busting myths about frequency lowering
TRANSCRIPT
Facts or myths about frequency lowering in hearing
aids
Michael Nilsson, Ph.D.
Head, Audiological Training and Education
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Who is Michael Nilsson?
26 years in hearing research and hearing device industry
Accomplishment
• Worked at four manufacturers, clinical
practice, university, research
• Adjunct faculty University of Utah,
Central Michigan University
Accomplishment
• Lead author of the Hearing In Noise
Test
• Co-author of two signal processing
patents
• Lectured on four continents
Degree
• BS, Cognitive Science…
• MS, Social Science
• PhD, Psychology
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Agenda
Fact or myth: Frequency lowering is a recent invention
Fact or myth: Frequency lowering can be accomplished by more than one
method
Fact or myth: Frequency lowering degrades sound quality
Fact or myth: Frequency compression contributes to sound deprivation
Fact or myth: Extended bandwidth devices remove the need for frequency
lowering
Fact or myth: Frequency lowering leads to dependence
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2
3
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1
Frequency lowering is a recent invention
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Fact or myth
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A history of frequency lowering
When did frequency lowering first get patented for the treatment of hearing
loss?
First patent: 1925 in the UK
When was the first commercially available product with frequency lowering?
First commercial product: 1972 Oticon TP72
When was the first commercially available digital product with frequency
lowering?
First digital commercial product: 2000 HD11 by RION
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Frequency lowering is a recent invention
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Fact or myth
2
Frequency lowering can be
accomplished by more than one method
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Fact or myth
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Kneepoint
Compression ratio
Frequency
transposition
Frequency
compression
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Frequency lowering Marketing names
2
Frequency lowering can be
accomplished by more than one method (or a hybrid of both)
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Fact or myth
3
Frequency lowering degrades sound
quality
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Fact or myth
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Perceptual dimensions relevant to Frequency Lowering
FrL default
FrL stronger
FrL weaker
Familiarity of low
and mid frequency
sounds
like \A\, \E\, \I\
Access of high
frequency
sounds
like \S\, \F\, \TH\
Familiarity of
high frequency
sounds
like \S\ & \SH\
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A trade off
Audible artifacts are more likely with more aggressive frequency
lowering
• Frequency transposition is more likely to generate artifacts than
compression because sounds are summed
• Creates dissonant sounds or side tones
More aggressive frequency lowering is meant for more severe losses
• The trade-off is between a less natural sound or not hearing
3
Frequency lowering degrades sound
quality
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Fact or myth
4
Frequency compression contributes to
auditory deprivation
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Fact or myth
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When are sounds audible?
Both fall below threshold
above 2000 Hz (speech
peaks are audible to about
2500 Hz – not shown)
Hearing Aids A and B are
both modern devices, and
are at maximum settings in
this region A severe sloping loss Hearing aid responses
and targets for speech
(input at 65 dB SPL)
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With a high frequency hearing loss… Deprivation at the cortex will occur
High Frequency information
cannot be processed
Information cannot be resolved due
to damaged regions of the cochlea
Information can be processed
Information can be resolved when
presented where hearing is still
possible
With Frequency lowering, more information gets to the cortex, just at
different frequencies… Deprivation at the highest frequencies will not be
changed
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Frequency lowering spectrograms
Original Signal Frequency lowering
High frequency HL
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Fitting recommendations
• Use a plurals test or logatome Test to determine whether high frequencies are
detectable
• If not, apply default frequency lowering
• Test for discrimination by comparing ‘s’ versus ‘sh’
• Increase frequency lowering as necessary for access and use of high frequency
content
• Adjust as necessary for sound quality while acclimatization occurs
4
Frequency compression contributes to
auditory deprivation
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Fact or myth
5
Extended bandwidth devices remove the
need for frequency lowering
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Fact or myth
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1 10 2 3 4 5 6 8
Frequency (kHz)
<<S>>
<<S>>
<<SH>>
<<SH>>
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Broadband response?
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Broadband response?
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Broadband response?
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Broadband response?
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Target Groups where Frequency lowering can be
implemented
Severe to Profound
• Audibility is the greatest gain
• Awareness is the biggest target
• Frequency lowering has the biggest potential to provide audibility with no
side effects if it doesn’t help
Mild to Moderate
• Extended bandwidth is often preferred, but distortion in damaged ear is
possible
– Not useful If high frequencies don’t sound tonal, or sound is harsh and
uncomfortable
• Target High Frequencies that are hard to reach with conventional
amplification
5
Extended bandwidth devices remove the
need for frequency lowering
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Fact or myth
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Frequency lowering leads to
dependence
6 Fact or myth
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The flexible auditory system
The auditory system is very flexible and adaptive
• We understand a wide range of voices
• People can communicate with even severe losses
It works very hard to compensate for any difficulty
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Speech variation in speech
‘The quick brown fox
jumped over the lazy dog’
• Two green lines mark 1500
and 4000hz
• The auditory system copes
with 1 octave variation all
the time without it sounding
unnatural
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With any change, the auditory system must learn to listen
again
Less information = dependence
More information = benefit
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Frequency lowering leads to
dependence (just like dependence on amplification)
6 Fact or myth
The need for acclimatization
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Use Frequency lowering when it adds audibility and clarity
• Variability across 1 octave is not a problem
• Will need some time to adjust
• Added audibility should eventually give listeners information
they can use to improve hearing
Shifts from 8 kHz to 3 kHz is >1 octave
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Agenda
Fact or myth: Frequency lowering is a recent invention
Fact or myth: Frequency lowering can be accomplished by more than one
method
Fact or myth: Frequency lowering degrades sound quality
Fact or myth: Frequency compression contributes to sound deprivation
Fact or myth: Extended bandwidth devices remove the need for frequency
lowering
Fact or myth: Frequency lowering leads to dependence
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2
3
4
5
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Together,
we change lives