Spectral centroid
• 6 harmonics: f0 = 100Hz• 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
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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.