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Psychoacoustics Riana Walsh [email protected] Relevant texts • Acoustics and Psychoacoustics, D. M. Howard and J. Angus, 2nd edition, Focal Press 2001 • An Introduction to the Psychology of Hearing, B.C.J. Moore, 5th edition, Academic Press, Elsevier 2004 • Fundamentals of Hearing, An Introduction, W. A. Yost, 4th edition, Academic Press 2000 • Listening An introduction to the Perception of auditory events, S. Handel, MIT Press 1989 Course outline • Structure and function of the auditory system; frequency selectivity of the auditory system; the perception of pitch, loudness and timbre; temporal perception; sound localisation; identification of auditory objects; streaming; organisation of auditory memory; pitch organisation; memory, attention, melody and rhythm. Hearing • Psychoacoustics – the study of hearing relationship between the physical properties of sound and the sensations they produce. • Hearing – the process that transforms sound waves into neural signals that can be interpreted by our brain • Sound waves – fluctuations in air pressure across time, created by the motion or vibration of an object (e.g. the vibration of vocal chords, oscillating violin string) - physical properties: frequency and amplitude. The peripheral auditory system • The peripheral auditory system consists of the outer, middle and inner ear. • In brief: The ear drum moves in and out in response to the pressure changes in sound waves – transmitted through the middle to the inner ear – transduced into neural sinals that are interpreted by the brain The path of sound waves through the outer, middle and inner ear • Sound waves travel down the auditory canal and cause the ear drum to vibrate. • The main function of the ossicles is the efficient transfer of sound waves from air to the fluids of the cochlea. • The ossicles of the middle ear vibrate in response to tympanic membrane vibration. They amplify and transmit these vibrations to the oval window. • Amplification is necessary as more energy is required to move the fluids (of the inner ear) than air (in middle ear). The middle ear • Achieved: difference in the effective areas of the ear drum and oval window; lever action of the ossicular chain • Difference in the area of the eardrum and oval window [pressure = force/area] • Middle ear (also acoustic) reflex – muscles attached to the ossicles contract upon exposure to intense sounds (>~80dB SPL) • Contraction of these muscles reduces the transmission of pressure through the ossicular chain – may prevent inner ear damage • Frequency dependent – most effective < 2 kHz • Minimum time for reflex 10-150ms (depends on intensity) – so reflex not effective for sounds with a sudden onset e.g. gunshots • This reflex may also function is the reduction of the audibility of self-generated sounds, such as speech. It has been shown to be activated just before vocalisation. The structure of the inner ear • The part of inner ear concerned with hearing is the fluid filled cochlea. • Reissner’s membrane and the basilar membrane (BM) divide the cochlea along its length. • The start of the cochlea (near oval window) is the base (basal end), the other end of the cochlea is the apex (apical end of the cochlea) • Motion of the basilar membrane in response to sound The basilar membrane response to sound • Movement of the stapes sets the oval window in motion – causes the BM to move. • Response of BM to sinusoidal stimulation – travelling wave, which moves from base to apex. • The position of the peak in the vibration pattern on the BM depends on the frequency of the sound – this is due to the mechanical properties of the BM • High (low) frequencies produce max. BM displacement near the base (apex) – frequency analysis – each point on the BM is sharply tuned BM response to sound • Each point on the BM is sharply tuned, responding with high sensitivity to a limited range of frequencies. • BM vibration is nonlinear – the magnitude of its response does not grow directly in proportion with the magnitude of the input • Linear for low input sound levels (<20dB SPL) and very high input sound levels (>90dB SPL) BM response to sound • Compressive nonlinearity at midrange levels – a large range of input sound levels is compressed into a smaller range of BM responses • Nonlinearity occurs at the base of the BM when the stimulating frequency is close to the BM point being monitored – compression only around the peak of the BM response pattern • The nonlinearity and sharp tuning of BM are physiologically vulnerable BM response to sound • Compression at the apical end is less than at the basal end – at the apical end compression does not seem to depend on the frequency of the input relative to that of the place (CF) being monitored • Frequency-to-place conversion – the distance from the apex to the point of displacement is proportional to the logarithm of the input frequency. • For input sounds with more than one frequency the BM vibration pattern depends on the frequency separation of the components Aside • Our central nervous system consists of the brain and spinal cord • Neurons are the building blocks of our central nervous system • Many different types of neuron (e.g. sensory neuron, interneuron, motor neuron) • Components of a typical biological neuron Structure of the neuron • Three main sections: dendrites, cell body, and the axon. • The function of the dendrites is to receive signals from other neurons at connection points called synapses. • The function of the axon is to transmit signals out of the cell body • The dendrite is separated from the transmitting axon by a narrow gap called a synapse Structure of the neuron • Most neurons have several dendrites to receive stimulation and only one axon to transmit nerve impulses • The axon releases chemicals, called neurotransmitters, into the synapse, and these diffuse across to the receiving dendrite and enter the cell body • The neurotransmitter may be excitatory or inhibitory - it may excite or inhibit the receiving neuron from firing. • The signals received are combined by the cell body • If the signal is above a certain threshold, the cell ‘fires’ producing a pulse that propagates down the axon and is passed on to other neurons • Towards the end of the axon are multiple branches (axon terminals) each terminating in a synapse • In this way a single neuron can excite or inhibit many other neurons