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Explore the impact of digital signal processing (DSP) technology versus analog in hearing aids, including features like directional microphones, noise reduction, and feedback cancellation.
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Does Higher Technology Result in Higher Levels of Benefit? BCASLPA October 22, 2004 Ruth Bentler www.shc.uiowa.edu
What features? • Dsp versus Analog • Directional Mics • Noise Reduction • Feedback Cancellation
Dsp versus Analog • Orange Juice or Tomato Juice • Cardiovascular or Weight-Bearing • Puppy or cable
Dsp versus Analog • Orange Juice or Tomato Juice • Cardiovascular or Weight-Bearing • Puppy or cable
Digital versus analog • Not a debate amongst (most) researchers • Easy to contrive the design • Often misleads the clinician • E.G. • Wood & Lutman (March 2004, IJA)
Abstract • Question: Are dsp hearing aids better than analog (linear) hearing aids? • Design: 100 first-time users, single- blinded, wore the HAs for 5 weeks each • APHAB, GHABP, QoL, Diary • REAR, Speech-in-Noise • Results…
Results • Better dsp performance at 75 dB inputs (4%); no difference at 65 dB input • No difference in QoL • No difference in use time • No difference in APHAB subscales (n=36) • Difference in Satisfaction subscale of GHABP in favor of dsp • 60 preferred dsp; 31 preferred analog
Conclusions • “Dsp provides significantly better speech recognition performance for raised speech in background noise than carefully fitted (“but not adjusted”) linear analog hearing aids.” • “Users report somewhat greater satisfaction…and less aversiveness to sound.
An indepth look at the facts… • Gain/output not controlled (audibility??) • Limiting versus peak clipping not controlled (distortion??) • Linear aids not adjusted to “comfort”, as were dsp aids (blinded??) • And, finally, features such as directional mics, noise reduction and feedback cancellation were active in the dsp circuits…
So, one more time… • The advantages of dsp hearing aids (to the end user) lie in the features, if they lie at all… • Manufacturer benefits? • Dispenser benefits?
What we know… • One and two mic designs • Low frequency compensation • Mic noise goes up (and up) • Many companies use mic noise algorithm
Quick Tutorial • Ways to build directivity into a hearing aid case: • Single mic with two ports • Two omni mics • Combination of omni & directional mics • Three mics • Mic array
OmniDirectional Microphone Polar plot for omnidirectional mic in free field Level of signal Angle of signal source
Polar Response Pattern Free field characteristics of different types of microphones (Knowles TB 21) Omnidirectional Cardioid Hypercardioid Supercardioid
Quick tutorial, cont. • Ways to implement directionality in the hearing aid case: • Fixed polar pattern • Program different polar patterns in different memories • Automatic directional mode • Adaptive directional mode
80 70 60 50 40 30 20 10 0 -10 100Hz 200 500 1kHz 2 5 10kHz Ways to quantify directivity • Front-to-back ratio (FBR) • Directivity Index (DI) • Theoretical • Free field • KEMAR Frequency, Hz
4.8 4.0 2.3 6.0 5.1 3.0 5.7 5.0 3.3 FF (BTE) KEMAR (BTE) Theoretical Cardioid Hypercardioid Supercardioid
4.8 4.1 2.7 6.0 5.6 3.3 5.7 5.4 3.5 FF (ITE) KEMAR (ITE) Theoretical Cardioid Hypercardioid Supercardioid
Laboratory Data • We have 30-40 years of lab data and trade-magazine evidence that • Directional mics can improve SNR • That enhancement is based on • # and placement of speakers • Type/level/distance of noise • Reverberation • Baseline comparison (unaided, BTE/ITE, Omni) • LF Compensation versus hearing levels
Laboratory Data • AND, that lab data do not relate very well to self-report data • E.g., Walden, Surr & Cord (Hearing Journal, 2003)…
Laboratory Data • And, we can’t predict directional advantage: • Ricketts & Mueller (JAAA, 2000) examined three studies for effect on directional advantage: • Slope of hearing loss • Amount of high frequency hearing loss • Aided omnidirectional advantage • In one study, found significant negative relationship between aided omnidirectional performance and directional advantage
Cord, Surr, Walden & Olsen (2002) Performance of directional microphone hearing aids in everyday life, JAAA, 295-307. • Called back users of directional mic hearing aids that fell into two groups • Those who used them regularly (deemed successful)(n=22) • Those who did not, and used the default omnidirectional mode (deemed unsuccessful)(n=26) • No predictive power in APHAB scores
Cord, Surr, Walden & Olsen (2002) • Microphone Performance Questionnaire (MPQ) indicated directional mics preferred when signal is in front (near) and noise is in back • All 48 participants reported being satisfied with their HAs in each mic configuration; although the directional mic used less often, equally satisfied with it when they did...
Walden and Walden (2004). Predicting success with hearing aids in everyday living, JAAA, 342-353. • Purpose of the study: Investigate relationship between two measures of hearing aid success (IOI-HA and HAUS) and demographic and audiometric measures. • No blinding; clinic appointment data (n=50) • Not really a comparison of mic conditions, although • IOI-HA showed statistically significant difference across the groups • Omni only (n=29) • Omni/Directional with a switch (n=21) • NO difference in HAUS across two groups
Walden, Surr, Cord, and Dyrland (2004). Predicting hearing aid microphone preference in everyday listening. JAAA, 365-396. • Purpose of the study: Define environments for which either the omnidirectional or directional mode was better (thus providing guidance): • Talker location • Noise location • Distance • Time • Ease of listening • (Indirectly assigned reverberation)
Cord, Surr, Walden & Dyrland (2004) • Beginning of a model! • Mean estimated use time was 61.8% for omni mode and 38.2% for directional mode. • Average use of the omni mode was 65% for 8 participants for whom the default setting was omni, and 58.9% for the 9 participants for whom the default was directional.
Do experienced/trained users of hearing aids with directional microphones report better amplification outcomes in daily life than users of hearing aids without directional microphones?
(Infamous) Valente, Fabry & Potts (1995).Recognition of speech in noise with hearing aids using dual microphones, JAAA, 440-449. • Purpose: To determine the effectiveness of a directional mic (two omni design) • Two sites (25 at each) • No blinding • Although not a comparison to omni design, PHAB (Site 1) and APHAB (Site 2) showed subjects performing above the mean benefit norms.
Preves, Sammeth, & Wynne (1999). Field trial evaluations of a switched directional/omnidirectional ITE hearing instrument (1999). JAAA, 273-284. • Purpose: To evaluate the usefulness of a switch-option directional microphone system • 10 blinded subjects (single-blinded cross-over design) wore aids for 2 trials • Self-report inventories (after non-equalized trial and equalized trial) • APHAB • Subjective Comments
Preves, Sammeth, & Wynne, continued • For Trial #1 (non-equalized) • APHAB: RV subscale showed directional mode significantly better (fewer reported problems) • Comments: If only one, 6/10 directional mode; Subjects “hesitant to give up” either mode • For Trial #2 (equalized) • APHAB: RV and BN RV subscale showed directional mode significantly better (fewer reported problems) • Comments: If only one, 6/10 directional mode; Subjects “hesitant to give up” either mode
Boymans and Dreschler (2000), Field trials using a digital hearing aid with active noise reduction and dual-microphone directionality, IJA, 260-268. • 16 subjects (single-blinded cross-over design) wore aids for four consecutive field trials • No noise reduction • Directional mics only • Noise reduction only • Directional mics plus noise reduction
Boymans and Dreschler (2000) continued • “Subjective” outcome measures • Paired comparisons (only in the lab) • APHAB