SPL (15.7 dB gain)
•The greatest pure tone output is 109.2 dB SPL, while the rms output of the complex is 97.6 dB SPL, about 12 dB less.
•In terms of loudness summation (from Zwicker, 1990)…
◊Normals will show greatest summation (≈ 10 to 15 dB) at moderate listening levels, less (about 5 dB) at levels
approaching LDL
◊Hearing loss appears to widen critical bands and show less summation, especially at high levels (≈ 3 dB summation)
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•So, the single pure tone at 109.2 dB SPL would probably sound louder than the composite to an impaired ear, but under
certain circumstances (i.e., regions of good hearing) the composite could be louder
•It is a good idea to check LDLs for both tonal and broader band signals
Bentler & Pavlovic (1989, JASA) studied loudness summation at threshold of discomfort for normal and hearing-impaired
listeners
•PTA = 38 dB, falling configuration, presbycusic listeners
•They were presented single tones (centered at 16 CBs) and multitone complexes (2, 4, 8, and 16 tones)
•Results…
Summation re: 1 Tone
----------------------#Components Normal Hearing Impaired
2
2.8 dB
4.6 dB
4
3.6 dB
6.0 dB
8
3.5 dB
7.0 dB
16
3.8 dB 7.0 dB
---------------------------------------•These results indicate that the 109.2 dB SPL pure tone would be louder, causing a greater level of discomfort than the
97.6 dB SPL complex tone
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card fld "Info…"
Strategy "A"
•Fill in audiogram (speech MCLs & LDLs) on order form, send it in with impressions
•Fit the aid, ask how it sounds, perform informal evaluation
•Trial for a few weeks (determine subjective impression of benefit)
•Troubleshoot complaints
Strategy "B"
•Same as "A" but send in REUG
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Strategy "C"
•Same as "B" but use software to…
◊Estimate LDL, RESR, 2-cc OSPL90
◊Use PV on tymps to identify any really unusual ears, anticipate high frequency loss due to canal effects
◊Use custom REUG to predict REIG via a selection procedure (i.e., NAL)
◊Calculate 2-cc use-gain and FOG with an appropriate reserve gain (i.e., 10 dB)
◊Pick a matrix
◊Order appropriate options
•When aid arrives, verify 2-cc values, set pots to hit target REIG & RESR
•At fitting…
◊Verify target REIG & RESR
◊Verify that RESR < LDL for pure tone, broad band, and narrower band complex stimuli
•Other possibilities, if appropriate…
◊Determine real-ear spectrum level of noise at a use-gain setting (i.e., noise complaints or good low frequency
sensitivity)
◊Check occlusion effect, REOG
•Pair with a student, enroll in AR program, consumer education emphasis
Strategy "D"
•Same as "C" but…
◊Measure MCLs & LDLs for narrow band stimuli
◊Use custom CORFIG & RECD
"C" versus "D"
•Strategy "C" has errors of prediction (Enrico Fermi)
•Strategy "D" has errors of measurement
◊equipment errors, blocked probe
◊different physical structure of aid or earphone used to estimate CORFIGs, RECD
◊tester errors, like poor probe placement, lack of attention to detail
◊subject errors, listening fatigue, inappropriate response to task
◊validity of test stimuli to real world listening judgments of LDL
•If prediction errors are greater than measurement errors, is the difference great enough to affect users' perceived benefit?
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card fld "Info…"
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The eardrum is the entry (reference point) to the auditory system. The SPL developed at the eardrum is dependent
upon…
•The characteristics of the ear canal
◊Length
◊Diameter
◊Volume
•The characteristics of the middle ear (including the eardrum)
Therefore…
•The same hearing aid will perform differently upon each listener
•The KEMAR response (average response) is only an approximation
•Individual differences exist and may need to be accounted for
The model…
•A computer simulation of a mathematical model based on…
◊Gardner & Hawley's model of the ear canal as an electrical transmission line
◊Zwislocki's electrical analog model of the middle ear
√i.e., Zwislocki coupler
√KEMAR ear
•de Jonge, ASHA, 1982; Hearing Journal, 1983.
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card fld "Info…"
Each of the responses was obtained using a 1.2 x 0.2 cm parallel vent with…
a. A very large canal (2.8 cm long, 1.5 cm diameter)
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b. A wide canal with a normal length (1.4 cm long, 1.5 cm diameter)
c. A normal canal (1.4 cm long, 0.75 cm diameter)
The larger canal volumes interact with the vent to form a Helmholtz resonator with lower frequency resonance peaks.
card fld "Therefore…"
Since the vent response depends on the canal/middle ear impedance…
•You cannot estimate real-ear vent effects from placing a vented aid on a 2-cc coupler
◊the vent must be plugged during 2-cc measurements
•Obtaining an RECD is problematic for a vented aid…
◊the vented real-ear response must be obtained with exactly the same aid (or mold), as the aid that will be worn
◊often this is not available
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card fld "Info…"
Each of the responses was obtained using a wide canal (1.4 cm long, 1.5 cm diameter) with…
a. A vent 1.2 cm long, 0.2 cm diameter
b. A vent 1.2 cm long, 0.2 cm diameter with a 100 ohm damper
c. same length vent, but the diameter is increased to 0.4 cm
•Damping eliminates the Helmholtz resonator vent peak. So does increasing the diameter of the vent, provided you can
tolerate the increased gain between 1000 Hz and 2000 Hz.
Card number_____ 115
card fld "Info…"
Effect of increasing vent size when there is a very large canal (2.8 cm long, 1.5 cm diameter)…
a. Vent diameter 0.2 cm (1.2 cm long x 0.2 cm wide)
b. Vent diameter 0.4 cm
c. Vent diameter 0.6 cm (an open mold condition
•Increasing vent diameter causes the resonant peak of the Helmholtz resonator to shift to higher frequencies.
•The responses are very peaky, almost like the old carbon microphone hearing aids
Card number_____ 116
card fld "Info…"
Slides…
Hearing Aid Lectures Handout
de Jonge
Page 53 of 55
•A0. Original image 4x6" scanned at 150 dpi (total pixels = 540,000) at 8 bit resolution (256 levels of gray, 48 dB dynamic
range).
◊ 0= black
◊255= white
Adobe Photoshop 2.5 was used to create visual effects. The image was transferred back to film via a film recorder.
•A1. Simulates recruitment. All pixels with levels < 128 were set to black. Output levels climbed rapidly to full
brightness (255).
•A2. 2:1 compression ratio. The original range of 0 to 255, was compressed into 128 to 255.
•A3. 4:1 compression ratio. The original range of 0 to 255, was compressed into 192 to 255.
Too much compression elimates contrast, and makes it difficult to see variations between objects.
Frequency vs. space analogy
For sound, high frequencies vary rapidly with time, low frequency information changes little with time.
For visual images, high frequency information is the detail that changes rapidly with distance. Low frequency
information is analogous to large objects that change little with distance.
•A4. High-pass filter effect, 5 pixels.
•A5. High-pass filter effect, 20 pixels.
•A6. Low-pass simulation, Gaussian blur, 10 pixels.
•A7. Low-pass simulation, Gaussian blur, 30 pixels.
•A8. "Noisy listening." Gaussian noise, 64.
•A9. "Noisy listening." Gaussian noise, 128.
•A10. "Simple attenuation." Brightness for all pixels reduced by 100.
Card number_____ 117
card fld "Info…"
Slides…
Note: Click on the slide with the <optionKey> to play the sound.
•B0. Sonogram Demo. 500 msec 1000 and 2000 Hz tones, 500 msec of white noise, 500 msec of 500 Hz square wave, three
250 msec 500 Hz tone bursts (250 msec silent interval.
IHAS CCR 15
•B1. "Go change your car color is red." Synthetic sentence sampled at 11 KHz with 8 bit resolution using SoundEdit
software.
IHAS CCR 1
Hearing Aid Lectures Handout
de Jonge
Page 54 of 55
•B2. CCR is given a high frequency emphasis (HPF, high pass filtered)…
----------------250 Hz -15 dB
500
-5
1K
+5
2K
+12
4K
+18
----------------IHAS CCR 2
•B3. CCR is mixed with white noise, 1/4 full scale amplitude (≈ +6 dB S/N)
IHAS CCR 3
•B4. HPF-CCR mixed with noise (pre-filtered, then noise added).
IHAS CCR 4
•B5. CCR mixed with noise, then both filtered (HPF).
IHAS CCR 5
•B6. CCR "hearing aid processed," HAP.
◊see graph
IHAS CCR 6
•B7. Same as B6, but amplitude doubled twice (+12 dB into clipping).
IHAS CCR 7
•B8. CCR, noise added, then HAP.
IHAS CCR 8
•B9. Same as B8, but then clipped by doubling the amplitude.
IHAS CCR 9
•B10. Same as B9, but amplitude doubled again (clipped further).
IHAS CCR 10
•B11. CCR, "… color is red." delayed 100 msec and added to itself to simulate reverberation.
IHAS CCR 11
•B12. Same as B11, but HAP.
IHAS CCR 12
•B13. Same as B11, but mixed with noise and then HAP.
IHAS CCR 13
•B14. Same as B13, but clipped 24 dB (4 amplitude doublings).
IHAS CCR 14
Speech is an extremely robust signal. Our central auditory nervous systems are quite good at separating noise and
distortion from the signal.
Hearing Aid Lectures Handout
de Jonge
Page 55 of 55