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

aifi

i1

iN

ai

i1

iN

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.