PSGY1010 Audition 2 Localisation and Auditory Scene Analysis PDF

PSGY1010 Cognitive Psychology 1 Audition II: Localisation and Auditory Scene Analysis Dr Chung Kai Li [email protected] The challenge of auditory scene analysis 2  Today’s lecture Learning objectives: ▪ Describe the cues used to determin...

PSGY1010 Cognitive Psychology 1 Audition II: Localisation and Auditory Scene Analysis Dr Chung Kai Li [email protected] The challenge of auditory scene analysis 2  Today’s lecture Learning objectives: ▪ Describe the cues used to determine the location of a sound source ▪ Understand the basic principles of auditory grouping that help us to make sense of the auditory scene 3 Sound localisation in action 4  Comparing location information in vision and audition ▪ Visual information for the relative location of objects is contained within the retinal image ▪ However, the place activated by a sound on the cochlea does not indicate its location 5  How do we localise sound? ▪ Our ability to ascribe a spatial position to sounds relies on binaural and monaural cues ▪ Binaural cues 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) ▪ Monaural cues work with one ear can help localise the elevation (up-down plane) and distance of a sound ▪ filter properties of the pinna (outer ear) ▪ intensity & reverberation The three dimensions of sound location 6  Interaural time differences (ITDs) ▪ The 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 (A), the distance to each ear is the same and there is no difference in time ▪ However, when the source is positioned to one side (B), the sound will reach the nearer ear first 7  Interaural time differences (ITDs) ▪ The range of ITDs encountered depends on the: ▪ speed of sound (typically constant ~330m/s through air) ▪ distance between the two ears (larger heads create bigger range of ITDs) ▪ The maximum ITD in humans is typically around 600μs (0.6ms) ▪ Requires precise signalling of timing (e.g. phase-locking) Typical ITDs for a range of azimuths ▪ Most useful for low frequency or abrupt-onset sounds 8  Interaural level differences (ILDs) ▪ The relative sound pressure level reaching the two ears also depends on the location of the source in azimuth ▪ A 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 (e.g. 6000Hz), but not low frequency sounds (e.g. 200Hz) 9 Interaural level differences (ILDs) (azimuth)  Physiology of binaural processing ▪ Processing of ITDs and ILDs starts within the brainstem in the superior olivary complex (superior olive) 11 Overview of ascending auditory pathways  Physiology of binaural processing ▪ 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 12  Strengths and weaknesses of binaural cues ▪ ITDs and ILDs provide complementary information about azimuth location ▪ ITDs work particularly well for low-frequency sounds ▪ ILDs provide information about high-frequency sounds ▪ However, 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 13  Monaural localisation cues - elevation Filter properties of the pinnae ▪ When sound reflects off the nooks and crannies of your external ear, the relative intensity of different frequencies sound waves 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 14  Monaural localisation cues - distance Relative intensity ▪ Sound intensity decreases with distance, so closer objects will tend to have greater amplitudes than farther ones Reverberation ▪ The way in which sound reflects off objects 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 15  Localising sounds within rooms ▪ Reflected sound poses a potential problem for localization ▪ with multiple sounds reaching the listener from different directions, how can we tell the true number and location of the sound source? Precedence effect ▪ similar sounds arriving in quick succession from different locations are localised according to the direction of the first sound ▪ provided the delay is short (