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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 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] Why study hearing? • Best example of speech recognition – Mimic human speech processing • Hearing aids/ Cochlear implants • Speech coding 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 Specifications Frequency range: 20Hz-20kHz Dynamic range: 0-130 dB JND frequency: 5 cents JND intensity: ~1dB Size of cochlea: smaller than a dime A N A T O M Y 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 Pinna /Auricle Outer ear Sound Localization Auditory Canal Horizontal localization Vertical localization Is sound on your right or left side? Interaural Time Difference (ITD) Interaural Intensity Difference (IID) 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) Illustration of interaural differences Left ear Right ear time sound onset left right Illustration of interaural differences Left ear Right ear time sound onset arrival time difference Illustration of interaural differences Left ear Right ear time sound onset ongoing time difference Illustration of interaural differences intensity difference Left ear Right ear time sound onset Thresholds Interaural time differences (ITDs) Threshold ITD 10-20 ms (~ 0.7 cm) Interaural intensity differences (IIDs) Threshold IID 1 dB D U P Interaural time differences (ITDs) Low frequencies L • Up to around 1500 Hz; sensitivity declines rapidly above 1000 Hz E • Smallest phase difference corresponds to the true ITD X Interaural intensity differences (IIDs) High Frequencies T • 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 H E O R Y 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 structure selective amplifies certain parts of the sound spectrum Pinna /Auricle Outer ear •Closed tube resonance: ¼ wave resonator •Auditory canal length 2.7cm •Resonance frequency ~3Khz •Boosts energy between 2-5Khz upto 15dB Auditory Canal A N A T O M Y Eardrum Middle Ear Ossicles Oval window Pressure variations are converted to mechanical motion Eardrum OssiclesOval Window Ossicles: Impedance matching Malleus, Incus, Stapes – 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) A N A T O M Y Inner Ear Semicircular Canals Cochlea Body's balance organs Accelerometers in 3 perpendicular planes •Hair cells detect fluid movements •Connected to the auditory nerve Inner Ear Semicircular Canals Cochlea Cochlea is a snail-shell like structure 2.5 turns 3 fluid-filled parts: •Scala tympani •Scala Vestibuli •Cochlear duct (Organ of Corti) (1) Organ of Corti (2) Scala tympani (3) Scala vestibulli (4) Spiral ganglion (5) auditory nerve fibres Inner Ear Semicircular Canals 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 The cochlea works as a frequency analyzer It operates on the incoming sound’s frequencies Place Theory 4mm2 1mm2 32-35 mm long • 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) 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) Auditory Neuron Auditory Area of Brain 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 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 Perception of Sound Threshold of hearing – How it is measured – Age effects Equal Loudness curves Bass loss problem Critical bands Frequency Masking Temporal Masking Threshold of Hearing Hearing area is the area between the Threshold in quiet and the threshold of pain 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 Threshold variation with age •Presbycusis •Hearing sensitivity decreases with age especially at High frequencies •Threshold of pain remains the same •Reduced dynamic range 4mm2 1mm2 32-35 mm long 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. 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 lowno bass Too hightoo much bass 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