The benefits of hearing aid performance features

1
The benefits of hearing aid performance features
Harvey Dillon National Acoustic Laboratories,
Australian Hearing, CRC for Cochlear Implant,
Speech Hearing Research
EHIMA, Brussels, May, 1999
2
Simulated hearing loss
Loss of high frequencies
(Boothroyd)
Loss of weak sounds
(Boothroyd)
Loss of resolution, weak sounds, in noise
(Moore)
3
Aim of this talk

• To describe the signal processing features of
  hearing aids that are currently available.
• Describe and quantify the benefit,
• Describe which types of clients it will benefit.

4
Programmability
25 of market in USA
10 years old
5
Programmability

• Benefit
• No direct benefit, but .
• Adjustments more likely to be done

6
Programmability

• No direct benefit, but .
• Adjustments more likely to be done

7
Programmability

• No direct benefit, but .
• Adjustments more likely to be done

8
Programmability
Immediate adjustment
Immediate solution
9
Programmability

• Patient selection
• Everyone

10
Digital signal processing
Microphone
Earphone
Processor
1 0 0 1 0 0 1 1 .
Analog to digital converter
Digital to analog converter
11
Digital signal processing
Benefits

• No direct benefit, but ..
• Some performance features only feasible with
  digital signal processing

12
Digital signal processing

• Patient selection
• Anyone who needs a feature that cannot be
  provided by analog hearing aids.
• (But eventually, everyone)

13
Tone controls
Bass
Treble
14
Tone controls

• Correct tone setting more critical than for
  normal hearers

Too much treble
15
Fitter-adjusted tone controls

• Patient selection
• Anyone for whom we cannot confidently predict the
  optimum tone control setting
• - that is, everyone.

16
Compression limiting versus peak clipping
To prevent excessive loudness

• Peak clipping chops the tops off sounds
• Compression limiting rapidly turns the volume
  down

Benefits Comfort, Sound quality
17
Peak clipping versus compression limiting
Compressed
Demos
18
Compression limiting

• Patient selection
• Everyone, except those who need higher output
  levels than can be achieved with compression
  limiting. (Peak clippers can provide 6-10 dB
  higher output levels when measured with speech
  signals.)

19
Telecoil
20
Telecoil

• Patient selection
• Anyone who has trouble on the phone, or who
  regularly attends a place that is looped, and,
• Is willing to have a hearing aid large enough to
  take a coil and switch (or remote control)

21
Wide dynamic range compression
Intense
Automatic
Non-linear
Non-linear
Moderate
Weak
Normal
22
Wide dynamic range compression

• For intense sounds
• ? More comfort ..
• For medium sounds
• ??? No advantage .
• For weak sounds
• ??? Intelligibiity increase ..

• Automatic operation
• convenient for anyone .
• essential for those with manipulation difficulty.

23
Increasing listening comfort
linear
Output (dB SPL)
CT62
65
Input (dB SPL)
24
Wide dynamic range compression
Intelligibility benefit
Moore, Johnson, Clark Pluvinage, 1992
25
Preferences Non-linear or linear
55
No of people
31
14
26
Preferences Non-linear or linear
No of people
27
Preferred Threshold
60
No of people
31
9
statistically significant
28
Reaction to intense sounds
29
Wide dynamic range compression

• Patient selection
• Most patients
• Unclear how to choose not to use WDRC
• Profound loss will limit the amount of
  compression that can be achieved

30
Multi-channel compression
Hearing loss varies across frequencies so should
compression
31
Multi-channel hearing aids
32
Multi-channel hearing aids

• Benefits (re single channel compression)
• Small ! 5 to 10 in intelligibility.
• But .. benefit at low and high levels, re
  single channel compression has not been evaluated.

33
Benefits of multi-channel
From Moore, Peters Stone (1999) JASA
105400-411
34
Multi-channel compression

• Patient selection
• Possibly most beneficial for people with
  moderately or steeply sloping hearing loss.
• Not harmful for anyone

More evidence urgently needed
35
The biggest problem noise
36
Suppressing noise adaptively
Hearing aid automatically alters its response
depending on the environment
37
Adaptive noise suppression
Graphic equaliser
Graphic equaliser
6 dB
12 dB
6 dB
12 dB
38
Noise Reduction
Signal
Input Spectral Level
Noise
Frequency
Gain
Signal
Output Spectral Level
Noise
39
Adaptive noise suppression

• Benefits
• A definite comfort advantage.
• Occasionally there may be an intelligibility
  advantage.
• Disadvantage
• May sometimes misinterpret what is the signal and
  what is the noise.

40
Adaptive noise suppression

• Patient selection
• Everyone, but benefit will be greatest for people
  who need gain at all frequencies, which implies
  low frequency hearing loss should be greater than
  about 30 dB HL.

41
Steady HF-noise
Impulsive HF-noise
Babble-noise
Keidser Grant
Traffic-noise
Speech in quiet
Less LF/more HF gain in dB
More LF/less HF gain in dB
NAL-response
42
Speech (60 dB) in air compressor noise
(SII0.7) Reduce annoyance Ease of understanding
Speech (65 dB) in quiet
Speech (70 dB) in traffic-noise (SII0.4) Reduce
annoyance Ease of understanding
Keidser Grant
43
Multi-memory
Different programs for different situations
44
Multi-memory hearing aids

• Patient selection
• Active lifestyle, with hearing aid use in a
  variety of situations
• High frequency loss gt 55 dB HL
• Low frequency gain gt 0 dB

45
Improving signal to noise ratio
46
Directional microphones
Benefit 5 dB improvement in SNR
50 improvement in speech understanding
Omni-directional
47
Directional microphones
Benefit 5 dB improvement in SNR
50 improvement in speech understanding
Omni-directional
Directional 5 dB SNR
48
Effect of decreasing noise
BKB sentences Moore, Johnson, Clark Pluvinage,
1992
49
The cardioid family
90
Super
60
120
150
30
-10
-20
-30
180
0
Hyper
330
210
300
240
270
Figure-8
50
Dual microphones

• Benefits
• directional when needed
• omni-directional when needed

51
Directivity Index
52
45 mm
53
Directional microphones

• Patient selection
• Any one can benefit from an improvement in SNR,
  from normal hearing to profound hearing loss.
• Hearing aid is only directional when it has gain.

54
Wireless / Direct audio input
55

• Size
• Servicing

56
Size Reliability
57
Wireless systems

• Patient selection
• Anyone will benefit from the SNR increase.
• Use if the logistics of the situation allow it.

58
Bilateral or binaural amplification
59
Bilateral or binaural amplification

• Clearer in quiet
• Clearer in noise
• Better localisation
• Greatest benefit for people with moderate to
  profound hearing loss
• Average 5 dB improvement in signal to noise
  ratio
• Avoiding auditory deprivation

60
Bilateral hearing aids

• Patient selection
• Anyone with a hearing loss in both ears, unless
  the loss in either ear is too profound to be
  aidable, or too mild to be aidable.
• Benefit is greatest for those with bilateral
  moderate losses or greater.

61
Feedback management

• Whistling occurs when amplification gt leakage
• Standard solutions
• plug ear tightly
• decrease high frequency amplification
• turn volume down

62
Feedback management
63
Unsophisticated aid
64
Feedback management
65
Feedback cancelling
-
Internal feedback path
66
Feedback cancelling

• Benefits
• Approximately 10 dB more gain before feedback
• More open ear -gt better own voice quality
• More gain -gt more intelligible speech
• Less whistling -gt less embarrassment

67
Feedback management and supression

• Patient selection
• Anyone who has trouble with feeback, particularly
• patients with profound loss,
• patients with near normal LF loss, and severe HF
  loss.

Frequency (Hz)
Hearing threshold (dB HL)
68
In-the-canal and Completely-in-the-canal

• Reduced wind noise
• Some directivity
• Reduced visibility

69
Soft ear shell materials

• Increased comfort
• Increased depth enables decreased occlusion
  effect
• Increased retention

SUPERIOR
INFERIOR
MEDIAL
LATERAL
FRONTAL SECTION
70
Importance of prescription

• Adjustment range is huge
• Benefits require good adjustment
• Accurate audiogram
• Good prescription procedure
• Knowledgable clinician for fine-tuning

71
Basis of the NAL-NL1 (non-linear) procedure

• For every input level above compression
  threshold
• Maximise predicted speech intelligibility
• Make loudness of speech equal to, or less than,
  normal

72
NAL-NL1 Vs IHAFF 22 S at MCL
73
NAL-NL1 Vs IHAFF NUMBER of Ss PREFERRING
74
Summary of features

• Programmability - enabling feature
• Digital signal processing - enabling feature
• Tone controls .......sound quality,
  ?
• Cmpression limiting .....sound
  quality
• Wide dynamic range compression .. 20 to
  50
• Multi-channel compression .... 5
• Directional microphones .... 50
• Wireless ... 100
• Bilateral hearing aids . 50
• Feedback management/cancelling quality,
  convenience, ?
• Small size in-ear location ...cosmetic,
  wind
• Adaptive noise suppression .. comfort
• Multi-memory ... comfort

75
CONFERENCE
Hearing aid amplification for the new millenium
Sydney, Australia November 15-19, 1999
76
Further details, and more, available inHearing
Aidsby Harvey DillonAvailable 1999Approx
500 pages covering hearing aid technology,
prescription, fitting, follow-up and evaluation