Speech-in-Noise & Directional Mics Directional microphone technology wasinvented by the military during WorldWar II. It’...
Speech-in-Noise & Directional MicsHearing instruments designed with thistechnology would, theoretically, provide abetter l...
Speech-in-Noise & Directional Mics As HI dispensing professionals, there aretwo things we must do for the hearingimpaired...
Speech-in-Noise & Directional Mics The amplified gain of hearing instruments isused to provide the audibility of sound fo...
Hearing Instruments MakeSoft Compromised Sound
Into LoudCompromised Sound
Speech-in-Noise & Directional Mics Directional microphones have proven toprovide a greater opportunity of speechrecogniti...
Speech-in-Noise & Directional MicsDIRECTIONAL MICROPHONES They do not increase the sound input fromthe front of the liste...
Speech-in-Noise & Directional MicsConventional•1 mic, 2 ports•on BTEs only•often no “on/off”Newer•2 omni mics•found on ITE...
In Any MicrophoneSound Moves DiaphragmDiaphramSource
DiaphramSource SourceSounds Hitting Both SidesCancel Each Other Out
Directional Microphone FunctionWhen Sounds Come From Front…)))The Diaphragm moves))FilterRearFront Direction of incoming s...
D)))))From the Front
D)))))From the FrontDiaphragm moves
D)From the Front
D)From the Front
Sound From BehindSound is cancelled by hitting both sides ofdiaphragmFront RearAcoustic Time Delay Network
Directional Microphone FunctionWhen Sounds Come From Rear…)The Diaphragm cannotmoveRearFront(((((((Filter slows soundDirec...
From Behind (((((
• ((((((From Behind
• ((((((From Behind
• ((((((From Behind
• ((((()From Behind
• ((((()From BehindDiaphragm doesnot move
Speech-in-Noise & Directional MicsDIRECTIONAL MICROPHONESWith this fundamental front—back operation,it was found that the ...
Speech-in-Noise & Directional MicsDIRECTIONAL MICROPHONESThey do not restorenormal hearingability—theystimulate theresidua...
Speech-in-Noise & Directional MicsDIRECTIONAL MICROPHONESThere is currently new and moreexpensive directional technology w...
Speech-in-Noise & Directional MicsDIRECTIONAL MICROPHONES ANSI has established a standard formeasurement of their effecti...
Speech-in-Noise & Directional MicsPOLAR PLOTS The directional performance graphs/chartsare represented as Polar Plots. D...
Speech-in-Noise & Directional MicsPOLAR PLOTS An increased DI number represents theeffectiveness of directional microphon...
Speech-in-Noise & Directional MicsPOLAR PLOTS These reduced intensity areas arerepresented as indentations into the perfe...
Speech-in-Noise & Directional MicsSomePerfectlyRoundedSymmetricalPolarPlots:OmnidirectionalCardioidSupercardioidHypercardi...
Speech-in-Noise & Directional MicsDIRECTIONAL MICROPHONES Most of today’s typical directionalmicrophones generally repres...
Speech-in-Noise & Directional MicsDIRECTIONAL MICROPHONES Their effectiveness is, of course, influencedby the frequencies...
Speech-in-Noise & Directional MicsDI and Frequencies The most important frequencies forunderstanding speech are: 1kHz - 4...
Speech-in-Noise & Directional MicsARTICULATION INDEX To determine the best performance forspeech understanding, dots with...
Speech-in-Noise & Directional MicsARTICULATION INDEX There are one hundred dots, with each dotrepresenting a one percent ...
Speech-in-Noise & Directional MicsARTICULATION INDEX
Speech-in-Noise & Directional MicsSPEECH UNDERSTANDING IN NOISE Normal hearing allows for an understandingof speech-in-no...
Speech-in-Noise & Directional MicsSPEECH UNDERSTANDING IN NOISEIn 1997, Meade Killion’s research discoveredthat with every...
Speech-in-Noise & Directional MicsCertainly, the five to six decibelimprovement in the signal-to-noise ratioexhibited by t...
• Directional microphones objectivelyimprove speech/noise performance• Digital noise reduction subjectivelyenhances comfor...
Adaptive DirectionalityThis technology is designed for the use of twin omni-directional microphones. It can:• Automaticall...
Speech-in-Noise & Directional Mics Adaptive directionality created withdual microphones has not shown anyfurther improvem...
Beam forming:Making Directional Mics Better YetMics with more than 2 ports•eg. 3 or more micsThis is Killion’s ArrayMicTM•...
Photo providedCourtesy M. Killion
1. Directional mictransmitter2. Ear-levelFM receiver
Speech-in-Noise & Directional MicsDIGITAL NOISE REDUCTION•Spectral subtraction•Phase cancellation•Spectral enhancement•Spe...
Noise reduction: Implementation fraught with flawsThe problem: Speech & noise are mixed together
36002400120000 300 0 300 0 300Time in msecHz/ba/ /da/ /ga/Critical Speech Cueson Spectrogram
Speech in Quiet
Speech in Noise
Spectral Subtraction1. The spectrum of speech & noise together is received2. During pauses in conversation, the spectrum o...
5075Frequency (kHz).1 1. 101. Speech Plus Noise Spectrum…Speech with its 6db/octave roll-offNarrow bands of noisedBSPL
5075Frequency (kHz).1 1. 102. Noise Spectrum b/w Pauses of Speech…dBSPL
5075Frequency (kHz).1 1. 10dBSPL 3. Subtract Noise Measured During Speech PausesFrom Speech + Noise SpectrumLeaves This Sp...
5075Frequency (kHz).1 1. 10dBSPL This Speech Spectrum…Really Isn’t Too Badly Altered from Original
5075Frequency (kHz).1 1. 10Problem is, Noise Spectrum is Often Wide…Intersects with Wide Speech Spectrum…So, this combined...
5075Frequency (kHz).1 1. 10Minus the Wide Noise Spectrum…Measured during pauses in speechdBSPL
5075Frequency (kHz).1 1. 10dBSPL Subtract Noise Measured During Speech PausesFrom Speech + Noise SpectrumLeaves Only This ...
5075Frequency (kHz).1 1. 10Former Wide Speech Spectrum Now Badly Altered…dBSPL
Phase Cancellation1. Exact time waveform of noise is measured.2. Inverted noise phase added to the original noisewaveform ...
Because in the headphones:• Speech is sent directly to the eardrum fromthe headphone.• Noise is sampled by the microphone ...
Why Phase Cancellation Can Work in HeadphonesBut Not in Hearing AidsSpeech entersdirectlyfrom headphoneNoise from outsidel...
SpectralEnhancement1. A digital algorithm detects spectral speech cues innoise such as vowel formants or high-frequencysib...
The challenge for SpectralEnhancement, is the high-frequencyconsonants:In noise:• The valleys b/w peaks of speech are fill...
The challenge for Spectral Enhancement,is the high-frequency consonantsIn noise:The valleys b/w peaks of speech are filled...
Speech Synthesis1. The digital algorithm detects spectral speech cues innoise.2. Once a particular speech sound is detecte...
Today’s Digital Hearing Aids Use:A weak form of Spectral Subtraction is an amplitudemodulation approach; this sometimes is...
For Speech:Mean Intensity is Not in Middle of Range255075Frequency (kHz).1 1K. 10Long Term Average Speech Spectrum (LTASS)...
Noise ReductionMost Digital Hearing Aids Use It...Sounds that don’t change in intensity are reducedSounds that do change i...
How often(eg. %)the intensityof thesoundis atsomeparticulardB levelSpeech (single talker)Decibel LevelNoiseSpeech has an o...
125 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000Gain(dB)Noise Reduction with OneChannel…J m d b iu...
125 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000Gain(dB)Noise Reduction with OneChannel…Reduces Ga...
125 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000Gain(dB) Noise Reduction withTwo Channels.Isn’t mu...
125 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000Gain(dB)Noise reduction wouldreduce half of the ga...
125 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 70001 2 3 4 5 6 7 8 9 10 11 12 13 14 15Center Hz of Ea...
125 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 70001 2 3 4 5 6 7 8 9 10 11 12 13 14 15Center Hz of Ea...
Digital Noise Reduction+Directional MicrophonesNoise reduction algorithms•give subjective comfort to clientDirectional mic...
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HIS 240 - Speech In-Noise and Directional Mics

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HIS 240 - Speech In-Noise and Directional Mics

  1. 1. Speech-in-Noise & Directional Mics Directional microphone technology wasinvented by the military during WorldWar II. It’s concept was refined andminiaturized into hearing instrumentsin the late 1960’s.
  2. 2. Speech-in-Noise & Directional MicsHearing instruments designed with thistechnology would, theoretically, provide abetter listening experience for the hearingimpaired. However, the early microphonesproduced so much of their own noise whenoperating that they were of limited utility.
  3. 3. Speech-in-Noise & Directional Mics As HI dispensing professionals, there aretwo things we must do for the hearingimpaired patient/client:1.Improve audibility2.Improve signal-to-noise ratio
  4. 4. Speech-in-Noise & Directional Mics The amplified gain of hearing instruments isused to provide the audibility of sound forthe hearing impaired. However, amplified audibility does notalways provide a greater opportunity forspeech recognition in difficult listeningenvironments i.e. speech-in-noise ability.
  5. 5. Hearing Instruments MakeSoft Compromised Sound
  6. 6. Into LoudCompromised Sound
  7. 7. Speech-in-Noise & Directional Mics Directional microphones have proven toprovide a greater opportunity of speechrecognition in noisy environments. In fact, refinements in the 1990’s providedfor the remote control of the directionalmicrophone ability by the patient/client.
  8. 8. Speech-in-Noise & Directional MicsDIRECTIONAL MICROPHONES They do not increase the sound input fromthe front of the listener—they reduce theinput of the sound behind them. It is hoped that the desired input signal is infront of the patient/client and not behindthem!
  9. 9. Speech-in-Noise & Directional MicsConventional•1 mic, 2 ports•on BTEs only•often no “on/off”Newer•2 omni mics•found on ITEs•routinely “on/off”
  10. 10. In Any MicrophoneSound Moves DiaphragmDiaphramSource
  11. 11. DiaphramSource SourceSounds Hitting Both SidesCancel Each Other Out
  12. 12. Directional Microphone FunctionWhen Sounds Come From Front…)))The Diaphragm moves))FilterRearFront Direction of incoming sound
  13. 13. D)))))From the Front
  14. 14. D)))))From the FrontDiaphragm moves
  15. 15. D)From the Front
  16. 16. D)From the Front
  17. 17. Sound From BehindSound is cancelled by hitting both sides ofdiaphragmFront RearAcoustic Time Delay Network
  18. 18. Directional Microphone FunctionWhen Sounds Come From Rear…)The Diaphragm cannotmoveRearFront(((((((Filter slows soundDirection of incoming sound
  19. 19. From Behind (((((
  20. 20. • ((((((From Behind
  21. 21. • ((((((From Behind
  22. 22. • ((((((From Behind
  23. 23. • ((((()From Behind
  24. 24. • ((((()From BehindDiaphragm doesnot move
  25. 25. Speech-in-Noise & Directional MicsDIRECTIONAL MICROPHONESWith this fundamental front—back operation,it was found that the greater the separationbetween the two microphone openings(ports), the more effective the speech-in-noisebecame.
  26. 26. Speech-in-Noise & Directional MicsDIRECTIONAL MICROPHONESThey do not restorenormal hearingability—theystimulate theresidual ability ofthe patient/client.HEARING LOSS IS PERMANENT!
  27. 27. Speech-in-Noise & Directional MicsDIRECTIONAL MICROPHONESThere is currently new and moreexpensive directional technology whichinvolves two separate microphoneswithin the hearing instrument i.e. “dualmicrophone processing”.
  28. 28. Speech-in-Noise & Directional MicsDIRECTIONAL MICROPHONES ANSI has established a standard formeasurement of their effectiveness. It iscalculated as the Directivity Index (DI). The greater the DI, the more effective it isregarding the separation of the signal-to-noise.
  29. 29. Speech-in-Noise & Directional MicsPOLAR PLOTS The directional performance graphs/chartsare represented as Polar Plots. Directional performance polar plots begin byrepresenting a Directivity Index (DI) aszero—(no DI). It’s graphical appearance is represented as aperfect 360 degree circle on the polar plot.
  30. 30. Speech-in-Noise & Directional MicsPOLAR PLOTS An increased DI number represents theeffectiveness of directional microphoneactivity. The DI number increases, as polar plotsreveal null points (areas where sound isreduced in intensity).
  31. 31. Speech-in-Noise & Directional MicsPOLAR PLOTS These reduced intensity areas arerepresented as indentations into the perfectcircle. These indentations are referred to as thenull areas of intensity.
  32. 32. Speech-in-Noise & Directional MicsSomePerfectlyRoundedSymmetricalPolarPlots:OmnidirectionalCardioidSupercardioidHypercardioid
  33. 33. Speech-in-Noise & Directional MicsDIRECTIONAL MICROPHONES Most of today’s typical directionalmicrophones generally represent aDirectivity Index (DI) of about 5dB to6dB.
  34. 34. Speech-in-Noise & Directional MicsDIRECTIONAL MICROPHONES Their effectiveness is, of course, influencedby the frequencies it/they receive. In other words the directivity index (DI)varies by the different input frequencies. In fact, many algorithms for today’sinstruments have automatic reduction of lowfrequency inputs when directionalmicrophone activity is initiated.
  35. 35. Speech-in-Noise & Directional MicsDI and Frequencies The most important frequencies forunderstanding speech are: 1kHz - 4kHz (themost important of these is 2kHz). One can simply take average DI’s of 4polar plots to calculate the overall microphoneDI for hearing instrument performance.
  36. 36. Speech-in-Noise & Directional MicsARTICULATION INDEX To determine the best performance forspeech understanding, dots withinthe “speech banana” were created. This was to identify the most effectiveinput frequencies to be “processed” bydirectional microphones.
  37. 37. Speech-in-Noise & Directional MicsARTICULATION INDEX There are one hundred dots, with each dotrepresenting a one percent contributiontowards speech intelligibility. You will notice that most of the dots arelocated within the higher frequencies of the“speech banana”.
  38. 38. Speech-in-Noise & Directional MicsARTICULATION INDEX
  39. 39. Speech-in-Noise & Directional MicsSPEECH UNDERSTANDING IN NOISE Normal hearing allows for an understandingof speech-in-noise ability to be achieved fiftypercent of the time; this should occur whenthe speech signal intensity is equal to thenoise signal intensity.
  40. 40. Speech-in-Noise & Directional MicsSPEECH UNDERSTANDING IN NOISEIn 1997, Meade Killion’s research discoveredthat with every one decibel of improvement ofthe speech signal over the noise signal, a tenpercent improvement was realized for theability to better understand speech in noise.
  41. 41. Speech-in-Noise & Directional MicsCertainly, the five to six decibelimprovement in the signal-to-noise ratioexhibited by today’s directionalmicrophones, can reflect a fifty to sixtypercent expected improvement inperformance for the patient/client—ifthey have residual hearing ability tostimulate.
  42. 42. • Directional microphones objectivelyimprove speech/noise performance• Digital noise reduction subjectivelyenhances comfort in noiseSolutions for speech understanding inNOISESpeech-in-Noise & Directional Mics
  43. 43. Adaptive DirectionalityThis technology is designed for the use of twin omni-directional microphones. It can:• Automatically shift from Omni to Dmic (Dependingupon listening environment).• Automatically switch among various polar plots(Depending on listening situation).• Shift polar plot nulls to origin of noise (Dependingon noise source direction).NOTE: It is not necessarily statistically better…Buthas advantages for those with poor manual dexterityand those who cannot tell when to use a feature or whatfeature should be used.
  44. 44. Speech-in-Noise & Directional Mics Adaptive directionality created withdual microphones has not shown anyfurther improvement in the SNR Multiple microphone (more than two)technology can provide greater than thefive to six decibels of signal-to-noiseratio improvement. However, is it practical?
  45. 45. Beam forming:Making Directional Mics Better YetMics with more than 2 ports•eg. 3 or more micsThis is Killion’s ArrayMicTM•heart is in right place•DI’s are about 7-10dB!Photo providedCourtesy M. Killion
  46. 46. Photo providedCourtesy M. Killion
  47. 47. 1. Directional mictransmitter2. Ear-levelFM receiver
  48. 48. Speech-in-Noise & Directional MicsDIGITAL NOISE REDUCTION•Spectral subtraction•Phase cancellation•Spectral enhancement•Speech synthesis
  49. 49. Noise reduction: Implementation fraught with flawsThe problem: Speech & noise are mixed together
  50. 50. 36002400120000 300 0 300 0 300Time in msecHz/ba/ /da/ /ga/Critical Speech Cueson Spectrogram
  51. 51. Speech in Quiet
  52. 52. Speech in Noise
  53. 53. Spectral Subtraction1. The spectrum of speech & noise together is received2. During pauses in conversation, the spectrum of noise isestimated.3. The spectrum of speech & noise is subtracted by thespectrum of noise only.4. Theoretically, this leaves just the spectrum of speech.Problem:• The wide noise spectrum intersects with the speechspectrum.• Thus, removing noise removes some of speech.
  54. 54. 5075Frequency (kHz).1 1. 101. Speech Plus Noise Spectrum…Speech with its 6db/octave roll-offNarrow bands of noisedBSPL
  55. 55. 5075Frequency (kHz).1 1. 102. Noise Spectrum b/w Pauses of Speech…dBSPL
  56. 56. 5075Frequency (kHz).1 1. 10dBSPL 3. Subtract Noise Measured During Speech PausesFrom Speech + Noise SpectrumLeaves This Speech Spectrum…
  57. 57. 5075Frequency (kHz).1 1. 10dBSPL This Speech Spectrum…Really Isn’t Too Badly Altered from Original
  58. 58. 5075Frequency (kHz).1 1. 10Problem is, Noise Spectrum is Often Wide…Intersects with Wide Speech Spectrum…So, this combined Speech + Noise Spectrum…dBSPL
  59. 59. 5075Frequency (kHz).1 1. 10Minus the Wide Noise Spectrum…Measured during pauses in speechdBSPL
  60. 60. 5075Frequency (kHz).1 1. 10dBSPL Subtract Noise Measured During Speech PausesFrom Speech + Noise SpectrumLeaves Only This Speech Spectrum…
  61. 61. 5075Frequency (kHz).1 1. 10Former Wide Speech Spectrum Now Badly Altered…dBSPL
  62. 62. Phase Cancellation1. Exact time waveform of noise is measured.2. Inverted noise phase added to the original noisewaveform cancels the noise.This phase+Oppositephase=
  63. 63. Because in the headphones:• Speech is sent directly to the eardrum fromthe headphone.• Noise is sampled by the microphone outsideof the headphone.Digital hearing aids:• Do not have this luxury.• As both the speech & noise inputs arepicked up by outside microphone.Phase Cancellation used innoise reduction headphones…Why not in hearing aids?
  64. 64. Why Phase Cancellation Can Work in HeadphonesBut Not in Hearing AidsSpeech entersdirectlyfrom headphoneNoise from outsideleaks into earcanal and mixeswith speechNoise from outsidepicked up bymicrophone andinverted in phase THIS CANCELS OUT THE NOISE
  65. 65. SpectralEnhancement1. A digital algorithm detects spectral speech cues innoise such as vowel formants or high-frequencysibilants.2. The algorithm deliberately enhances or amplifies thesespectral speech cues. This is just a different approachfrom noise reduction.
  66. 66. The challenge for SpectralEnhancement, is the high-frequencyconsonants:In noise:• The valleys b/w peaks of speech are filledw/noise the peaks are thus less prominent• The low-frequency vowels are more intense,so these still stand out--it is easier to enhancethese.
  67. 67. The challenge for Spectral Enhancement,is the high-frequency consonantsIn 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.
  68. 68. Speech Synthesis1. The digital algorithm detects spectral speech cues innoise.2. Once a particular speech sound is detected itthen 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 soundunnatural.
  69. 69. Today’s Digital Hearing Aids Use:A weak form of Spectral Subtraction is an amplitudemodulation approach; this sometimes is a combo offrequency 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 eachchannel the noise has a fairly flat waveform. Over timethe speech waveform fluctuates rapidly.If noise is sensed within a channel,then gain is reduced some 5-20dB.
  70. 70. For Speech:Mean Intensity is Not in Middle of Range255075Frequency (kHz).1 1K. 10Long Term Average Speech Spectrum (LTASS)dBSPLTHIS IS BECAUSE SPEECH HAS ABNORMAL DISTRIBUTION OF INTENSITY
  71. 71. Noise ReductionMost Digital Hearing Aids Use It...Sounds that don’t change in intensity are reducedSounds that do change in intensity (speech) are not reduced
  72. 72. How often(eg. %)the intensityof thesoundis atsomeparticulardB levelSpeech (single talker)Decibel LevelNoiseSpeech has an odd distribution of intensity
  73. 73. 125 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000Gain(dB)Noise Reduction with OneChannel…J m d b iuazvrn onge lph gchshkfsth
  74. 74. 125 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000Gain(dB)Noise Reduction with OneChannel…Reduces Gain Over All Speech Hz’s!All Speech Sounds DropJ m d b iuazvrn onge lph gchshkfsth
  75. 75. 125 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000Gain(dB) Noise Reduction withTwo Channels.Isn’t much better?J m d b iuazvrn onge lph gchshkfsth
  76. 76. 125 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000Gain(dB)Noise reduction wouldreduce half of the gain overthe Speech frequencies!Vowels Would DropConsonants Would NotJ m d b iuazvrn onge lph gchshkfsth
  77. 77. 125 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 70001 2 3 4 5 6 7 8 9 10 11 12 13 14 15Center Hz of Each BandGain(dB)Noise reduction with lots ofbands/channels...J m d b iuazvrn onge lph gchshkfsth
  78. 78. 125 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 70001 2 3 4 5 6 7 8 9 10 11 12 13 14 15Center Hz of Each BandGain(dB)Would reduce gain over smaller Hz regionsJ m duazvrnnge lphgchsh kfsthob i
  79. 79. Digital Noise Reduction+Directional MicrophonesNoise reduction algorithms•give subjective comfort to clientDirectional microphones•gives objective improvement in speech receptionTogether they make a good team!•a twin-headed approach to speech-in-noise

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