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Chapter 12

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McMaster University
Gautam Ullal

Psych 2E03: Sensory Processes Chapter 12: Sound Localization and the Auditory Scene Auditory Localization - When you perceive objects located at different positions based on their sounds, you are experiencing auditory space - Auditory space extends around your head in all directions, existing wherever there is a sound - These feats of locating objects in space based on their sound are examples of auditory localization - Researchers have studied people’s ability to localize sounds by determining how well people can locate the position of the sound along the azimuth coordinate, extends from left to right, the elevation coordinate, extends up and down, and the distance coordinate, specifies how far a sound source is from the listener - There’s variability between different listeners’ ability to localize sounds, but when measurements from a number of people are averaged, listeners can localize sounds that are directly in front of them most accurately and sounds that are off to the side and behind least accurately - To determine the location, the auditory system carries out a calculation that makes use of location cues created by the way sound interacts with the listener’s head and ears - Cues for location – signals reaching the ears:  Binaural cues:  The two binaural cues – interaural time difference and interaural intensity difference are based on a comparison of the sound signals reaching left and right ears  Interaural time difference: difference between the times that the sound reaches the left and right ears  Interaural level difference: is based on the difference in the sound pressure level of the sound reaching the two ears  Head creates a barrier that reduces the intensity of sounds, occurring for high-frequency sounds, but not for low-frequency sounds  High-frequency sound waves are disrupted by the head (as they are more closely spaced), which creates a decrease in sound intensity on the far side of the head (acoustic shadow)  This effect on frequency of the interaural level difference has been measured by using small microphones to record the intensity of the sound reaching each ear in response to a sound source located at different positions relative to the head: level is hardly affected by changes in location for frequencies below 1,000Hz, but the level is greatly affected by locations for higher frequencies  Monaural cue:  Before sound stimulus enters the auditory canal, it is reflected from the head and back and forth within the various folds of the pinnae  Effect of interaction with the head and pinnae has been measured by placing small microphones inside a listeners ear and comparing the frequencies coming from the sound source  Difference between the sound from the source and the sound actually entering the ears is called the head-related transfer function because the head and pinnae decrease the intensity of some frequencies and enhance others  HRTF is called spectral cue because information for location is provided by the spectrum of frequencies that occurs at each location - Cues for location – effects on behaviour:  Virtual auditory space technique: sound stimuli can be presented in two ways  Free-field presentation: involves presenting sounds at various positions around a listener’s head, by moving speakers to different positions in space, and testing the listener in the dark  Headphone presentation: involves presenting sounds trough headphones, which gives the experimenter precise control over the stimuli  Wearing the headphones eliminates the spectral cues that are created by sound bouncing around inside the pinna, and the result is that sounds are perceived to originate inside the head (internalization)  Internalization can be eliminated by measuring characteristics of a person’s HRTF, programming these characteristics into a computer and having the computer process the sound that is delivered to the ears through headphones (externalization)  Creation of perception of an external sound source when using headphones is virtual auditory space  Judging azimuth locations:  Researchers have used VAS technique to determine whether ITDs cause listeners to perceive sounds as coming from different locations  Listeners were fairly accurate at judging the locations of these sounds  All three cues were set to indicate a particular location  Experiment that kept ITD constant and varied ILD and HRTF: no matter what direction the ILD or spectral cues indicated, judgments were based on ITD, resulting only when the sound included low frequencies  ITD dominant cue for low frequency sounds  Varying ILD has little effect on the perception of location for low-frequency sounds but is effective at higher frequencies  Results of many experiments indicate that ITD and ILD are used for judging location along the azimuth coordinate, with ITD being effective at low-frequencies and ILD being effective at high-frequencies - Judging elevation and other ambiguous locations:  ITD and ILD provide ambiguous information about the elevation of a sound source  ITD or ILD of zero can occur when sound is at a number of different locations  Spectral cues can be used to judge elevation  Smoothing out nooks and crannies of the pinnae with moulding compound makes it difficult to locate sounds along elevation coordinate  Localization changes when the mould is worn for several weeks, and when it is removed: localization is poor for the elevation coordinate immediately after the mould is inserted, but locations can still be judged at locations along the azimuth coordinate  As listeners continued to wear the moulds localization performance improved, learned to a
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