Psychoacoustics

Using listening experiments to understand the relationship between

perceived sensation and the physical features of a stimulus

•

Sensations:•

loudness (includes masking), pitch, roughness, fusion

Ideally, the physiology should explain psychoacoustical observations

Extraction of features from the temporal and spatial pattern of neural activity such as:

pitch, loudness, timbre, other cues

loudness �� total neural activity caused by the sound•

timbre �� spatial distribution pattern of neural activity•

pitch �� position of local peaks in the spatial distribution•

Auditory cortex: perception, cognition

Lecture-Oct11-dTuesday, October 12, 2010

8:31 AM

lectures-hearing-2 Page 1

Screen clipping taken: 10/13/2010, 9:52 AM

L (sone) = k x Ip

Steven’s law

p=0.3

k depends on frequency

Loudness perception

Absolute thresholds and equal loudness contours

Tones of equal SPL but different frequencies

do not sound equally loud.

•

The bandpass effect is attributed to the

transfer function of the outer-middle ear

and to a drop in the number of hair cells

towards the extremes of the BM.

For a sound to be perceived as twice as loud

(unit: sone), its intensity must be increased

by a factor of 10.

•

Loudness sensation comes from total neural

activity which is a nonlinear function of

stimulus intensity (see intensity coding by nerve

fiber).

Growth of loudness is different for different

frequencies.

•

<= Measured via listening experiments

Lecture-Oct14-aWednes day, October 13, 2010

8:38 AM

lectures-hearing-2 Page 2

Computational model of hearing

Screen clipping taken: 10/13/2010, 8:43 PM

Outputs of the computational model for a tone stimulus

Loudness summation

lectures-hearing-2 Page 3

Masking: an auditory phenomenon in which the presence of one sound

raises the hearing threshold for another sound in its frequency or time vicinity.

Simultaneous Masking

Masking threshold depends on SPL & frequency & nature of Masker

•

nature of the maskee•

Based on our study of physiology, simultaneous masking is explained by

our limited ability to distinguish small changes in the BM excitation pattern

produced by a sound.

•

A target will be masked if its addition to the masker does not change

the level at the output of any auditory filter by more than 1 dB.

For instance -•

Screen clipping taken: 10/13/2010, 9:56 AM

Excitation pattern: two sine tones

D = (0.25)*I

Lecture-Oct14-bWednesday, October 13, 2010

8:23 PM

lectures-hearing-2 Page 4

Screen clipping taken: 10/13/2010, 9:57 AM

Band-narrowing experiment of Fletcher

Screen clipping taken: 10/13/2010, 9:54 AM

Temporal Masking

Audibility of a tone in the

presence of a noise maskerApply a narrow band of noise. Then apply a tone in

the centre of noise band. The threshold amplitude

for the audibility of the tone keeps increasing with

the noise bw until critical bw is reached.

lectures-hearing-2 Page 5

Frequency perception: 2. Superposition of 2 tones

Frequency perception: 1. Discriminating pure tones.

Increasing separation slowly leads to a transition from beats/roughness to perception of 2 separate tones when the excitation regions on the BM are separated sufficiently. Note: if the 2 tones are fed separately to the two ears, there is no beats/roughness! (no overlap on BM)

JND for pitch of pure tones is ~2 Hz up to 1 kHz. After this, it is about 1-2% of the

frequency.

lectures-hearing-2 Page 6

Speech Perception

Speech, as we know, can be represented as acoustic patterns that vary

in frequency, intensity and time.

But, we now see that neither the wideband nor narrowband

spectrogram provide suitable perceptual representations…..

The spectrogram displays the time-frequency components according to

their physical intensity levels while the human ear's sensitivity to the

different components is actually influenced by several auditory

phenomena:

the widely differing sensitivities of hearing in the low, mid and high

frequency regions in terms of the threshold of audibility

•

the decreasing frequency resolution, with increasing frequency, across

the audible range

•

the nonlinear scaling of the perceived loudness with the physically

measured signal intensity

•

auditory masking by which strong signal components suppress the

audibility of relatively weak signal components in their t-f vicinity

•

Lecture-Oct14-cWednesday, October 13, 2010

8:48 PM

lectures-hearing-2 Page 7