Transcript Psychoacoustics - University of Limerick

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