Title: Recent advances in the design and fitting of nonlinear hearing aids
1Recent advances in the design and fitting of
non-linear hearing aids
Harvey Dillon National Acoustic Laboratories,
Australian Hearing, CRC for Cochlear Implant and
Hearing Aid Innovations
Manchester, July, 2000
Dillon H, NAL
2Aim of this talk
- To describe available non-linear signal
processing features of hearing aids - their benefits,
- their limitations,
- their prescription and fitting.
Dillon H, NAL
3This talk can be down-loaded from the Australian
Hearing Web-site, from July 5
- www.hearing.com.au
- Research_at_NAL.GOV.AU
Dillon H, NAL
4Programmability
40 of market in USA
11 years old
Dillon H, NAL
5Programmability
- No direct benefit, but .
- Adjustments more likely to be done
Dillon H, NAL
6Programmability
- No direct benefit, but .
- Adjustments more likely to be done
Dillon H, NAL
7Programmability
- No direct benefit, but .
- Adjustments more likely to be done
Dillon H, NAL
8Programmability
Immediate adjustment
Immediate solution
Dillon H, NAL
9Programmability
- Patient selection
- Everyone
Dillon H, NAL
10Digital signal processing
Microphone
Earphone
Processor
1 0 0 1 0 0 1 1 .
Analog to digital converter
Digital to analog converter
Dillon H, NAL
11Digital signal processing
Benefits
- No direct benefit, but ..
- Some performance features only feasible with
digital signal processing
Dillon H, NAL
12Digital signal processing
- Patient selection
- Anyone who needs a feature that cannot be
provided by analog hearing aids. - (But eventually, everyone)
Dillon H, NAL
13Compression 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
Dillon H, NAL
14Peak clipping versus compression limiting
Compressed
Dillon H, NAL
15Compression 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.)
Dillon H, NAL
16Wide dynamic range compression
Intense
Automatic
Non-linear
Moderate
Weak
Normal
Impaired
Dillon H, NAL
17Wide 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.
Dillon H, NAL
18Output dB SPL
65
Input dB SPL
Dillon H, NAL
19Preferences Non-linear or linear
55
No of people
31
14
Source Dillon et al (1998) Aust J Audiol.
Dillon H, NAL
20Preferences Non-linear or linear
No of people
Source Barker Dillon, 1999, Ear Hearing
Dillon H, NAL
21Preferred Threshold
60
No of people
31
9
statistically significant
Source Barker Dillon, 1999, Ear Hearing
Dillon H, NAL
22Increasing listening comfort
linear
Output (dB SPL)
CT62
65
Input (dB SPL)
Dillon H, NAL
23Reaction to intense sounds
Dillon H, NAL
24Wide 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
Dillon H, NAL
25Squelch or expansion
Gain (dB)
Input level
90
80
Output (dB SPL)
70
60
30
40
50
60
70
80
90
Input level
Dillon H, NAL
26Expansion or squelch
- Patient selection
- Patients who complain about internal noise
- Perhaps patients who complain about low-level
external sounds
Dillon H, NAL
27Multi-channel compression
Hearing loss varies across frequencies so should
compression
Dillon H, NAL
28Multi-channel hearing aids
Dillon H, NAL
29Multi-channel hearing aids
- Benefits (re single channel compression)
- Small ! 5 to 10 in intelligibility at
most. - But .. benefit at low and high levels, re
single channel compression has not been evaluated.
Dillon H, NAL
30Benefits of multi-channel
From Moore, Peters Stone (1999) JASA
105400-411
Dillon H, NAL
31Preferred no. of channels
Average input level
Keidser Grant, in preparation
Dillon H, NAL
32Preferred number of channels
Keidser Grant, in preparation
Dillon H, NAL
33Preferred number of channels
Keidser Grant, in preparation
Dillon H, NAL
34Multi-channel compression
- Patient selection
- Probably beneficial for people with moderately or
steeply sloping hearing loss. - Not too harmful for anyone
More evidence urgently needed
Dillon H, NAL
35The biggest problem noise
Dillon H, NAL
36Suppressing noise adaptively
Hearing aid automatically alters its response
depending on the environment
Dillon H, NAL
37Adaptive noise suppression
Dillon H, NAL
38Dillon H, NAL
39Noise Reduction
Signal
Input Spectral Level
Noise
Frequency
Gain
Signal
Output Spectral Level
Noise
Dillon H, NAL
40Steady 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
Dillon H, NAL
41Adaptive 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.
Dillon H, NAL
42Adaptive 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.
Dillon H, NAL
43Multi-memory
Different programs for different situations
Dillon H, NAL
44Multi-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
- Able to understand and use the controls
Dillon H, NAL
45Improving signal to noise ratio
Dillon H, NAL
46Effect of decreasing noise
BKB sentences Moore, Johnson, Clark Pluvinage,
1992
Dillon H, NAL
47Directivity Index
Dillon H, NAL
48Directional microphones
Benefit 5 dB improvement in SNR
50 improvement in speech understanding
Dillon H, NAL
49Dual microphones
- Benefits
- directional when needed
- omni-directional when needed
Dillon H, NAL
50The cardioid family
90
Super
60
120
150
30
-10
-20
-30
180
0
Hyper
330
210
300
240
270
Figure-8
Dillon H, NAL
5145 mm
Dillon H, NAL
52Directional 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.
Dillon H, NAL
53Feedback management
- Whistling occurs when amplification gt leakage
- Standard solutions
- plug ear tightly
- decrease high frequency amplification
- turn volume down
Dillon H, NAL
54Feedback management
Dillon H, NAL
55Unsophisticated aid
Dillon H, NAL
56Feedback management
Dillon H, NAL
57Feedback cancelling
-
Internal feedback path
Dillon H, NAL
58Feedback 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
Dillon H, NAL
59Feedback 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)
Dillon H, NAL
60Two linear benefits
Dillon H, NAL
61Wireless / Direct audio input
Dillon H, NAL
62Dillon H, NAL
63Dillon H, NAL
64Wireless systems
- Patient selection
- Anyone will benefit from the SNR increase.
- Use if the logistics of the situation allow it.
Dillon H, NAL
65Bilateral or binaural amplification
Dillon H, NAL
66Bilateral 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.
Dillon H, NAL
67Non-linear selection procedures
- IHAFF / Contour / VIOLA
- LGOB
- Scaladapt
- Fig6
- DSL i/o
Dillon H, NAL
68Normal Hearing (average)
Hearing Impaired (individual)
Dillon H, NAL
69The problem
- IHAFF, LGOB, ScalAdapt, FIG6, DSLi/o
- Based on loudness normalization
- One frequency at a time
- Not the right starting point
- Results conflict with NAL-R
Dillon H, NAL
70NAL-NL1
Dillon H, NAL
71Basis of the NAL-NL1 (nonlinear) procedure
- For every input level above compression
threshold - Maximise predicted speech intelligibility
- Make loudness of speech equal to, or less than,
normal
Dillon H, NAL
72Basis of calculations
- Intelligibility
- based on a modification of the Speech
Intelligibility Index method - allows for all the effects of hearing loss on
intelligibility
- Loudness
- based on a model by Moore Glasberg
- allows for the effects of hearing loss
Dillon H, NAL
73Demonstration of software
Dillon H, NAL
74NAL-NL1 vs IHAFF (4/4)
Average input level
Dillon H, NAL
75Speech recognition test (BKB)
Dillon H, NAL
76Summary of features
- Programmability - enabling feature
- Digital signal processing - enabling feature
- Tone controls .......sound quality,
? - Compression 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
- Volume control.comfort,
intelligibility
Dillon H, NAL
77Thats all Folks
Dillon H, NAL
78Web-site address WWW.hearing.com.au
Dillon H, NAL
79For further information . .
.
Dillon H, NAL
80Further details, and more, available inHearing
Aidsby Harvey DillonAvailable
2000(sometime)Approx 580 pages covering hearing
aid technology, prescription, fitting, follow-up
and evaluationBoomerang Press
Dillon H, NAL