{ published research technology selection, fitting and assessment. - - PDF document

10/21/2013 1

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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

 HI Industry research background – Starkey

Hearing Technologies, 2010‐2012

 Realities of hearing aid features don’t always fit

with marketing spin, conventional wisdom or published research

Disclosure Today’s Topics

Frequency Lowering DNR D‐Mics

 Directional

microphones

 Digital Noise

Reduction

 Frequency Lowering

What, How, Why…

 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

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Foundations…

Critical terms and concepts

Recruitment

 “an abnormally rapid

growth of loudness for sounds presented at levels greater than the threshold of detection”

 Abnormally rapid?  Growth of loudness?

Loudness

 Perceptual correlate

• f intensity

 Sound must be

audible to have loudness

 Change in loudness is

affected by:

 Magnitude of intensity

change

 Duration of intensity

change

Loudness

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The compressor of a hearing aid can be thought

• f as a loudness control

system. Why do we care?

Objective Subjective

Gain vs. Response

 Gain = (Output – Input)  Device gain ≠ change

in audibility

 Response = Intensity

REIG vs. REAR

 Real Ear Insertion Gain  REIG ≠ Audibility  Real Ear Aided Response  REAR > Threshold = Audibility

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Why do we care?

 We need both gain and

audibility to produce benefit that is: Objective and Subjective

Channels vs. Bands

 Channel = A subset of the

bandwidth for signal analysis and processing

 Band = A subset of the bandwidth

where you can control gain

Loudness and Gain

The compressor of a hearing

aid can be thought of as a loudness control system.

Signal processing features

are gain control systems.

The main course

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Today’s Topics

Frequency Lowering DNR D‐Mics

 Directional

microphones

 Digital Noise

Reduction

 Frequency Lowering

Directional Microphone Technology

How? Why? What?

 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

10/21/2013 6 How do they work?

Polar Response Pattern

 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.

A little like this…

 Fixed directional  Automatic directional  Adaptive directional  Automatic adaptive directional

The implementations

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 Switches between omnidirectional and fixed

directional

 When to switch is governed by sound

environment analysis

Automatic directional

Dual Omni‐directional

Change time delay, change response

 Vary the time delay, vary the polar response

pattern

 Adjust response pattern to maximize overall

SNR

Adaptive Directional

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Adaptive Null‐Steering Automatic Adaptive Directional

 Adds the low noise

benefit of an

• mnidirectional

response pattern

 When conditions are

appropriate – e.g., high SNR, low level listening.

DI on the head Directional Benefit

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) (3.5 – 5.5 dB)

(Ricketts, 2008)

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Caveat: Microphone Drift

Degraded Pattern due to 0.6‐dB Sensitivity Mismatch Normal Hypercardioid Pattern DI = 6.0 dB DI = 4.0 dB

Nulls are lost. DI drops by 2 dB.

The take home message

 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

Digital Noise Reduction

How? Why? What?

The theory

 Identify which parts of

sound are speech, and which parts are noise.

 Don’t amplify the

noise.

 Simple, right?  …um, no.

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 Fast vs. Slow acting  Noise reduction vs. Speech preservation

The implementation What it does

0 .5 1 1 .5 2 2 .5

• 10
• 8
• 6
• 4
• 2

2 4 6 8 10

T IM E , s SIGNAL VALUE

0 .5 1 1 .5 2 2 .5

• 10
• 8
• 6
• 4
• 2

2 4 6 8 10

T IM E , s SIGNAL VALUE  Identify Noise  Calculate Noise spectrum  For a given Time & Frequency:  Turn gain up when Speech  Turn gain down when Noise

0.5 1 1.5 2 2.5

• 40
• 30
• 20
• 10

10 20 30

TIME, s SNR, dB

Identify Noise

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Identify Speech

0.5 1 1.5 2 2.5

• 40
• 30
• 20
• 10

10 20 30

TIME, s SNR, dB

0.5 1 1.5 2 2.5

• 40
• 30
• 20
• 10

10 20 30

TIME, s SNR, dB

TC = 0.02; Slope = 0.45; Offset = 10

Apply Gain Rules In Running Speech

Noise Reduction

Speech Preservation “Strict” Detection “Lenient” Detection

A Balancing Act

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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)

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The take home message

 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

Frequency Lowering

How? Why? What?

The theory

 Some listeners may not

benefit from HF audibility

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Theory

 Off‐frequency listening  ‘Sensory overload’  Distortion  Adverse effects on

speech understanding

Frequency Lowering

/S/

The implementations

 Three Current

Implementations

 Frequency

Compression

 Transposition  Feature Synthesis

1 2 3 4 5 6 4 5 6 Non‐Linear Frequency Compression

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Frequency (Hz)

Frequency Compression

Increased audibility Decreased bandwidth at all times Reduced sound quality

Technique

Frequency Transposition

1 2 3 4 5 6

Frequency Transposition

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Frequency Transposition

• 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

Feature Synthesis Feature Synthesis

• Consonant sounds

replicated in real time

• Bandwidth

preserved

• Quality usually

preserved

The evidence

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Frequency Lowering: Adults (Frequency Compression)

(Glista et al., 2009)

*

Frequency Lowering: Adults

(Glista, Scollie, Bagatto, Seewald, Johson, 2009)

Frequency Lowering: Adults (Frequency Transposition)

(Kuk, Peeters, Keenan, & Lau, 2007) Consonant Recognition

(Galster, Valentine, Dundas & Fitz, 2011)

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Assessing benefit

 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?

The take home message

 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

Summary

Frequency Lowering DNR D‐Mics

 Directional

microphones

 Digital Noise

Reduction

 Frequency Lowering

Positive effects

 Directionality  DNR  Frequency lowering  All can provide

measurable benefit for appropriately selected and fit patients

10/21/2013 19

Negative Effects?

 Can occur  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 be audible to be perceptible
• 3. Magnitude of perception is dependent on the loudness

growth curve

Gain, Audibility & Magnitude of Perceptual effects

The 3 concepts are linked

Direct vs. Amplified Path

 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