Evidence base for hearing aid features:

Transcription

1 Evidence base for hearing aid features: { the ʹwhat, how and whyʹ of technology selection, fitting and assessment. Drew Dundas, PhD Director of Audiology, Clinical Assistant Professor of Otolaryngology UCSF Medical Center Disclosure HI Industry research background Starkey Hearing Technologies, Realities of hearing aid features don t always fit with marketing spin, conventional wisdom or published research DNR D Mics Directional microphones Digital Noise Reduction Frequency Lowering What the technology is intended to accomplish How it actually works Why you might want to recommend it Assessing benefit: Objective Subjective The take home message Frequency Lowering Today s Topics What, How, Why 1

2 Critical terms and concepts an abnormally rapid growth of loudness for sounds presented at levels greater than the threshold of detection Abnormally rapid? Growth of loudness? Foundations Recruitment Perceptual correlate of intensity Sound must be audible to have loudness Loudness Change in loudness is affected by: Magnitude of intensity change Duration of intensity change Loudness 2

3 The compressor of a hearing aid can be thought of as a loudness control system. Objective Why do we care? Subjective Gain = (Output Input) Device gain change in audibility Response = Intensity Real Ear Insertion Gain REIG Audibility Real Ear Aided Response REAR > Threshold = Audibility Gain vs. Response REIG vs. REAR 3

4 We need both gain and audibility to produce benefit that is: Objective and Subjective Channel = A subset of the bandwidth for signal analysis and processing Band = A subset of the bandwidth where you can control gain Why do we care? Channels vs. Bands The compressor of a hearing aid can be thought of as a loudness control system. Signal processing features are gain control systems. Loudness and Gain The main course 4

5 DNR D Mics Directional microphones Digital Noise Reduction Frequency Lowering Why? What? How? Frequency Lowering Today s Topics Directional Microphone Technology Noise is an unwanted competitor. It can also drive the compressor level estimate. This can result in decreased signal audibility, as well as poor SNR. The theory 5

6 How do they work? Displays relative sensitivity of the mic at different angles. Convention: Up is the look direction. Convention: Where the line gets close to the center, the mic is less sensitive. Polar Response Pattern Fixed directional Automatic directional Adaptive directional Automatic adaptive directional A little like this The implementations 6

7 Switches between omnidirectional and fixed directional When to switch is governed by sound environment analysis Automatic directional Dual Omni directional Vary the time delay, vary the polar response pattern Adjust response pattern to maximize overall SNR Change time delay, change response Adaptive Directional 7

8 Adds the low noise benefit of an omnidirectional response pattern When conditions are appropriate e.g., high SNR, low level listening. Adaptive Null Steering Automatic Adaptive Directional Typically 20% 35% when: The sound source of interest is in front and nearby Competing noise is mainly behind or surrounds the listener Reverberation is moderate or less The instrument has a high average directivity index (DI) ( db) DI on the head Directional Benefit (Ricketts, 2008) 8

9 Caveat: Microphone Drift Normal Hypercardioid Pattern DI = 6.0 db Nulls are lost. DI drops by 2 db. Degraded Pattern due to 0.6 db Sensitivity Mismatch DI = 4.0 db Directional Mics are good for almost everyone, but They are not magic If you don t have audibility, they can t help. if there is a vent, they cannot provide benefit if you are not at least 0dB insertion gain The take home message Why? What? How? Identify which parts of sound are speech, and which parts are noise. Don t amplify the noise. Simple, right? um, no. Digital Noise Reduction The theory 9

10 Fast vs. Slow acting Noise reduction vs. Speech preservation SIGNAL VALUE The implementation TIME, s What it does Identify Noise Calculate Noise spectrum For a given Time & Frequency: Turn gain up when Speech Turn gain down when Noise SNR, db Identify Noise TIME, s 10

11 30 30 TC = 0.02; Slope = 0.45; Offset = SNR, db 0-10 SNR, db TIME, s Identify Speech Apply Gain Rules TIME, s Strict Detection Noise Reduction Speech Preservation In Running Speech Lenient Detection A Balancing Act 11

12 Comfort and Annoyance (Palmer, Bentler, Mueller, 2006) * Acceptance of Background Noise (Mueller, Weber, Hornsby, 2006) * DNR makes noise more acceptable Cognitive Benefits (Sarampalis et al., 2009) 12

13 Effects of DNR: Enhanced comfort Some listeners may experience enhanced speech understanding in noise May make HAs more acceptable May free up cognitive resources for other tasks Why? What? How? The take home message Frequency Lowering Some listeners may not benefit from HF audibility The theory 13

14 Off frequency listening Sensory overload Distortion Adverse effects on speech understanding /S/ Theory Frequency Lowering Three Current Implementations Frequency Compression Transposition Feature Synthesis Non Linear Frequency Compression The implementations

15 Increased audibility Decreased bandwidth at all times Reduced sound quality Frequency (Hz) Frequency Compression Frequency Transposition Technique Frequency Transposition 15

16 Increased audibility Speech cue confusion Reduced sound quality Preserve bandwidth Identify HF consonant sounds Generate a spectral analogue at a lower frequency Provide appropriate audibility of the analogue re: concurrent speech sounds Frequency Transposition Feature Synthesis Consonant sounds replicated in real time Bandwidth preserved Quality usually preserved Feature Synthesis The evidence 16

17 Frequency Lowering: Adults * Frequency Lowering: Adults (Frequency Compression) (Glista et al., 2009) (Glista, Scollie, Bagatto, Seewald, Johson, 2009) Consonant Recognition Frequency Lowering: Adults (Frequency Transposition) (Kuk, Peeters, Keenan, & Lau, 2007) (Galster, Valentine, Dundas & Fitz, 2011) 17

18 Probe mic measures can be useful with bandwidth limited techniques Demonstrates change in audibility Problem: how much audibility is appropriate? What targets do you aim for? Provides measurable real world benefit Tuning for listener preference is necessary Like fitting targets, one size does not fit all Recent research suggests that preserving bandwidth is preferred even in patients with suspected dead regions Sound quality matters Assessing benefit The take home message DNR D Mics Directional microphones Digital Noise Reduction Frequency Lowering Directionality DNR Frequency lowering All can provide measurable benefit for appropriately selected and fit patients Frequency Lowering Summary Positive effects 18

19 Can occur Negative Effects? Choose settings carefully, using verification and patient perceptions as guides 1. Have to be applying gain with D Mics and DNR to change the output signal 2. Must The be audible 3 concepts to be perceptible are linked 3. Magnitude of perception is dependent on the loudness growth curve Gain, Audibility & Magnitude of Perceptual effects Comparing sealed coupler measurements to real world is not always realistic Direct sound arriving through the vent may reduce signal processing effects In challenging cases, and more severe hearing losses, consider less open fittings to maximize effect Questions? drew.dundas@ucsfmedctr.org Direct vs. Amplified Path 19