Chapter 12 Sound Localization and the Auditory Scene 1 Auditory Localization a Auditory space when you perceive objects located at different positions based on their sounds there is a sound their sound i Extends around your head in all directions existing wherever b Auditory localization these feats of locating objects in space based on c To study this ability to localize sounds researachers have determined how well people can locate the position of a sound in three dimensions i Azimuth extends from left to right ii Elevation extends up and down iii Distance the sound source from the listener d Figure 12 2 with the 227 loud speakers controlled measurements show that sounds directly in front are easiest to localize and sounds off to the side and behind the head are harder to localize e Problems with auditory system i In the ear the birds tweet and the cat s meow stimulate the cochlea based on their sound frequencies and the place that is activated in the cochlea provides information that determines the sound s pitch and timbre ii Because the place activated on the cochlea does not indicate sound s location the auditory system must use other information to determine location iii Location cues information it uses that are created by the way sound interacts with the listener s head and ears f Binaural Cues for Sound Location sounds that are off to the side reach one ear before the other and are louder at one ear than the other i Interaural time difference ITD based on the fact that there is a difference in when a sound reaches the left and right ears 1 2 3 if the source is located directly in front of the listener at A the distance to each ear is the same and the sound reaches the left and right ears simultaneously if the source is located off to the side at B the sound reaches the right ear before it reaches the left ear since ITD becomes larger as sound sources are located more to the side the magnitude of the ITD can be used as a cue to determine a sound s location ITD is an effective cue for location of low frequency sounds ii Interaural Level Difference ILD based on the difference in the sound pressure level level of the sound reaching the two ears 1 Difference in level between the two ears occurs because the head creates a barrier that reduces the intensity of sounds that reach the far ear 2 This reduction of intensity at the far ear occurs for high frequency sounds but not for low frequency sounds 3 Objects can have a large effect on the wave if the obejct is larger than the distance between the waves but has a small effect if its size is smaller than the distance between the waves boat and cattail ex 4 So high frequency small compared to size of the head are disrupted by the head this disruption in sound of intensity on the far side of the head is called acoustic shadow 1 ITD and ILD give information that allows people to judge location along the azimuth coordinate but provide ambiguous information about the elevation of a sound source different elevations but same distance to ears so does not help 2 Similar ambiguous information is provided when sound source is off to the side 3 These places of ambiguity are illustrated by the cone of confusion all points have the same ILD and ITD same distance from left and right ears 4 Monaural cues cue that depends on information from iii Using Binaural Cues for Perceiving Azimuth Locations i Spectral cue primary monaural cue for localization 1 Information for localization is contained in differences in the distribution of frequencies that reach the ear from different locations ii These differences are caused by the fact that before the sound stimulus enters the auditory canal it is reflected from the head and within the various folds of the pinnae iii Using a microphone and measuring frequencies of sounds coming from different direction in both ears at elevations 15 degrees above and below the head 1 Sounds coming from these two locations would have the same ILD and ITD but the difference in the way sounds bounce around within the pinna create different frequency spectra for the two locations iv The importance of the pinna has been demonstrated by showing that smoothing out the nooks ad cranies of the pinnae with molding compound makes it difficulty to locate sounds along the elevation coordinate g Monaural Cue for Localization only one ear v The idea that localization can be affected by using a mold to change the inside contours of the pinnae was also demonstrated 1 Paul Hofman and coworkers a Determined how localization changes when the mold is worn for several weeks and when what happens when the mold is removed b Sounds were presented at positions indicated by the intersections of the blue grid Average localization performance is indicated by the red grid the overlap between the two indicates that localization was accurate c Method after measuring initial performance Hofman fitted her listeners with molds that altered th shape of the pinnae and therefore changes the spectral cue d Results localization performance is poor for the elevation coordinate immediately after the mold is inserted but locations can still be judges at locations along the azimuth coordinate i This is what we expect if binaural cues are used for judging azimuth location and spectral cues are responsible for judging elevation locations ii Performance improved as days passed until by 19 days localization had become reasonably accurate iii When the molds were removed localization remained excellent immediately after removal of the ear molds iv Apparently training with the molds created a new set of correlations between spectral cues and location but the old correlation was still there as well 1 This may be due to separate set of neurons involved in each set of spectral cues 2 Overall a ITD and ILD work for juding azimuth location ITD for low frequencies and ILD for higher frequencies b Spectral cues work best for judging elevation especially at higher frequencies c Moving our heads help us avoid cone of confusion 2 The Physiology of Auditory Localization a Narrowly Tunes ITD Neurons i The idea that there are neurons that respond best to a specific ITD has been suggested by experiments that have found neurons in the inferior colliculus and superior olivary nuclei that respond to a narrow range of ITDs ITD tuning curves for narrowly tuned neurons ii 1 The neurons associated with the curves the left fire when sounds reaches the left ear first and the ones on the