Localisation and Auditory Scene Analysis

Question 1

Localising sound - binaural cues

Answer 1

Binaural - requires comparison of singles in left and right ears and are vital for signalling location of a sound in azimuth (left and right plane)

• interaural time differences (ITD)
• interaural level differences (ILD)

Question 2

Localising sounds monaural cues

Answer 2

Work with one ear and localise elevation (up and down plane) and distance of sound

• filter properties of pinnae
• intensity and reverberation

Question 3

Binaural - interaural time differences

Answer 3

Relative time at which sound arrives at two ears depends on its location in azimuth
A sound source thats straight ahead = distance to each ear is the same therefore no difference in time ( immediately behind, in front, above)
Sound source to the right = difference distances therefore sound reaches left ear later than right

Question 4

What does range of ITD’s depend on?

Answer 4

Speed of sound (constant through air)
Distance between two ears (larger heads = larger ITD’s)

Maximum in humans = 0.6ms
Requires precise signalling of timing - phaselocking therefore most useful for low frequencies and abrupt onset sounds

Question 5

Binaural - interaural level differences

Answer 5

Relative sound pressure level reaching two ears also depends on location of source in azimuth

• reduction in sound levels for the far ear, due to acoustic shadow created by head
• therefore occurs for high frequency sounds (6000Hz) but not for low (200Hz)

Question 6

What is the physiology of binaural processing?

Answer 6

Processing of ITD’s and ILD’s start within the brainstem in superior olivary complex (superior olive)
Binaural localisation cues processed by different neurones located in superior olive
- lateral superior olive (LSO) sensitive to ILD’s
- medial superior olive (MSO) neurones sensitive to ITD’s

Question 7

Strengths and Weaknesses of binaural cues

Answer 7

+ ITD’s (low frequencies) and ILD’s (high frequencies) provide complementary information about azimuth location
- provide ambiguous information about elevation and distance of sound source

Question 8

Monaural localisation cue - elevation

Answer 8

Elevation:
filter properties of pinnae - sound reflects off parts of external ear, the relative intensity of different frequency sound wave changes - changes sound source elevation (and azimuth)
- individuals different ear shapes and patterns filter frequencies in different ways
- experiments with artificially altering ear shape with plastic moulds impair the ability to localise sound elevation but brain adapts after period of time

Question 9

Monaural cue - distance

Answer 9

Relative intensity
- sound intensity decreases with distance, closer objects will tend to have greater amplitude than further

Reverberation

• the way in which sound reflects off objects provides cue to distance
• multiple reflectors combine to produce a persistence of sound - reverberation

The distance of source alters relative intensity and timing of direct and reverberant sounds

Question 10

Reverberation potential problems - precedence effect

Answer 10

Not sure where the actual location of the sound source
Precedence effect - when similar sounds arriving in quick succession from different locations = localised according to direction of first sound - provided the delay is short, only single sound is perceived (less 10-20ms)

Question 11

Scene analysis in natural environment

Answer 11

The natural environment has multiple sound sources
Auditory system undergoes scene analysis to make sense of sounds:
- segregates components of sound that comes from different sound sources
- group components of sound that come from same sound source

Question 12

What are the strategies for auditory grouping?

Answer 12

• similar to visual e.g., common fate, proximity
• spectrum grouping = combining different frequency sound components that occur at same time
• sequential grouping = combining sequences of sounds over time

Question 13

Spectrul grouping

Answer 13

grouped single sound as probably the same sound event
Harmonicity = if component is mistuned to other components it will be heard as separate sound
Common frequency change (common fate) - frequency components that change together tend to be grouped together

Question 14

Sequential grouping

Answer 14

Auditory stream segregation

• Changing characteristics of sounds can bias us to either group the sequence or segregate into multiple streams
• Similarity of pitch - often similar when from same source. Increasing stream difference promotes stream segregation
• Temporal proximity - sound s that occur in rapid progression tend to be produced by same source
• similarity of timbre grouping
• continuity - sounds that stay constant or change smoothly are often produced same source - perceived as continuous even when interrupted by noise
• phoenic restoration - interrupting speech with silence makes it difficult to understand - gaps containing noise makes it easier to understand