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Meena Ramani 04/10/06

EEL 6586 Automatic Speech Processing. Meena Ramani 04/10/06. 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 04/10/06

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  1. EEL 6586 Automatic Speech Processing Meena Ramani 04/10/06

  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. Lecture 1:The incredible sense of hearing “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. Why study hearing? • Best example of speech recognition • Mimic human speech processing • Hearing aids/ Cochlear implants • Speech coding

  5. Interesting facts • The stapes or stirrup is the smallest bone in our body. • It is roughly the size of a grain of rice ~2.5mm • Eardrum moves less than the diameter of a hydrogen atom • For minimum audible sounds • Inner ear reaches its full adult size when the fetus is 20-22 weeks old. • The ears are responsible for keeping the body in balance • Hearing loss is the number one disability in the world. • 76.3% of people loose their hearing at age 19 and over

  6. Specifications Frequency range: 20Hz-20kHz Dynamic range: 0-130 dB JND frequency: 5 cents JND intensity: ~1dB Size of cochlea: smaller than a dime

  7. A N A T O M Y

  8. Pinna /Auricle Outer ear Auditory Canal • Focuses sound waves (variations in pressure) into the ear canal • Pinna size: • Inverse Square Law • Larger pinna captures more of the wave • Elephants: hear low frequency sound from up to 5 miles away • Human Pinna structure: • Pointed forward & has a number of curves • Helps in sound localization • More sensitive to sounds in front • Dogs/ Cats- Movable Pinna => focus on sounds from a particular direction

  9. Pinna /Auricle Outer ear Auditory Canal • Horizontal localization Sound Localization • Vertical localization Is sound on your right or left side? Interaural Time Difference (ITD) Interaural Intensity Difference (IID)

  10. Interaural differences - The signal needs to travel further to more distant ear - More distant ear partially occluded by the head Two types of interaural difference will emerge - Interaural time difference (ITD) - Interaural intensity difference (IID)

  11. left right Illustration of interaural differences Left ear Right ear time sound onset

  12. Illustration of interaural differences Left ear Right ear time sound onset arrival time difference

  13. ongoing time difference Illustration of interaural differences Left ear Right ear time sound onset

  14. Illustration of interaural differences Left ear intensity difference Right ear time sound onset

  15. Thresholds Interaural time differences (ITDs) Threshold ITD  10-20 ms (~ 0.7 cm) Interaural intensity differences (IIDs) Threshold IID  1 dB

  16. D U P L E X T H E O R Y Interaural time differences (ITDs)  Low frequencies • Up to around 1500 Hz; sensitivity declines rapidly above 1000 Hz • Smallest phase difference corresponds to the true ITD Interaural intensity differences (IIDs)  High Frequencies • The amount of attenuation varies across frequency • below 500 Hz, IIDs are negligible (due to diffraction) • IIDs can reach up to 20 dB at high frequencies

  17. Pinna /Auricle Outer ear Auditory Canal • Horizontal localization Sound Localization • Vertical localization Is sound above or below? Pinna Directional Filtering • Pinna amplifies sound above and below differently • Curves in structureselective amplifies certain parts of the sound spectrum

  18. Pinna /Auricle Outer ear Auditory Canal • Closed tube resonance: ¼ wave resonator • Auditory canal length 2.7cm • Resonance frequency ~3Khz • Boosts energy between 2-5Khz upto 15dB

  19. A N A T O M Y

  20. Eardrum Middle Ear Ossicles Oval window Impedance matching • Acoustic impedance of the fluid is 4000 x that of air • All but 0.1% would be reflected back Amplification • By lever action < 3x • Area amplification [55mm2 3.2mm2] 15x Stapedius reflex • Protection against low frequency loud sounds • Tenses muscles stiffens vibration of Ossicles • Reduces sound transmitted (20dB) Pressure variations are converted to mechanical motion Eardrum OssiclesOval Window Ossicles: Malleus, Incus, Stapes

  21. A N A T O M Y

  22. Semicircular Canals Inner Ear Cochlea • Body's balance organs • Accelerometers in 3 perpendicular planes • Hair cells detect fluid movements • Connected to the auditory nerve

  23. Semicircular Canals Inner Ear Cochlea • Cochlea is a snail-shell like structure 2.5 turns • 3 fluid-filled parts: • Scala tympani • Scala Vestibuli • Cochlear duct (Organ of Corti) • Organ of Corti • Scala tympani • Scala vestibulli • Spiral ganglion • auditory nerve fibres

  24. Semicircular Canals Inner Ear Cochlea • Organ of Corti • Basilar membrane • Inner hair cells and outer hair cells (16,000 -20,000) • IHC:100 tiny stereocilia • The body's microphone: • Vibrations of the oval window causes the cochlear fluid to vibrate • Basilar membrane vibration produces a traveling wave • Bending of the IHC cilia produces action potentials • The outer hair cells amplify vibrations of the basilar membrane

  25. The cochlea works as a frequency analyzer It operates on the incoming sound’s frequencies

  26. 4mm2 1mm2 32-35 mm long Place Theory • Each position along the BM has a characteristic frequency for maximum vibration • Frequency of vibration depends on the place along the BM • At the base, the BM is stiff and thin (more responsive to high Hz) • At the apex, the BM is wide and floppy (more responsive to low Hz)

  27. Tuning curves of auditory nerve fibers • To determine the tonotopic map on Cochlea • Apply 50ms tone bursts every 100ms • Increase sound level until discharge rate increases by 1 spike • Repeat for all frequencies Response curve is a BPF with almost constant Q(=f0/BW)

  28. Auditory Neuron Carries impulses from both the cochlea and the semicircular canals Connections with both auditory areas of the brain Neurons encode • Steady state sounds • Onsets or rapidly changing frequencies Auditory Area of Brain

  29. Auditory Neurons Adaptation • At onset, auditory neuron fiber firing increases rapidly • If the stimulus remains (a steady tone for eg.) the rate decreases exponentially • Spontaneous rate: Neuron firings in the absence of stimulus Neuron is more responsive to changes than to steady inputs

  30. Perception of Sound Threshold of hearing • How it is measured • Age effects Equal Loudness curves Bass loss problem Critical bands Frequency Masking Temporal Masking

  31. Threshold of Hearing Hearing area is the area between the Threshold in quiet and the threshold of pain

  32. Bekesy Tracking • STEPS: • Play a tone • Vary its amplitude till its audible • Then tone’s amplitude is reduced to definitely inaudible and the frequency is slowly changed • Continu\e

  33. 4mm2 1mm2 32-35 mm long Threshold variation with age • Presbycusis • Hearing sensitivity decreases with age especially at High frequencies • Threshold of pain remains the same • Reduced dynamic range

  34. Equal Loudness Curves Loudness is not simply sound intensity! Factor of ten increase in intensity for the sound to be perceived as twice as loud.

  35. The Bass Loss Problem For very soft sounds, near the threshold of hearing, the ear strongly discriminates against low frequencies. For mid-range sounds around 60 phons, the discrimination is not so pronounced For very loud sounds in the neighborhood of 120 phons, the hearing response is more nearly flat. Eg. Rock music Too lowno bass Too hightoo much bass

  36. Elephants • Sound Production • A a typical male elephant’s rumble is around an average minimum of 12 Hz, a female's rumble around 13 Hz and a calf's around 22 Hz. • Produce sounds ranging over more than 10 octaves, from 5 Hz to over 9,000 Hz • Produce very gentle, soft sounds as well as extremely powerful sounds. (112dB recorded a meter away) • Hearing • Wider tympanic membranes • Longer ear canals (20 cm) • Spacious middle ears. Low frequency detection

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