Lecture 6 - Localisation and auditory scene analysis

Question 1

How do we localise sound?

Answer 1

• Binaural cues
• Monaural cues

Question 2

What are some aspects of binaural cues?

Answer 2

-Require comparison of signals in left and right ears and are vital for signalling location of a sound in azimuth (left-right plane).
-Interaural time differences (ITDs).
-Interaural level differences (ILDs).

Question 3

What are some aspects of monaural cues?

Answer 3

-Work with one ear and can help localise the elevation (up-down plane) and distance of a sound.
-Filter properties of the pinna (outer ear).
-Intensity and reverberation.

Question 4

What are interaural time differences (ITDs)?

Answer 4

-Relative time at which a sound arrives at the two ears depends on its location in azimuth (left-right).
-If the sound source is straight ahead, the distance to each ear is the same and there is no difference in time.
-When the sound source is positioned to one side, the sound will reach the nearer ear first.
-The range of ITDs depends on speed of sound (typically 330m/s through air) and distance between the two ears (larger heads create a bigger range).
-Maximum ITD in humans is around 600μs (0.6ms).
-Requires precise signalling of timing.
-Most useful for low frequency or abrupt-onset sounds.

Question 5

What are interaural level differences (ILDs)?

Answer 5

-Relative sound pressure level reaching the two ears also depends on the location of the source in azimuth.
-Reduction in sound level occurs for the far ear due to the acoustic shadow created by the head.
-This reduction occurs for high-frequency sounds but not low frequency sounds.

Question 6

What is the physiology of binaural processing?

Answer 6

-Processing of ITDs and ILDs starts within the brainstem in the superior olivary complex (superior olive).
-Binaural localisation cues processed by different types of neurons, located in different parts of the superior olive.
-The lateral superior olive (LSO) contains neurons that are sensitive to ILDs
-The medial superior olive (MSO) contains neurons that are sensitive to ITDs.

Question 7

What are some strengths of binaural cues?

Answer 7

-ITDs and ILDs provide complementary information about azimuth location.
-ITDs work particularly well for low-frequency sounds.
-ILDs provide information about high-frequency sounds.

Question 8

What are some weaknesses of binaural cues?

Answer 8

• they provide ambiguous information about elevation and tell us nothing about distance.
  -Cone of confusion – set of points from which a sound source will produce identical ITDs and ILDs.

Question 9

Monaural localisation cues - elevation: What are the filter properties of the pinnae?

Answer 9

-When sound reflects off the nooks and crannies of your external ear, the relative intensity of different frequencies sound wave changes.
-This changes with sound source elevation (and azimuth*).
-Individuals have different ear shapes and will filter the frequency content of complex sounds in slightly different way.
-Artificially altering ear shape with plastic moulds impairs the ability to localise sound elevation.

Question 10

Monaural localisation cues - distance: what is relative intensity?

Answer 10

Sound intensity decreases with distance, so closer object will tend to have greater amplitudes than farther ones.

Question 11

Monaural localisation cues - distance: what is reverberation?

Answer 11

-The way in which sound reflects off object also provides a cue to distance.
-Multiple reflections combine to produce a persistence of sound called reverberation.
-The distance of a source alters the relative intensity and timing of direct and reverberant sounds.

Question 12

What is the precedence effect?

Answer 12

-Similar sounds arriving in quick succession from different locations are localised according to the direction of the first sound.
-Provided the delay is short (<10 -20ms), only a single sound is perceived.

Question 13

What is auditory scene analysis?

Answer 13

-Natural environments contain multiple sound sources, so the auditory system needs to make sense of the mixture of component sounds that makes up the auditory scene.
-It must segregate the components of the sound coming from different sound sources.
-It needs to group the components of the sound that come from the same sound source.

Question 14

What are some strategies for auditory grouping/segregation?

Answer 14

-Follow many analogous principles e.g common fate, proximity, similarity, continuity.
-We also need to consider spectral grouping – combining different frequency sounds components that occur at the same time.
-Also need to consider sequential grouping – combining sequences of sounds over time.

Question 15

What does spectral grouping consist of?

Answer 15

-Harmonicity
-Common frequency change

Question 16

What is harmonicity?

Answer 16

-If a component is mistuned to other components, it will be heard as a separate sound.

Question 17

What is common frequency change?

Answer 17

– Analogous to the principle of common fate, frequency components that change together tend to group together.

Question 18

What is the sequential grouping?

Answer 18

-Auditory stream segregation - the process of organising sounds over time into separate perceptual events.
-Changing the characteristics of the sounds can bias us to either group the sequence or segregate it into multiple streams.

Question 19

How does similarity of pitch create sequential grouping?

Answer 19

-Sounds with similar pitch are often produced by the same source.
-Increasing frequency difference promotes stream segregation.

Question 20

How does temporal proximity create sequential grouping?

Answer 20

-Sounds that occur in rapid progression tend to be produced by the same source.
-Increasing presentation rate also promotes stream segregation.

Question 21

How does similarity of timbre create sequential grouping?

Answer 21

-Sound sources often have distinct timbre (e.g musical instruments), providing a good cue for stream segregation.

Question 22

How does continuity create sequential grouping?

Answer 22

-Sounds that stay constant or change smoothly are often produced by the same source.
-Perceived as continuous even when interrupted by noise.

Question 23

How does phonemic restoration create sequential grouping?

Answer 23

-This effect is not restricted to tones but can also occur with speech.