Lecture 11 Flashcards
Regardless of the technology, features, accessories, or forms, hearing aids, we must achieve:
Audibility and comfort
What is the LTASS?
- Long-term average speech spectrum
- We need to look at speech spectrum (where speech falls on the audiogram)
What is vocal effort?
Variations of the LTASS
What is speech mapping?
- Depending on frequency and audiogram configuration we get a computer-generated gain target for each frequency
- Different target points to hit for our gain
What are the underlying principles of the HA prescriptive method?
- Prescribed gain value (target gain), is calculated (using a formula) for each frequency (based on audiometric data)
- The audiometric data will indicate which target we need to match
How was gain calculated before digital HAs?
- Before the inception of digital devices, target gain was calculated at each audiometric frequency using tables for calculation
- Done prior to the hearing aid selection
How is gain calculated today?
Today, digital hearing aids have flexible frequency responses and the prescriptive formulas are included within the fitting software and verification equipment.
What does a linear prescriptive formula provide?
A linear prescriptive formula gives a single fixed amount of gain regardless of the input up to the point of saturation
What does a non-linear prescriptive formula provide?
A non-linear prescriptive formula gives different amount of gain dependent of the input
- Target gain for soft, average, and loud speech
- Compression
What are 2 philosophies underlying prescriptive formulas?
- Loudness normalization
- Loudness equalization
What is loudness normalization?
Gain prescribed = gain need for the person with hearing loss to rate the sound level the same as it would be rated by people with normal hearing
What is loudness equalization?
Gain prescribed = gain need to amplify all speech frequency bands to perceived equal loudness (consonants just as aloud as the vowels)
What are the 2 current researched, valid, and verified prescriptive methods?
- NAL-NL2 (National Acoustic Lab)
- DSL v5 (Desired Sensation Levels)
What is the NAL-NL2?
- NAL-NL2 (National Acoustic Lab): the generally preferred methods for adult patients
- Loudness Equalization strategy
What is the DSL v5?
- DSL v5 (Desired Sensation Levels): the generally preferred methods for pediatric patients
- Loudness Normalization strategy (keeping balance between how a normal hearing listener would hear sound)
What are prescriptive methods the starting point for?
Prescriptive methods are the starting point to determine gain and frequency response (always manipulate based on patient preference)
What are the 6 historical prescriptive methods?
- 1/2 gain rule
- POGO (prescription of gain and output)
- Berger
- NAL-R (first of the NAL)
- CID (central institute for the Deaf)
- DSL (first of the DSL)
These were linear methods
Explain the 1/2 gain rule
- 1940s
- Gain prescribed = hearing threshold X 0.5 for each frequency
- Didn’t consider the speech spectrum, was not suitable for severe to profound loss
- (Example: If a threshold was 105 dB HL, gain (105 x 0.5 = 53 dB of Rx gain)
- Input of 50 dB, adding 53 dB of gain = 103 dB output
- Not audible
- Not a fantastic method, but one of the first methods used
Explain the POGO
- 1980s
- POGO (Prescription of Gain and Output)
- Frequency specific gain calculation (gain has variation depending on the frequency)
- Gain = (Threshold x 0.5) -10 dB at 250 Hz
- Gain = (Threshold x 0.5) -5 dB at 500 Hz
- Gain = Threshold x 0.5 at other frequencies
- Still, no consideration of the speech spectrum, and may not be suitable for severe to profound hearing losses
Explain the berger method
- 1980s
- Speech spectrum considered (one of the first methods where this was considered)
- Variable gain at different frequencies with an emphasis on those important for speech discrimination (we can weight frequency gain differently to help hear specific speech)
Explain the NAL
- 1970s
- NAL and NAL-R(revised) NAL-RP(revised for profound losses)
- National Acoustic Labs (first established as the Commonwealth Acoustics Lab) began to service war vets and children in 1947
- Formula attempting to all speech frequency band to equal loudness (to maximize intelligibility)
- Revisions included adjustments for
- LF gain reduction to minimize the effects of background noise
- Minimize excessive gain in HF for steeply sloping losses
Explain the CID
- 1980s
- Central Institute for the Deaf Method (CID)
- Speech spectrum introduced with frequency-specific, measured patient MCLs (this was the first time patient MCL was considered)
- Amplify average speech spectrum to the MCL from frequencies between 500-4000 Hz
- 250 Hz, speech spectrum was amplified to half-way between threshold and MCL in an attempt to reduced BGN
Explain the DSL
- 1980s
- Desired Sensation Levels- work out of Western University and Dr. Richard Seewald
- Rationale was similar to the CID method
- Initially developed for pediatric patients to create a loudness normalization approach with providing good audibility for speech.
- More initial gain compared to the first versions of NAL (a little bit louder for pediatric patients)
- Gain: what is required to amplify the average speech spectrum to a desired level above thresholds without exceeding LDL
- This method came about because of rubella (which caused acquired pediatric HL)
- This was a normalization method
Explain the NAL-NL2
- Speech intelligibility and loudness comfort
- Latest version released in 2011, idealized for fitting hearing instruments with WDRC
- Comparative to the NAL-NL1 version, hearing aid users were prescribed an average of 3dB less gain overall
- Focus on tonal and non-tonal language (Tonal languages pitch contours of syllables distinguish one word from another, whereas in non-tonal languages pitch is used to convey intonation. Li et al, 2021)
- Gender differences (males prescribed more gain that females)
Explain the DSL-v5
- Speech intelligibility and loudness comfort
- The DSL method, coming out of work from University of Western Ontario (in the mid 1980s) aimed to provide clinicians with a systematic, science-based approach to pediatric fittings (text, page 533)
- Optimize speech recognition for children with hearing loss, bring speech to a desired sensation level to maximize intelligibility
- Speech is amplified across a broad range of frequencies to support auditory learning and make speech cues audible
DSL-v5: age & etiology
- Age and Etiology: Patient age and cause of hearing loss is considered
- Higher listening level for paediatric (presumed congenital) than for adults (assumed acquired). A reduction of approximately 7dB is applied for adults
DSL-v5: compression characteristics
The compression threshold is lower for milder losses and higher for more severe losses
DSL-v5: listening environments
Listening environments: DSL targets assume that communicative speech is taking place in a quieter environments
DSL-v5: bilateral fittings
Bilateral Fittings: correction factor when fitting bilateral technology, reducing speech targets by 3dB. Bilateral sloping losses get a boost in the mid frequencies and a decrease in high frequency gain
NAL-NL2 vs DSL: experience level
NAL-NL2: More initial gain for experienced users, less initial gain for new users
DSL: No correction for gain based on experience lvel
NAL-NL2 vs DSL: gender
NAL-NL2: Irrespective of hearing loss or experience, reduces gain on average for females than males (2dB)
DSL: No adjustments in gain based on gender
NAL-NL2 vs DSL: bilateral vs unilateral fittings
NAL-NL2: Gain correction changes for bilateral fittings based on input (-2dB for low imput and up to 6dB for high input)
DSL: -3dB decrease across input levels for bilateral fittings vs unilateral fittings
NAL-NL2 vs DSL: listening in noise
NAL-NL2: No correction factors for listening in noise
DSL: -3 to -5dB reduction for low importance frequencies (to do this, the REM system has to allow this)
NAL-NL2 vs DSL: loudness discomfort levels
NAL-NL2: Patient specific levels not taken into consideration
DSL: Alters gain if patient-specific are specified
What does NAL-NL2 take into consideration?
An increased number of channels (more channels = slightly less gain) to mitigate the channel summation effect
What formula is better?
- The best comparison we have in study was conducted by Johnson and Dillon (2011) - not real humans, just math calculations:
- 7 different audiometric types (5 SNHL, and 2 conductive loss profiles)
- Compared predicted loudness perception, cochlear excitation, audibility, speech intelligibility, conversation ability in quiet and background noise
- Results: overall loudness was similar, similar speech intelligibility in quiet and BGN
Proprietary formulas (manufacturer specific formulas)
- Many manufacturers have their own prescriptive formulas that are often based on DSLv5 or NAL-NL2 but with brand specific variations to suit the specific products that they offer
- Some examples: eSTAT, WFA (Starkey), Phonak Adaptive Digital, WFR (Widex Fitting Rationale), VAC (Oticon Voiced-Aligned Compression), Audiogram+ (Resound)
- Benefits and Pitfalls?
- Manufacturers get this data from their HA users
- The manufacturer specific formulas can be good, but they aren’t specifically verified
Verification of hearing aid targets with ____ rationales
Prescriptive
When do we need to verify HA targets with prescriptive formulas?
- When verifying HA targets with prescriptive formulas, we also need to do that when there are audiogram changes for patients
- When audiogram changes, reverify the prescription
