Chapter 4: The physics and biology of audition

Key Terms in this Chapter

amplitude

The maximum height of a wave, measured from its mean value to its maximum value.

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auditory association cortex

Two concentric bands of cortex encircling primary auditory cortex, containing cells responsive to auditory stimuli.

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azimuth

Direction in the horizontal plane, relative to straight ahead.

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basilar membrane

The flexible structure on the cochlear partition that houses mechanoreceptors; it forms part of the organ of Corti.

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cochlea

The spiral-shaped structure in the inner ear containing mechanoreceptors for hearing.

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cochlear nuclei

The mass of nerve cells in the brainstem where auditory nerve fibres terminate.

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cochlear partition

The flexible structure separating the scala vestibuli and scala tympani.

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complex wave

A wave that can be decomposed into a collection of simple, sinusoidal waves.

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decibels (dB)

A measure of the difference between two quantities, based on the logarithm of their ratio (so equal ratios between the quantities correspond to equal dB differences).

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dynamic range

In auditory nerve fibres, it is the difference between the minimum SPL to which a fibre responds, and the SPL at the fibre's maximum firing rate.

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ear canal

The S-shaped tube leading from the meatus to the tympanic membrane; it is approximately 0.6 cm wide and 2.5 cm long.

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Fourier analysis

The mathematical procedure by which a complex signal is decomposed into its magnitude spectrum and phase spectrum.

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Fourier spectrum

A representation of the magnitude of individual frequency components present in a signal such as a sound wave; it is also known as the magnitude spectrum.

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frequency

The number of cycles (periods) of a wave per unit of time or distance.

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frequency filter

Any process that modifies the frequency content of signals passing through it.

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'frequency-to-place' conversion

The fact that the place of maximum displacement on the basilar membrane depends on the frequency of sound vibration.

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fundamental frequency

The lowest sinusoidal frequency in a complex wave.

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Harmonic frequency

A sinusoidal component of a complex wave, having a frequency that is an integer multiple of the fundamental frequency.

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impedance

The degree of resistance offered by a medium to an oscillating signal.

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impedance matching

The boost in pressure at the oval window provided by the mechanical properties of the middle ear, it matches up the differing acoustic impedances of air and inner-ear fluid.

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inner ear

The fluid-filled organ lying in the temporal bone, containing mechanoreceptors for hearing and balance.

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inner hair cells

The mechanoreceptors on the basilar membrane that provide afferent signals to the auditory system when the membrane is displaced.

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inter-aural level difference (ILD)

A difference in the intensity of an auditory stimulus arriving at the one ear relative to the other.

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inter-aural time difference (ITD)

A difference in the time of arrival of an auditory stimulus at one ear relative to the other.

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Linear filters

A frequency filter that modifies the amplitude of input frequency components, but does not introduce components that were not present in the input.

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meatus

The opening into the ear canal.

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medial geniculate nucleus

The obligatory relay station for all ascending projections in the auditory system, en route to the cortex.

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middle ear

The air-filled cavity containing the bones and associated supporting structures that transfer sound energy from the outer ear to the inner ear.

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non-linear filter

A frequency filter that distorts signals by adding new frequencies or by failing to respond at very low or high amplitudes.

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organ of Corti

The complex structure that forms part of the cochlear partition; it contains mechanoreceptors sandwiched between two flexible membranes.

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oto-acoustic emissions

A sound emitted from the ear, either spontaneously or following auditory stimulation.

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outer ear

The outermost components of the auditory system: the pinna, meatus, and ear canal.

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outer hair cells

Motile hair cells spanning the gap between the basilar membrane and tectorial membrane; they control the mechanical coupling between the two membranes.

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oval window

The point of entry for sound energy into the inner ear; it is a small membrane-covered opening in the cochlea.

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phase

A measure of the timing or position of a wave relative to a fixed point of reference, or to another wave.

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Phase locking

The firing of hair cells in synchrony with the variation of pressure in a sound wave.

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phase spectrum

A representation of the relative phases of individual frequency components present in a signal.

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pinna

The external ear visible on each side of the head; its shell-like inner part (concha) funnels sound into the meatus.

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Primary auditory cortex

The cortical destination of projections from the medial geniculate nucleus.

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scala tympani

The tube running along the cochlea that contains the round window at its base.

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scala vestibuli

The tube running along the cochlea that contains the oval window at its base.

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sine wave

A wave whose height varies smoothly so that it conforms to a mathematical sine function of time or distance (adjective: sinusoidal).

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Sound pressure level (SPL)

A decibel measure of sound pressure relative to a fixed reference pressure.

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spectrogram

A graphical representation of changes in the frequency content of a signal over time.

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superior olive

The complex of cells in the brainstem receiving projections from the cochlear nucleus; it contains binaural neurons that provide information about the location of sound sources.

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tectorial membrane

The flexible structure lying on top of the basilar membrane.

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tonotopic organisation

An organised arrangement of neurons, so that cells with similar frequency preferences lie close together in a neural structure.

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transfer function

A function that describes a linear filter's frequency response, in terms of the degree of attenuation at each frequency.

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travelling wave

The ripple of displacement that travels along the basilar membrane when sound vibrations are applied to the oval window.

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two-tone suppression

Suppression of an auditory nerve fibre's response to a tone during presentation of a second tone.

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tympanic membrane

The flexible cone-shaped membrane extending across the inner end of the ear canal; it transmits sound energy to the middle ear.

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