Download Sound Localisation - University of Limerick

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Spectral
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• 6 harmonics: f0 = 100Hz
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• E.g. 1: Amplitudes: 6; 5.75; 4; 3.2; 2; 1
• [(100*6)+(200*5.75)+(300*4)+(400*3.2)+(500*2
)+(600*1)] / 21.95
• = 265.6Hz
• E.g. 2: Amplitudes 1; 2; 6; 5.75; 4; 3.2
• [(100*1)+(200*2)+(300*6)+(400*5.75)+(500*4)+
(600*3.2)] / 21.95
• = 301.86Hz
Masking
• A sound may become inaudible due to the
presence of one or more other sounds
• Explained in terms of an increase in the hearing
threshold of the weaker sound
• Formal definition:
• “The process (or amount) by which the threshold
of audibility for one sound is raised by the
presence of another (masking) sound”
• Amount – measured in dB
Masking
• A sound is most easily masked by another sound
that has frequency components close to it
• Related to the BM frequency resolution – our
ability to separate the components of a complex
sound
• Masking occurs if the frequency selectivity of the
ear is insufficient to separate the signal and the
masker
Types of masking
• Simultaneous masking – signal present at
the same time as the masker
• Backward masking – signal present before
the masker
• Forward masking – signal present after the
masker
• Asa trk 23-25
Mechanism of simultaneous
masking
• Two conceptions:
• The masker swamps the neural activity
evoked by the signal
• The masker suppresses the activity which
the signal would evoke if presented alone –
two-tone suppression
Forward masking
• The amount of forward masking is greater
the nearer in time to the masker the signal
occurs
• limited to signals which occur within about
200ms after the cessation of the masker
• Influenced by the relation between the
frequencies of the signal and masker
forward masking
• Some explanations:
• BM response rings after end of masker - temporal
overlap of vibration patterns on the BM – for
small delay times between masker and signal
• fatigue in the auditory nerve or higher centres –
reduces the response to the signal after the masker
• The auditory processes underlying forward and
backward masking are not well understood
Sound Localisation
Sound Localisation
• Two ears
• To determine the direction and distance of a sound
source
• Locate sounds in the horizontal plane, the vertical
plane (elevation) and distance – for each of these
we use a number of different cues:
• Interaural time difference (ITD)
• Interaural level difference (ILD)
• Pinna and head cues - head-related transfer
function (HRTF), head movement, movement of
sound source
Locating sounds in the azimuth
• Azimuth – locations on an imaginary circle
that extends around us in a horizontal plane,
measured in angle degrees
• Locating a sound source in the azimuth:
• Interaural time difference (ITD)
• Interaural level difference (ILD)
Interaural time difference (ITD)
• Time difference between the sound arriving
at both ears.
• ITD approx. range: 0 for a sound straight
ahead to about 690 µs for a sound at 90°
azimuth (directly opposite one ear)
• Location of sound source for max ITD?
• Location of sound source for min ITD?
ITD
• Medial superior olives – first brain stem
region where inputs from both ears
converge – contributes to detection of ITD –
neurons here respond to timing differences
between inputs of both ears
Interaural level difference (ILD)
• Difference in level (intensity) between a
sound arriving at one ear versus the other
• Properties:
• Sounds are more intense at the ear closer to
the source
• Largest at 90°, -90° and min. at 0° and 180°
ILD
• Head blocks high-frequency sounds much
more than low-frequency sounds,
• low frequency sounds have a wavelength
which is long compare with the size of the
head – sound bends around the head
• ILDs are greatest for high frequency sounds
ILD
• Neurons sensitive to intensity differences
are found in the lateral superior olives
Summary ITD / ILD
• Frequency dependency only for pure tones
– not for complex tones
• Sounds with more than one frequency –
comparisons across frequency of ITD and
ILD – most common ITD / ILD
• ITD and ILDs are not sufficient to tell us
completely where a sound is coming from.
Summary ITD / ILD
• Do not indicate if the sound is from the front or
back, or higher / lower (elevation)
• Head movement, movement of the sound source
• Other cues:
• Direction-dependent filtering of the head and
pinnae
• important for judgements of vertical location and
front / back discrimination
Pinnae and head cues
• Spectral changes by head and pinnae used to judge
location of a sound.
• Spectral changes by the pinnae are limited to
frequencies > 6 kHz - head, torso may modify the
spectrum at lower frequencies
• The head and pinnae modify the spectra of sounds
in a way that depends on where the sound is –
form a complex direction-dependent filter
Pinnae and head cues
• Characterised by measuring the spectrum of the
sound source and the spectrum of the sound
reaching the eardrum – ratio of these two,
expressed in dB, gives Head Related Transfer
Function (HRTF)
• HRTFs differ across individuals, due to head and
pinnae shape and sizes.
• Listeners can use these changes in intensity across
frequency to learn where a sound comes from.
• Visual feedback
• The Precedence / Haas effect
Auditory distance perception
• Determine how far away a sound is
• Cue: relative intensity of a sound – become less
intense with greater distance
• Cue: spectral composition of sounds – high
frequencies dampen (decrease in energy) more
than low frequencies for far away sounds – sound
of close vs far away thunder
• Cue: relative amounts of direct vs. reverberant
energy – a closer sound – more direct energy, also
time delay between direct and reflected sound
Auditory distance perception
• Change in intensity as listener moves toward the
sound source
• Relies on many cues:
• In order to estimate the distance of a sound source
the listener can combine absolute intensity,
changes in intensity with distance (a moving
source), spectral composition, and relative
amounts of direct and reflected energy.