Background noise the problem and some solutions

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"Background noise - the problem and some
solutions"
Harvey Dillon NAL CRC for Cochlear Implant and
Hearing Aid Innovations
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The biggest problem noise
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The underlying problem
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Frequency resolution
SPL
freq
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Frequency resolution
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Temporal resolution
SPL
time
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Temporal resolution
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Directional microphones
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Effect of decreasing noise
BKB sentences Moore, Johnson, Clark Pluvinage,
1992
H. Dillon NAL, CRC for CI and HAI
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BKBs Keidser, Ching, et al 2002
13 per dB
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Directional microphones
H. Dillon NAL, CRC for CI and HAI
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Directional microphones
Benefit 5 dB improvement in SNR
50 improvement in speech understanding
Omni-directional
Directional 5 dB SNR
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Directional microphone
Front port tube
Rear port tube
Diaphragm
Acoustic damper
Hearing aid case
Figure 2.6 Diagram showing the sound paths in a
directional microphone.
Source Dillon (2001) Hearing Aids
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Polar diagrams
Front
H. Dillon NAL, CRC for CI and HAI
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Directional microphone
H. Dillon NAL, CRC for CI and HAI
Front
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Directional microphone
Front
H. Dillon NAL, CRC for CI and HAI
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Correcting microphone drift
Front
?
-

?
T
Adaptor
Output
Source Dillon (2001) Hearing Aids
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90
Cardioid on head
60
120
Omni on head
150
30
-10
-20
-30
0
180
330
210
300
240
270
Figure 2.8 Directional sensitivity of (a) an
omnidirectional (solid line) and (b) a
directional (dotted line) microphone, mounted on
the head at 2 kHz. Data adapted from Knowles,
TB21.
Source Dillon (2001) Hearing Aids
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Directivity index

• Variations
• 2-D metric
• derived 3-D metric
• measured 3-D metric

Front
Directivity index
AIDI DI weighted across frequency by
Articulation Index weights
H. Dillon NAL, CRC for CI and HAI
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Directivity indexes
Source Dillon (2001) Hearing Aids
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Link-It
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What DI is needed for normal hearing in noise?
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Types of directional arrays
Fixed versus adaptive
Additive versus subtractive
End-fire versus broadside
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Super-directional microphone
Second-order subtraction
d
Front
-

?
T
H. Dillon NAL, CRC for CI and HAI
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45 mm
Source Raicevich (1996)
H. Dillon NAL, CRC for CI and HAI
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Directivity index - hand-held
H. Dillon NAL, CRC for CI and HAI
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Subtractive array
-
Directional at low and mid freq -
Directivity limited to 6 dB - Low-frequency
cut - Higher internal noise

?
T
Output
Source Dillon (2001) Hearing Aids
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Source Dillon (2001) Hearing Aids
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Widrow LMS noise reduction
Speech Noise

?
Delay
-
Noise
Figure 7.5 The Widrow Least Mean Squares
adaptive noise reduction scheme, based on a
reference microphone that picks up only the
noise. The fixed delay compensates for the delay
inherent in the adaptive filter.
Source Dillon (2001) Hearing Aids
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Two-sided array Griffiths Jim adaptive noise
reduction
Right

?


?
-
Left
Source Dillon (2001) Hearing Aids
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Microphone array benefit
Source Hoffman et al (1994)
H. Dillon NAL, CRC for CI and HAI
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Reverberation
Total field
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Reverberation plus noise
Total field
Noise
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Directional mics - when and whereQuestion
3 What are the characteristics of the everyday
listening situations that favor either the OMNI
or the DIR microphone mode?

• Answer Environmental characteristics that
  favor
• Omnidirectional Directional
  
  
  
• Talker located other than front Talker located
  in front
• Background noise absent, very Moderate
  background noise
  soft, or very loud present
• In cars or out-of-doors Talker distance
  lt 20 feet
• Smaller rooms Larger Rooms
• 
  People as background noise

B Walden
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Demonstrating directional advantage
S
N
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Demonstrating directional advantage

• Start with directional mic off
• Use continuous discourse
• Use a SNR of approx -3 dB
• SNR can be set adaptively
• a few words here and there
• Switch on directional mics

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Directional microphone
Speech
Demos
Noise
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Frontal frequency response
Figure 7.2 Frontal sensitivity of a two-port (or
two-microphone) subtractive directional
microphone relative to the sensitivity of an
equivalent single-port microphone. The parameter
shown is the port spacing. The internal delay
needed to produce a cardioid polar response has
been assumed.
Source Dillon (2001) Hearing Aids
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Combined amplified and vent-transmitted sound
paths
Source Dillon (2001) Hearing Aids
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Thats all folks
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Adaptive noise suppression

• (For comfort)

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Adaptive noise suppression
6 dB
12 dB
6 dB
12 dB
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Noise Reduction
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Spectral Subtraction
Phase

Magnitude
?
F.T.
I.F.T.
-
Avg noise Spectrum
Switch
Averager
Speech/non-speech detector
Figure 7.10 A Spectral Subtraction noise
reduction system incorporating a Fourier
Transform to calculate the power spectrum, a
speech/non-speech detector to enable the average
spectral power of the noise to be estimated, and
an Inverse Fourier Transform to turn the
corrected spectrum back into a waveform.
Source Dillon (2001) Hearing Aids
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Wireless systems

• (for super intelligibility)

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Wireless / Direct audio input
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Wireless microphone
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Wireless microphone
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Wireless microphone
Babble
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Wireless microphone
Babble
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Summary Possible solutions to noise

• Move closer
• Electronic filtering
• Directional microphones
• FM (wireless) systems

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More information?

• About NALs research
• www.nal.gov.au
• About the book Hearing Aids
• www.boomerangpress.com.au

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Super-directional microphone
500 Hz Subtractive
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Super-directional array

• Head diffraction

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Super-directional array

• Head diffraction

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Superdirectional microphone
Noise
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Superdirectional microphone
Noise
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Superdirectional microphone
Noise