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ELEC3104: Mini-Project –
Cochlear Signal Processing
Modified SOLO taxonomy framework 2 ✓ The Structure of Observed Learning Outcomes (SOLO) taxonomy is a framework for analysing students’ depth of knowledge. ✓ It describes 6 Hierarchical levels (Levels 0 to 5) of increasing complexity in student’s understanding of topics studied. ✓ The taxonomy encourages students to think about which level they are currently with their learning and what they need to do in order to progress to the next level. ✓ SOLO taxonomy has been adapted for this course to include Pass, Credit, Distinction and High Distinction levels to help the students to understand the different levels (Levels 0 to 5) on the learning curve and what they need to do to progress. ✓ Students may choose to stay on in a particular level without moving to the next level. ✓ The mini-project is designed to provide an increasing complexity from Pass (Level 2) to High distinction (Level 5) levels as shown below. Different Stages of the Modified SOLO Taxonomy ELEC3104: Project Outline 3 ✓ This mini project (individual) will focus on understanding and modelling the spectral analyses carried out by the human cochlea.d modelling the spectral analyses carried out by the human cochlea. SOLO – Level 1 ▪ Introduction to Human Auditory System and MATLAB coding fundamentals SOLO – Level 2 (Pass Level) ▪ Implementation of a parallel FIR filter bank model of the cochlea for analysis and synthesis purposes. SOLO – Level 3 (Credit Level) ▪ Implementation of a parallel IIR filter bank model of the cochlea for spectral analysis. SOLO – Level 4 (Distinction Level) ▪ Implementation of a parallel IIR filter bank model of the cochlea for pitch detection. SOLO – Level 5 (High Distinction Level) ▪ Incorporate mechanisms into the parallel IIR filter bank cochlea model that makes the filterbank adaptive. Additional Information: In addition to the information provided to you in these slides, you are strongly encouraged to find and view animations and videos that describe the functioning of the peripheral auditory system and the cochlea in particular. Visualisation in the form of these animations will be very helpful in understanding cochlear signal processing. Eg: Cochlear Animation - https://www.youtube.com/watch?v=dyenMluFaUw Prof. Eliathamby Ambikairajah, School of EE&T Term 3, 2021 4 ELEC3104: Mini-Project – Cochlear Signal Processing SOLO – Level 1: Introduction to Human Auditory System and MATLAB coding fundamentals Introduction to the Human Auditory System 5 ✓ The human auditory system is responsible for converting pressure variations caused by the sound waves that reach the ear into nerve impulses that are interpreted by the brain. ✓ The Human Auditory System is designed to assess frequency (pitch) and amplitude (loudness). ✓ The peripheral auditory system is divided into the Outer Ear, Middle Ear, and Inner Ear. ✓ The peripheral auditory system and in particular the cochlea can be viewed as a real-time spectrum analyser. ✓ The primary role of the cochlea is to transform the incoming complex sound wave at the ear drum into electrical signals. ✓ The human ear can respond to minute pressure variations in the air if they are in the audible frequency range, roughly 20 Hz - 20 kHz L Electrical signals. Sounds Level Faint 20dB (A faint Whisper is 30dB) Soft (Quiet) 40dB Moderate 60dB (normal conversation) Loud 80dB (alarm clocks, vacuum cleaners) Very Loud 90dB(Blenders);110dB (Concerts, car horns) Uncomfortable 120dB (jet planes during take off) Painful and dangerous 130dB(Jackhammers); 140dB(Gunshots) *Use hearing protection ✓ Over 85 dB for extended periods can cause permanent hearing loss ✓ Zero decibels (0 dB) represent the absolute threshold of human hearing, below which we cannot hear a sound. Outer Ear (Air Vibration): A resonator 6 ✓ The pinna surround the ear canal and functions as sound wave reflectors and attenuators . ✓ The sound waves enter a tube-like structure called ear canal and it serves as a sound amplifier. ✓ The sound waves travel through the canal and reach the eardrum and cause it to vibrate ✓ The length (L) of the human ear canal is 2.8 cm (and 7 mm in diameter) ✓ Speed of sound (c) = 340.3 m/sec ; ✓ The resonant frequency (f) of the canal is = 4 = 3,038Hz. ✓ The human outer ear is most sensitive at about 3kHz and provides about 20dB (decibels) of gain to the eardrum at around 3000Hz. Outer ear is a low-Q bandpass filter (Representative figure only) Middle Ear: An Impedance Matcher & an Amplifier 7 ✓ Middle ear transforms the vibrating motion of the eardrum into motion of the stapes via the two tiny bones, the malleus and incus . ✓ The pressure of the sound waves on the oval window is around 25 times higher than on the eardrum. ✓ Since the sound Intensity () is proportional (∝) to the square pressure (2) , the sound intensity increases 625 times (or 28dB) ✓ Middle ear converts acoustic energy to mechanical energy and mechanical energy to hydraulic energy Outer Ear Middle Ear To cochlea Sound input dB f dB f The combined frequency response of the outer and middle ear is a band-pass response, with its peak dominated near 3 kHz Middle Ear Gain function d B Inner Ear 8 ✓ The inner ear consists of the cochlea responsible for converting the vibrations of sound waves into electrochemical impulses which are passed on to the brain via the auditory nerve. ✓ The cochlea is a spiral shaped structure which is about 3.5 cm in length if uncoiled. ✓ The cochlea is divided along its length by the basilar membrane (BM) which partitions the cochlear into two fluid canals (scala vestibuli and scala tympani). ✓ The BM terminates just reaching the helicotrema, so there is a passage way between the scala vistibuli and the scala tymapni equalising the difference in pressure at the ends of the two scalas. A longitudinal section of an uncoiled cochlea Basilar Membrane (Hydro Dynamical process) 9 ✓ The Basilar Membrane varies in width and stiffness along its length. ✓ At basal end it is narrow and stiff where as towards the apex it is wider and more flexible. ✓ Each point along the basilar membrane has a characteristic frequency, (), to which it is most responsive. ✓ The maximum membrane displacement occurring at the basal end for high frequencies (20 kHz) and at the apical end for low frequencies (70Hz) . ✓ When the vibrations of the eardrum are transmitted by the middle ear into movement of the stapes, the resulting pressure differences between the cochlear fluid chambers, generate a travelling wave that propagates down the cochlea and reach maximum amplitude of displacement on the basilar membrane at a particular point before slowing down and decaying rapidly ✓ The location of the maximum amplitude of this travelling wave varies with the frequency of the eardrum vibrations 3.5 cm Base Apex = 3.5 cm Basilar membrane 0.05155 cm If is the distance of a point on the basilar membrane from the stapes, then the frequency, (), that produces a peak at this point is given by: = 20000.0 10 −0.667 0 ≤ ≤ 3.5 • It is evident that a 20 kHz tone at the stapes will cause the BM to vibrate at a point = 0. • A 70 Hz tone will excite the BM at a point x = 3.5 cm (i.e. at the apex) The basilar membrane is a resonant structure that vibrates, vertically in sympathy with pressure variations in the cochlear fluid. Basilar Membrane 10 ✓ Different frequencies stimulate different areas of the basilar membrane ✓ When a tone (single sinusoid) is applied, the cochlear fluid oscillates in phase with the stimulating frequency causing a travelling wave pattern of the vibration on the basilar membrane ✓ There will be one place where the resonant frequency of the membrane matches the stimulus frequency and this place will show the maximum amount of vibration ✓ By measuring vibration at particular points on the membrane for a range of stimulus frequencies we can plot the frequency response of each place on the membrane ✓ The essential function of the basilar membrane is to act as a frequency analyser (a set of band-pass filters each responding to a different frequency region) resolving an input sound at the eardrum into its constituent frequencies Organ of Corti 11 ✓ Attached to the basilar membrane and running its entire length is the organ of corti containing some 30,000 sensory hair cells. ✓ The hairs (cilia) of these cells stick up from the organ of corti and are in contact with overlying Tectorial Membrane ✓ There are two types of sensory hair cells: ▪ One row of inner hair cells, whose cilia float freely in the fluid-filled region called subtectorial space ▪ Three rows of outer hair cells whose cilia are attached to the tectorial membrane ✓ Most of the afferent fibres (neurons which carry signals to the brain) come from inner hair cells, ✓ The efferent fibres (which receive signals from the brain) go mainly to outer hair cells. ✓ When the basilar membrane deflects, due to pressure wave in the cochlear fluid, the tectorial membrane move and shear which causes the hairs of the outer hair cells to bend and also cause the fluid flow in the subtectorial space. ✓ This in turn triggers the inner hair cells to transmit nerve impulses along the afferent fibres and eventually to brain. ✓ The motion of each part of the basilar membrane as detected by the inner hair cells is transmitted as neural description to the brain. A simplified Diagram of a Human Auditory System OUTER & MIDDLE EARs SOUND BASILAR MEMBRANE INNER HAIR CELLS OUTER HAIR CELLS AFFERENT FIBRES EFFERENT FIBRES AUDITORY PROCESSING (Brain) Inner Ear (Cochlea) Nerve Fibres Organ of Corti Mechanical to Neural Transduction (Electro Chemical) 12 ✓ The mechanical displacement to electrical energy transduction process takes place in the inner hair cells ✓ Bending of the inner hair cell cilia due to basilar membrane displacement produces a change in the overall resistance (reduces it) of the inner hair cell, thus modulating current flow through the hair cell. ✓ The modulation being directly proportional to the degree of bending of the cilia and the bending of the cilia is one direction only; in effect a half wave rectification of the basilar membrane displacement takes place. ✓ Bending of the cilia releases neurotransmitter which passes into synapses of one or more nerve cells which fire to indicate vibration ✓ The amount of firing is thus related to the amount of vibration ✓ Since the neurotransmitter is only released when the cilia are bent in one direction, firing tends to be in phase with basilar membrane movement Here bending the inner hair cell cilia is simulated by charging of the capacitor and returning to the initial position of the cilia is equivalent to discharging the capacitor. Inner Hair Cell model Cochlear Modelling: Cascade and Parallel Models 13 ✓ The basic model of the cochlea is a transmission line model (cascade model) in which the basilar membrane is modelled as a cascade of 128 low pass filters, notch filters and resonators as shown above. ✓ Each digital filter section in the model above represents a section of the basilar membrane (tuned to a specific frequency) with 128 sections representing the entire basilar membrane Magnitude response of the cascaded filter bank model
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