HIS 240 - Dir Mics and Dig Noise Reduction 2011

Directional Mics & Digital Noise Reduction

Ted Venema PhDConestoga College

Speech in Noise+

Hearing Aids=

Problems in Noise

Why?

Two things we must do for hearing loss:

1. Improve audibility

2. Improve signal-to-noise ratio

Hair Cells: A Closer Look

Inner hair cells

Outer hair cells

Picture from Australian HearingHear & Now, issue 4, 1998

Normal Outer Hair Cells

From Yost, WAFundamentals of Hearing(1994)

When Stereocilia Are Gone, Cells Often Gone

From Yost, WAFundamentals of Hearing(1994)

An Outdated View of the Organ of Corti

IHC OHC OHC OHC

Afferentfibers Efferent

fibers

Tectorial Membrane

A Simple View of the Organ of Corti

Basilar MembraneFig 1-3, Venema, T. Compression for Clinicians 2nd edition, Thomson Delmar Learning 2006

NormalNormal Inner Inner

& & Outer Outer

Hair CellsHair Cells

Fig 1-7, Venema, T. Compression for Clinicians 2nd edition, Thomson Delmar Learning 2006

DamagedDamagedHair Hair CellsCells

(mostly(mostlyouter)outer)


Hearing Aids?They

don’t

grow

new

hair

cells

Note how embedded

OHCs actually pull tectorial membrane

down

Outer Hair Cells: The Outer Hair Cells: The ActiveActive Cochlear Cochlear MechanismMechanism

Here’s a “Passive” Traveling Wave

BaseApexApex

Basilar Membrane

A wave without outer hair cells


BaseApex

Basilar Membrane

Outer Hair Cells Sharpen the Peak!They are the “muscles” of the cochleaThey usually get damaged first

Lows Highs

Outer Hair Cell Contributions to the Traveling Wave1. Amplify2. Sharpen

BasilarMembrane Displacement

BasilarMembrane

TravelingWave

Envelope ofTraveling Wave

OHCsSharpenPeak


This is why hearing aids for ears aren’t like glasses for eyesThis is why hearing aids for ears aren’t like glasses for eyes

Natural shape of fluid wave:2 peaks from 2 tones close in Hz

Hair cell damage results in:smaller rounded peaks

Hearing aids make wave bigger:but cannot sharpen it

It’s the “sharpening” thing that’s the main challenge


Loss of outer hair cells•dulls the traveling wave peak•soft sounds no longer naturally amplified

Loss of inner hair cells•mixed speech & noise sent on to brain•hearing aids make mixed up sound louder

Hair cell damageHair cell damage& &

Speech in NoiseSpeech in Noise

Hearing Aids MakeSoft Compromised Sound

Into LouderCompromised Sound

Directional mics•objectively improve speech/noise performance

Digital noise reduction •subjectively enhances comfort in noise

Solutions for Speech in NoiseSolutions for Speech in NoisePresently, there are two...Presently, there are two...

DNR does not at this timeDNR does not at this time

Objectively, reliably, statistically

Improve speech reception in background noise

DNR does improveDNR does improve

Subjective listening comfort in noise

On the other hand,On the other hand,

If you can bring a hearing aid to the speaker’s lips

You increase the signal-to-noise ratio (SNR)

This does improve speech reception in background noise!

For example, an FM system…For example, an FM system…

Isn’t that exactly what it does??

So does a directional microphone, but to a lesser extent

For those with normal For those with normal hearing...hearing...

Speech & noise have to be of similar intensity•to understand 50% of the speech

Signal to Noise Ratios (SNRs) to get 50%•may differ from lab to lab…

But for some performance for some test•let’s call this “X”

An additional 5 dB SNR is required •for person to “get” 50% speech in noise

Magic number is around “5”•directional mics try to hit this number

Each additional 1 dB SNR•results in up to 10% speech improvement

For mild-to-moderate SNHL, For mild-to-moderate SNHL, however...however...

HL (PTA) SNR Required30 440 550 660 770 980 1290 18

SNRs for Various Degrees of HLSNRs for Various Degrees of HLKillion, Hearing Review, Dec 1997Killion, Hearing Review, Dec 1997

Directional Microphones: Directional Microphones: Old & NewOld & New

ConventionalConventional•1 mic, 2 ports•on BTEs only •often no “on/off”

NewerNewer•2 omni mics•found on ITEs•routinely “on/off”

Directional Microphones Directional Microphones

In Any MicrophoneIn Any MicrophoneSound Moves Diaphragm

Diaphram

Source

Diaphram

Source Source

Sounds Hitting Both SidesSounds Hitting Both Sides Cancel Each Other Out

Front Rear

Time Delay

Directional Mic FunctionDirectional Mic Function When Sounds Come From Front

Directional Microphone FunctionDirectional Microphone Function When Sounds Come From Front…

)

)

)

The Diaphragm moves

)

)

Filter

RearFront Direction of incoming sound


D )))))

From the Front

D )))))

From the Front

Diaphragm moves

D )

From the Front

Sound From BehindSound From BehindSound is cancelled by hitting both sides of diaphragmSound is cancelled by hitting both sides of diaphragm

Front Rear

Acoustic Time Delay Network

Directional Microphone FunctionDirectional Microphone Function When Sounds Come From Rear…

)

The Diaphragm cannot move

)

RearFront

((

((

( (((

Filter slows sound

Direction of incoming sound


From Behind

• (((((

• (((((

(

From Behind

• ((((()

From Behind

• (((((

)

From Behind

Diaphragm does not move

Directional Microphones & Hz ResponseDirectional Microphones & Hz Response

Many directional mics have several positions•omni-directional•directional equalized to omni-directional response•directional with low-cut response


Some PerfectlyRoundedSymmetricalPolar Plots:

OmnidirectionalCardioidSupercardioidHypercardioid

Directional Microphones: Directional Microphones: Polar PlotsPolar Plots


Directional index (DI)•ratio of sensitivity to front sounds•compared to other surrounding sounds

Omnidirectional mics don’t favor front sound•they have DI of 0 dB

Hearing aid directional mics •try to achieve a DI of about 5

Quantifying polar plotsQuantifying polar plots

Most important Hz’s for understanding speech •1kHz - 4kHz•most important of these Hz’s is 2kHz

Can simply take average DI’s of 4 polar plots•to come up with overall DI for speech

Can have great DI’s for 125 Hz, but...Can have great DI’s for 125 Hz, but...Speech Hz’s are the bottom lineSpeech Hz’s are the bottom line

Articulation IndexArticulation Index

Mueller&

Killion,1990


•To arrive at a single number in dB•Shows SNR improvement for speech

AI-DI: AI-DI: Different WeightsDifferent WeightsApplied to the DI at each HzApplied to the DI at each Hz

Eg: 500Hz = 20% (5.5) = 1.11000Hz = 23% (4.5) = 1.042000Hz = 33% (4.0) = 1.324000Hz = 24% (3.5) = .84

AI-DI = 4.3AI-DI = 4.3

Basically, however…Basically, however…

Can simply add the DI’s from the 4 polar plots•divide by 4 and get a simple average DI•who would recognize the difference!!!

Weighting method: 500Hz = 20% (5.5) = 1.11000Hz = 23% (4.5) = 1.042000Hz = 33% (4.0) = 1.324000Hz = 24% (3.5) = .84

AI-DI = 4.30AI-DI = 4.30

Average Method: 500Hz = 25% (5.5) = 1.3751000Hz = 25% (4.5) = 1.1252000Hz = 25% (4.0) = 1.04000Hz = 25% (3.5) = .875

AI-DI = 4.375AI-DI = 4.375

In General, DI’s Are Used To:In General, DI’s Are Used To:

Predict the SNR improvement for speechPredict the SNR improvement for speech

That might be provided That might be provided

By a specific directional microphoneBy a specific directional microphone

AI-DI•different importance given to DI at each Hz•gives a single number in dB•shows SNR improvement for speech

e.g. AI-DI of 4 dB•compared to typical omni-directional mic•can result in up to 40% better speech scores•actually, even if 20% that’s good!

Putting it all together - for speechPutting it all together - for speech

Dmics & Vents (Fabry 2007, HR Products Report)

Open-fit BTEs•Provide minimal occlusion with .8mm tube fitting•Or with > 3mm vent in mold

Venting:•Reduces low-Hz gain where Dmics most effective

Open-fit BTEs with vented molds•Reduce gain below 750H

Open-fit BTEs with non-occluding molds•Reduce gain below 1500Hz

Acoustic Effects of Open Fit Thin Tube BTEs

Courtesy Bernafon

Vent = 2 way street; sound goes in & out

Thin Tube Effect

Courtesy Bernafon

Smaller diameter reduces high Hz resonance peaksMore electronic gain therefore needed

Amplified sound & Vent Interactions

Courtesy Bernafon

Open fit can give pleasing mixture of in/out sounds

But at similarlevels thesedon’t mix well

Leads to Doubling orCancelling

“Comb filter”effect

Dmics & Vents Cont’d…

In Dual mic Dmics:•Signal subtraction process produces internal mic noise•Closer mic spacing also increases internal noise, but•Also increases high-Hz directionality

Normal Dmic spacing of about 10mm: •Gives compromise b/w noise & high-Hz directionality

Less spacing (5-8mm) increases high-Hz directionality•From about 4000 to 6500Hz

Summary:•Less Dmic effect, but higher Hz’s of directionality

Adaptive DirectionalityUsed with twin omni-directional mics

Can automatically shift from Omni to Dmic •Depending on listening environment

Can automatically switch among various polar plots•Depending on listening situation

Can shift polar plot nulls to origin of noise•Depending on noise source direction

Not necessarily statistically better…•But has advantages; for those with poor manual dexterity•Those who cannot tell when to use what feature

ReSound’s “Asymmetrical Dmic FittingBinaural

Omni-directional:Not very intelligible

Binaural Directional:

Decrease audibilityFor side & behind

Asymmetric Fitting:

Side & behind audibleCan shift attention

to anyone

Beamforming: Beamforming: Making Directional Mics Better YetMaking Directional Mics Better Yet

Mics with more than 2 ports•eg. 3 or more mics

This is Killion’s ArrayMicTM

•heart is in right place•DI’s are about 7-10dB!

But would you wear it?

Photo provided Courtesy M. Killion

Photo provided Courtesy M. Killion

Beamforming: An Example

1. Directional mic transmitter

2. Ear-level FM receiver

Digital Noise Reduction

Noise reduction: Implementation fraught with flaws

Problem: Speech & noise are mixed together

3600

2400

1200

00 300 0 300 0 300

Time in msec

Hz

/ba/ /da/ /ga/

Critical Speech Cues on Spectrogram


Speech in Quiet


Speech in Noise


Types of Digital Noise Reduction

•Spectral subtraction

•Phase cancellation

•Spectral enhancement

•Speech synthesis

Spectral Subtraction1. Spectrum of Speech & noise together is measured

2. During pauses in conversation, spectrum of noise estimated

3. Spectrum of speech & noise minus spectrum of noise

4. Theoretically, this leaves just spectrum of speech

Problem:• Noise spectrum is wide; intersects with speech spectrum• So removing noise removes some of speech

50

75

Frequency (kHz).1 1. 10

1. Speech Plus Noise Spectrum…Speech with its 6db/octave roll-offNarrow bands of noise

dB

SP

L

50

75


2. Noise Spectrum b/w Pauses of Speech…d

B S

PL

50

75


dB

SP

L3. Subtract Noise Measured During Speech Pauses

From Speech + Noise SpectrumLeaves This Speech Spectrum…

50

75


dB

SP

LThis Speech Spectrum…Really Isn’t Too Badly Altered from Original

50

75


Problem is, Noise Spectrum is Often Wide…Intersects with Wide Speech Spectrum…So, this combined Speech + Noise Spectrum…

dB

SP

L

50

75


Minus the Wide Noise Spectrum…Measured during pauses in speech

dB

SP

L

50

75


dB

SP

LSubtract Noise Measured During Speech Pauses

From Speech + Noise SpectrumLeaves Only This Speech Spectrum…

50

75


Former Wide Speech Spectrum Now Badly Altered…d

B S

PL

Phase Cancellation1. Exact time waveform of noise is measured

2. Inverted noise phase added to original noise waveformcancels noise

This phase

+

Opposite phase

=

Because in headphones:• speech sent directly to eardrum from headphone• noise sampled by microphone outside of headphone

Digital hearing aids:• do not have this luxury• both speech & noise picked up by outside microphone

Phase Cancellation used in noise reduction headphones…

Why not in hearing aids?

Why Phase Cancellation Can Work in Headphones But Not in Hearing Aids

Speech enters directly from headphone

Noise from outsideleaks into ear canal and mixeswith speech

Noise from outsidepicked up by microphone andinverted in phase THIS CANCELS OUT THE NOISE

Spectral Enhancement

1. Digital algorithm detects spectral speech cues in noise

• such as vowel formants or high-Hz sibilants

2. Deliberately enhance or amplify these spectral speech cues

• Just a different approach from noise reduction

Challenge for Spectral Enhancement, however is the high-Hz Consonants

In noise:valleys b/w peaks of speech are filled w/noisepeaks thus less prominent

Low-Hz vowels more intenseso these still stand out; easier to enhance these

Speech Synthesis

1. Digital algorithm detects spectral speech cues in noise

2. Once a particular speech sound is detected• then adds a similar synthesized speech sound

Requires a stored collection of speech sounds• difficult to digitally recognize speech sounds• complexity overwhelming• synthesized speech can sound unnatural

Today’s Digital Hearing Aids Use:

A weak form of Spectral Subtractionan amplitude modulation approachsometimes a combo of Hz & duration modulation too

Subtracting noise spectrum from noise + speech spectrumremoves too much speech

DSP algorithms characterize waveforms in each Channelnoise has fairly flat waveform over timespeech waveform fluctuates rapidly

If noise sensed in a channelthen gain reduced some 5-20dB

For Speech:Mean Intensity is Not in Middle of Range

25

50

75


Long Term Average Speech Spectrum

dB

SP

L

THIS IS BECAUSE SPEECH HAS ABNORMAL DISTRIBUTION OF INTENSITY

Noise Reduction Noise Reduction Most Digital Hearing Aids Use It...Most Digital Hearing Aids Use It...

Sounds that don’t change in intensity are reduced

Sounds that change in intensity (speech) are not reducedFig 7-13, Venema, T. Compression for Clinicians 2nd edition, Thomson Delmar Learning 2006

How often(eg. %)

the intensityof thesound is at some

particular dB level

Speech (single talker)

Decibel Level

Noise

Speech is a weird sound…Speech is a weird sound…It has an odd distribution of intensityIt has an odd distribution of intensity

125 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000

Gai

n (d

B)

Noise Reduction with One Channel…Noise Reduction with One Channel…

J m d b i

u

a

zv

rn onge l

ph gch

sh

k

fs

th

125 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000

Gai

n (d

B)

Noise Reduction with One Channel…Noise Reduction with One Channel…Reduces Gain Over All Speech Hz’s!

All Speech Sounds DropAll Speech Sounds DropJ m d b i

u

a

zv

rn onge l

ph gch

sh

k

fs

th


125 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000

Gai

n (d

B)

Noise Reduction with Noise Reduction with Two Channels isn’t much Two Channels isn’t much

betterbetter

J m d b i

u

a

zv

rn onge l

ph gch

sh

k

fs

th

125 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000

Gai

n (d

B)Noise reduction wouldNoise reduction would reduce reduce

gain over 1/2 Speech Hz’s!gain over 1/2 Speech Hz’s!

Vowels Would DropVowels Would Drop

Consonants Would NotConsonants Would NotJ m d b i

u

a

zv

rn onge l

ph gch

sh

k

fs

th

125 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Center Hz of Each Band

Gai

n (d

B)

Noise reduction with lots of bands/channels...Noise reduction with lots of bands/channels...

J m d b i

u

a

zv

rn onge l

ph gch

sh

k

fs

th

125 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Center Hz of Each Band

Gai

n (d

B)

Would reduce gain over smaller Hz regionsWould reduce gain over smaller Hz regions

J m d

ua

zv

r

n nge l

ph

g

chsh k

f

s

th

o

b i


Digital Noise ReductionDigital Noise Reduction++

Directional Microphones Directional Microphones

Noise reduction algorithmsNoise reduction algorithms•give subjective comfort to client

Directional microphonesDirectional microphones•gives objective improvement in speech reception

Together they make a good teamTogether they make a good team•a twin-headed approach to speech in noise

ConclusionConclusion

The cochlea is a fascinating place And there’s lots more to learn

HIS 240 - Dir Mics and Dig Noise Reduction 2011

Technology

Transcript of HIS 240 - Dir Mics and Dig Noise Reduction 2011