his 240 - speech in-noise and directional mics

Speech-in-Noise & Directional Mics

Directional microphone technology was invented by the military during World War II.

It’s concept was refined and miniaturized into hearing instruments in the late 1960’s.


Hearing instruments designed with this technology would, theoretically, provide a better listening experience for the hearing impaired. However, the early microphones produced so much of their own noise when operating that they were of limited utility.


As HI dispensing professionals, there are two things we must do for the hearing impaired patient/client:

1.Improve audibility2.Improve signal-to-noise ratio


The amplified gain of hearing instruments is used to provide the audibility of sound for the hearing impaired.

However, amplified audibility does not always provide a greater opportunity for speech recognition in difficult listening environments i.e. speech-in-noise ability.

Hearing Instruments MakeSoft Compromised Sound

Into LoudCompromised Sound


Directional microphones have proven to provide a greater opportunity of speech recognition in noisy environments.

In fact, refinements in the 1990’s provided for the remote control of the directional microphone ability by the patient/client.


DIRECTIONAL MICROPHONESThey do not increase the sound input

from the front of the listener—they reduce the input of the sound behind them.

It is hoped that the desired input signal is in front of the patient/client and not behind them!


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

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

In Any MicrophoneSound Moves Diaphragm

Diaphram

Source

Diaphram

Source Source

Sounds Hitting Both Sides

Cancel Each Other Out

Directional Microphone Function When Sounds Come From Front…

)

)

)

The Diaphragm moves

)

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Filter

RearFront Direction of incoming sound

D )))))

From the Front

D )))))

From the Front

Diaphragm moves

D )

From the Front

Sound From BehindSound is cancelled by hitting both sides of

diaphragm

Front Rear

Acoustic Time Delay Network

Directional 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

With this fundamental front—back operation, it was found that the greater the separation between the two microphone openings (ports), the more effective the speech-in-noise became.


DIRECTIONAL MICROPHONESThey do not

restore normal hearing ability—they stimulate the residual ability of the patient/client.

HEARING LOSS IS PERMANENT!


DIRECTIONAL MICROPHONES

There is currently new and more expensive directional technology which involves two separate microphones within the hearing instrument i.e. “dual microphone processing”.


DIRECTIONAL MICROPHONESANSI has established a standard for

measurement of their effectiveness. It is calculated as the Directivity Index (DI).

The greater the DI, the more effective it is regarding the separation of the signal-to-noise.


POLAR PLOTSThe directional performance graphs/charts

are represented as Polar Plots. Directional performance polar plots begin

by representing a Directivity Index (DI) as zero—(no DI).

It’s graphical appearance is represented as a perfect 360 degree circle on the polar plot.


POLAR PLOTSAn increased DI number represents

the effectiveness of directional microphone activity.

The DI number increases, as polar plots reveal null points (areas where sound is reduced in intensity).


POLAR PLOTSThese reduced intensity areas are

represented as indentations into the perfect circle.

These indentations are referred to as the null areas of intensity.


Some PerfectlyRoundedSymmetricalPolar Plots:

OmnidirectionalCardioidSupercardioidHypercardioid


DIRECTIONAL MICROPHONESMost of today’s typical directional microphones generally represent a Directivity Index (DI) of about 5dB to 6dB.


DIRECTIONAL MICROPHONESTheir effectiveness is, of course, influenced

by the frequencies it/they receive. In other words the directivity index (DI)

varies by the different input frequencies. In fact, many algorithms for today’s

instruments have automatic reduction of low frequency inputs when directional microphone activity is initiated.


DI and FrequenciesThe most important frequencies for

understanding speech are: 1kHz - 4kHz (the most important of these is 2kHz).

One can simply take average DI’s of 4 polar plots to calculate the overall microphone DI for hearing instrument performance.


ARTICULATION INDEX To determine the best performance for speech understanding, dots within the “speech banana” were created.

This was to identify the most effective input frequencies to be “processed” by directional microphones.


ARTICULATION INDEXThere are one hundred dots, with

each dot representing a one percent contribution towards speech intelligibility.

You will notice that most of the dots are located within the higher frequencies of the “speech banana”.


ARTICULATION INDEX


SPEECH UNDERSTANDING IN NOISE

Normal hearing allows for an understanding of speech-in-noise ability to be achieved fifty percent of the time; this should occur when the speech signal intensity is equal to the noise signal intensity.


SPEECH UNDERSTANDING IN NOISE

In 1997, Meade Killion’s research discovered that with every one decibel of improvement of the speech signal over the noise signal, a ten percent improvement was realized for the ability to better understand speech in noise.


Certainly, the five to six decibel improvement in the signal-to-noise ratio exhibited by today’s directional microphones, can reflect a fifty to sixty percent expected improvement in performance for the patient/client—if they have residual hearing ability to stimulate.

• Directional microphones objectively improve speech/noise performance

• Digital noise reduction subjectively enhances comfort in noise

Solutions for speech understanding in NOISE


Adaptive Directionality

This technology is designed for the use of twin omni-directional microphones. It can: • Automatically shift from Omni to Dmic (Depending

upon listening environment). • Automatically switch among various polar plots

(Depending on listening situation). • Shift polar plot nulls to origin of noise (Depending

on noise source direction). NOTE: It is not necessarily statistically better…But has advantages for those with poor manual dexterity and those who cannot tell when to use a feature or what feature should be used.


Adaptive directionality created with dual microphones has not shown any further improvement in the SNR

Multiple microphone (more than two) technology can provide greater than the five to six decibels of signal-to-noise ratio improvement.

However, is it practical?

Beam forming: Making Directional Mics Better

YetMics 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!

Photo provided Courtesy M. Killion

Photo provided Courtesy M. Killion

1. Directional mic transmitter

2. Ear-level FM receiver


DIGITAL NOISE REDUCTION

•Spectral subtraction

•Phase cancellation

•Spectral enhancement

•Speech synthesis

Noise reduction: Implementation fraught with flaws

The problem: Speech & noise are mixed together

3600

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Time in msec

Hz

/ba/ /da/ /ga/

Critical Speech Cues on Spectrogram

Speech in Quiet

Speech in Noise

Spectral Subtraction

1. The spectrum of speech & noise together is received2. During pauses in conversation, the spectrum of noise is

estimated. 3. The spectrum of speech & noise is subtracted by the

spectrum of noise only.4. Theoretically, this leaves just the spectrum of speech. Problem:

• The wide noise spectrum intersects with the speech spectrum.

• Thus, removing noise removes some of speech.

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75

Frequency (kHz).1 1. 10

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

dB

SP

L

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

B S

PL

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dB

SP

L3. Subtract Noise Measured During Speech Pauses

From Speech + Noise SpectrumLeaves This Speech Spectrum…

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dB

SP

LThis Speech Spectrum…

Really Isn’t Too Badly Altered from Original

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Problem is, Noise Spectrum is Often Wide…Intersects with Wide Speech Spectrum…

So, this combined Speech + Noise Spectrum…

dB

SP

L

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Minus the Wide Noise Spectrum…

Measured during pauses in speech

dB

SP

L

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dB

SP

LSubtract Noise Measured During Speech Pauses

From Speech + Noise SpectrumLeaves Only This Speech Spectrum…

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Former Wide Speech Spectrum Now Badly Altered…

dB

SP

L

Phase Cancellation1. Exact time waveform of noise is measured.

2. Inverted noise phase added to the original noise waveform cancels the noise.

This phase+

Opposite phase

=

Because in the headphones:• Speech is sent directly to the eardrum from the headphone.

• Noise is sampled by the microphone outside of the headphone.

Digital hearing aids:• Do not have this luxury.• As both the speech & noise inputs are 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. A digital algorithm detects spectral speech cues in noise such as vowel formants or high-frequency sibilants.

2. The algorithm deliberately enhances or amplifies these spectral speech cues. This is just a different approach from noise reduction.

The challenge for Spectral Enhancement, is the high-frequency consonants:

In noise:• The valleys b/w peaks of speech are filled

w/noise the peaks are thus less prominent• The low-frequency vowels are more intense,so these still stand out--it is easier to enhance these.

The challenge for Spectral Enhancement, is the high-frequency consonants

In noise:The valleys b/w peaks of speech are filled w/noisepeaks thus, they become less prominent.

The low-frequency vowels are more intenseSo, these still stand out. It is easier to enhance these.

Speech Synthesis

1. The digital algorithm detects spectral speech cues in noise. 2. Once a particular speech sound is detected it then adds a similar synthesized speech sound.

NOTE: It requires a “stored collection” of speech sounds. This may result into:

• A difficulty to digitally recognize speech sounds.• Overwhelming complexity. • A synthesized speech sound which can sound unnatural.

Today’s Digital Hearing Aids Use: A weak form of Spectral Subtraction is an amplitude modulation approach; this sometimes is a combo of frequency and “time-of-duration” modulation.

By subtracting the noise spectrum from the noise + speech spectrum, it may remove too much speech.

DSP algorithms have characterized waveforms. In each channel the noise has a fairly flat waveform. Over time the speech waveform fluctuates rapidly.

If noise is sensed within a channel,then gain is reduced some 5-20dB.

For Speech:Mean Intensity is Not in Middle of Range

25

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Frequency (kHz).1 1K. 10

Long Term Average Speech Spectrum (LTASS)d

B S

PL

THIS IS BECAUSE SPEECH HAS ABNORMAL DISTRIBUTION OF INTENSITY

Noise Reduction Most Digital Hearing Aids Use It...

Sounds that don’t change in intensity are reduced

Sounds that do change in intensity (speech) are not reduced

How often(eg. %)

the intensityof thesound is at some

particular dB level

Speech (single talker)

Decibel Level

Noise

Speech has an odd distribution of intensity

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Gai

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Noise Reduction with One Channel…

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Noise Reduction with One Channel…

Reduces Gain Over All Speech Hz’s!

All Speech Sounds DropJ m d b i

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B)Noise Reduction with

Two Channels. Isn’t much better?

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B)Noise reduction would reduce half of the gain

over the Speech frequencies!

Vowels Would Drop

Consonants Would NotJ m d b i

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

Center Hz of Each Band

Gai

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Noise reduction with lots of bands/channels...

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Center Hz of Each Band

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Would reduce gain over smaller Hz regions

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Digital Noise Reduction+

Directional Microphones

Noise reduction algorithms• give subjective comfort to client

Directional microphones• gives objective improvement in speech reception

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

his 240 - speech in-noise and directional mics

Technology

noise directional micspolar

noise ability

noise whenoperating

noise ratio

loudcompromised sound

adirectivity index di

sound input fromthe

hearing loss