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Meena Ramani 04/12/06

EEL 6586 Automatic Speech Processing. Meena Ramani 04/12/06. Topics to be covered. Lecture 1: The incredible sense of hearing 1 Anatomy Perception of Sound Lecture 2: The incredible sense of hearing 2 Psychoacoustics Hearing aids and cochlear implants.

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Meena Ramani 04/12/06

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  1. EEL 6586 Automatic Speech Processing Meena Ramani 04/12/06

  2. Topics to be covered Lecture 1: The incredible sense of hearing 1 Anatomy Perception of Sound Lecture 2: The incredible sense of hearing 2 Psychoacoustics Hearing aids and cochlear implants

  3. The incredible sense of hearing-2 “Behind these unprepossessing flaps lie structures of such delicacy that they shame the most skillful craftsman" -Stevens, S.S. [Professor of Psychophysics, Harvard University]

  4. How do we hear?

  5. Threshold of Hearing

  6. Equal loudness curves

  7. The Bass Loss Problem Rock music Too lowno bass Too hightoo much bass

  8. Threshold variation with age

  9. The Audiogram

  10. The Audiogram (contd.) Pure tone audiogram [250 500 1K 2K 4K 6k] Hz <20 dB HL is Normal Hearing

  11. Loudness Growth Curve

  12. Otoacoustic emissions • The ear produces some sounds! • OHC-outer hair cell • Used to test hearing for infants & check if patient is feigning a loss

  13. 500 & 502 Hz 500 & 520 Hz Monoaural beats If two tones are presented monaurally with a small frequency difference, a beating pattern can be heard Interaction of the two tones in the same auditory filter Waveform: 150 Hz + 170 Hz

  14. 500 Hz - left 520 Hz - right binaural Binaural beats Beating can also be heard when the tones are presented to different ears! Beating arises from neural interaction Only perceived if the tones are sufficiently close in frequency

  15. The case of the missing fundamental Telephone BW: 300-3400 Hz How do we know the pitch? • Primary Auditory cortex • Pitch sensitive neurons [Bendor and Wang, Nature 2005] • Neuron responds to fundamental and harmonics • What are the I/Ps to these neuron? • How do spikes represent periodic, temporal and spectral information?

  16. Auditory-periphery model (Zhang et al. ~2001) Matlab code available Feed it a wav file Spits out PSTH <post stimulus time histogram>

  17. Critical bands • Equally loud, close in frequency • Same IHCs • Slightly louder • Equally loud, separated in freq. • Different IHCs • Twice as loud • Psychoacoustic experiments

  18. Critical Band (cont.) • Proposed by Fletcher • How to measure? • S/N ratio vs noise BW • CB ~= 1.5mm spacing on BM • 24 such band pass filters • BW of the filters increases with fc • Logarithmic relationship • Weber’s law example • Bark scale

  19. Critical bands for HI

  20. “You know I can't hear you when the water is running!” MASKING

  21. Frequency Masking • Masking occurs because two frequencies lie within a critical band and the higher amplitude one masks the lower amplitude signal • Masking can be because of broad band, narrowband noise, pure and complex tones • Low frequency broad band sounds mask the most • Eg. Truck on road, water flowing • Masking threshold • Amount of dB for test tone to be just audible in presence of noise

  22. Temporal Aspects of Masking • Simultaneous Masking • Pre-Stimulus/Backward/Premasking • 1st test tone 2nd Masker • Poststimulus/Forward/Postmasking • 1st Masker 2nd test tone

  23. Temporal Aspects of Masking (contd.) Simultaneous masking • Duration >200ms constant test tone threshold • Assume hearing system integrates over a period of 200ms Postmasking • Decay in effect of masker for 100ms • More dominant Premasking • Takes place 20ms before masker is on!! • Each sensation is not instantaneous , requires build-up time • Quick build up for loud maskers • Slower build up for softer maskers • Less dominant effect

  24. Temporal masking for HI

  25. EEL 6586 Automatic Speech Processing Meena Ramani 04/14/06

  26. What do the hearing impaired hear?

  27. Facts on Hearing Loss in Adults • One in every ten (28 million) Americans has hearing loss.  • The vast majority of Americans (95% or 26 million) with hearing loss can have their hearing loss treated with hearing aids.  • Only 6 million use HAs • Millions of Americans with hearing loss could benefit from hearing aids but avoid them because of the stigma.

  28. Types of Hearing aids Behind The ear In the Ear Completely in the canal In the Canal

  29. Anatomy of a Hearing Aid • Microphone • Tone hook • Volume control • On/off switch • Battery compartment

  30. Hearing Aid Fitting Ear Mold Measurements

  31. Acclimatization effect Auditory cortex brain plasticity Time for the HI to reuse the HF information: Acclimatization effect How does this affect HA fitting? • Multiple fitting sessions • Initial fitting should be optimum one

  32. So doc, what is the fitting methodology employed by the hearing aid company to compensate for my hearing loss? Not-so-average Joe (PhDEE/Speech person) CONFIDENTIAL ?

  33. So, do you want your HA to: • Always be comfortably loud • Equalize loudness across frequencies • Normalize loudness • …? ? Which fitting methodology is the best?

  34. Existing HL compensation algorithms Rationale • Adhoc: Half Gain, POGO • Make speech comfortable: NAL-R • Loudness normalization: IHAFF, Fig 6 • Loudness equalization: DSL

  35. Spectrograms and sound files Sensorineural hearing loss [10 20 30 60 80 90] dB HL Speech level= 65 dBA Section Two

  36. Performance metrics Speech Intelligibility Speech Intelligibility (SI): The degree to which speech can be understood Subjective Measures HINT Objective Measures AI, STI Speech Quality: “Does the speech match your expectations?” Speech Quality Objective Measures PESQ Subjective Measures MOS

  37. Performance metrics (contd.) • Objective speech quality measure • Perceptual Evaluation of Subjective Quality (PESQ) • Subjective speech quality measure • Mean Opinion Score (MOS) • Subjective speech intelligibility measure • Hearing In Noise Test (HINT) Reference signal Comparison signal Score

  38. Hearing In Noise Test (HINT)

  39. Location: Shands speech & hearing clinic (sound proof booth) Subjects: 15 HI people PTA: 40-70 dB HL 15 normal hearing people Tools used: Matlab HINT and MOS GUIs Subjective listening experiments Audiograms of the HI patients

  40. Subjective HINT and MOS scores for RBC:hearing impaired, cell phone speech RBC has a 7 dB improvement in SI when compared to DSL MOS scores reveal that RBC has a quality rating of ‘Good’

  41. Subjective HINT and MOS scores for RBC:normal hearing, cell phone speech RBC has a 12 dB improvement in SI when compared to DSL MOS scores reveal that RBC has a quality rating of ‘Good’

  42. Cochlear Implants The first fully functional Brain Machine Interface (BMI) Definition: A device that electrically stimulates the auditory nerve of patients with severe-to-profound hearing loss to provide them with sound and speech information

  43. Who is a candidate? • Severe-to profound sensorineural hearing loss • Hearing loss did not reach severe-to-profound level until after acquiring oral speech and language skills • Limited benefit from hearing aids

  44. Worldwide: Over 100,000 multi-channel implants At Univ of Florida: Implanted first patient in 1985 Currently follow over 400 cochlear patients CI statistics

  45. Technical and Safety Issues • Magnetic Resonance Imaging • Surgical issues

  46. How does the Cochlea encode frequencies?

  47. Example: New Freedom

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