PsychoacousticsPsychoacoustics

ROY D. TIPONES, ECEROY D. TIPONES, ECE

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 (ear drum) 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 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

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

� 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