Sound localization psychophysics

Transcription

1 Sound localization psychophysics Eric Young A good reference: B.C.J. Moore An Introduction to the Psychology of Hearing Chapter 7, Space Perception. Elsevier, Amsterdam, pp (2004). Sound localization: what is it good for? 1

2 Where s the bird? What are the cues for sound localization? There are two binaural cues: ITD: interaural differences in time of arrival of the sound ILD: interaural differences in the loudness (sound level) of the sound Note that ITD can be separated into two cues: 1) the ITD of the envelope of the sound (ITD above) and 2) the ITD of the details of the waveform (the fine structure, IPD at right) ITD and IPD are numerically approximately equal. However IPD cues are much stronger perceptually. 2

3 ITDs are primarily a cue for azimuth in that they vary little with elevation. Middlebrooks and Green, 1990 ILDs are also mainly a cue for azimuth, although at higher frequencies, they provide additional information. Middlebrooks and Green,

4 How are different cues integrated? ITDs are used at low frequencies and ILDs at high frequencies. Blue curves show the cues measured at the ears for a speaker at the MAA. Green curves show the minimum detectable ITD and ILD, based on headphone experiments. ITD matches ILD matches spectral cues here? IPD! Mills, 1972 What is the interaction of cues when they don t correspond? Are ITDs and ILDs perceptually equivalent? It is possible for subjects to adjust the ITD of a sound (ordinate) so as to center the image of a sound presented with a certain ILD (abscissa)? Note ITDs are more effective at low frequencies Harris,

5 But that doesn't mean that ITD and ILD produce equivalent percepts. Here subjects discriminated a 500 Hz tone with 0 ITD from a similar tone with one of five ITDs as the ILD of the second tone varied. The discriminability (d') is plotted versus the ILD. Perfect trading would have resulted in d' minima near 0. Note that as the ITD increases, the "best" ILD (the minimum of the curve) gives a stimulus with higher and higher discriminability. So ITD and ILD are not perceptually equivalent, despite the trading ratio experiment. X! Hafter and Carrier, 1972 What are the cues for elevation and front/back? Because the head is approximately symmetrical, locations along a cone of confusion all produce roughly the same binaural cues ITD and ILD. Thus ITD and ILD provide little information about elevation and there is a confusion about front vs back. 5

6 The ambiguity is resolved by spectral cues produced by the external ear. The amplitude of sound at the eardrum is modified by reflections (interference patterns) in the pinna. The pattern of modification, plotted below, varies with the direction of the sound source Two sound paths through the pinna Shaw Evidence that pinna acoustics are important for location in elevation: occluding the cavities of the pinnae decreases performance in a sound source elevation task Gardner and Gardner,

7 In order to use spectral cues accurately, the stimuli must be broadband. With narrowband stimuli (1/6 octave noiseband), the percept of elevation depends on the frequency content of the stimulus, and not its source direction. 6 khz noise sounds like above and 8 khz noise sounds like below in this subject. Middlebrooks, 1992 Cue trading revisited: broadband noise is presented over headphones that simulate virtual-space by incorporating HRTFs. The result is good localization with all cues present. However, when ITD cues are set to 0, -45 or 90, it is clear that the ITD cues dominate the others. That is, the localization in azimuth follows the ITD cues and the elevation performance is degraded. This result occurs only if the stimulus contains lowfrequency energy. X! Wightman and Kistler 7

8 Binaural unmasking: Using localization to reduce interference or masking: the cocktail party effect. Binaural masking level differences are a part of the explanation. The results are from tone detection experiments in a noise masker. The relative phases in the two ears of the tone (S) and noise (N) are given by the subscripts. Noise with a a different interaural phase (different location) is less effective in masking a tone (by up to 15 db!). The effect is strongest at low frequencies, but continues at high frequencies (ordinate is N 0 S 0 / N π S 0 ). This corresponds roughly to the strength of phase-locking in the auditory nerve. 8

9 An important deficit in hearing impairment is the loss of binaural unmasking. Note the speech reception thresholds (SRT, the signal/ noise ratio at threshold for speech intelligibility) are worse in impaired listeners for various noise conditions. Bronkhorst Plomp 1989 Using localization to suppress echoes: the precedence effect 9

10 With reverberation, the first sound that arrives (black Xs) is more accurate than subsequent sound (gray dots). Direction to which ITDs point for a 580 Hz tone at three directions. The first few ms of information are more accurate in the cases with reverberation. Data from a MSO model with 4 ms sequential analysis bins. Shinn-Cunningham et al Precedence decreases the information about location for the second of two stimuli, presumed to be an echo. 10