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You will need to listen to these demonstrations over headphones. (see AUDIO FILES for information on the *.wav files)
When two tones are played simultaneously and a listener is asked to determine if the interaural time difference of one tone has changed, it is more difficult to detect this interaural time shift than when the tone is presented in isolation. This "binaural interference" occurs even when the two tones are very far apart in frequency. It is as if, the acuity for detecting a change in the spatial location of one tone is interfered with when another tone is also present. The early work on binaural interference suggested that binaural interference might mean that spatial cues may not be useful for sound source segregation. Recent experiments (see references below) suggest that while acuity for spatial processing may be impaired when two tones are simultaneously presented, spatial separation still allows one to segregate the two tones.
For the eight Target Tonal Presentations (T:1 through T:8) below, a 450-Hz tone is presented three times each time for 200 ms. The first and third times the tone is presented without any interaural differences (diotically). The second or middle presentation contains an interaural time difference which increases from 30 microseconds to 240 microseconds in 30 microsecond steps from T:1 to T:8. Start with T:1 and decide for which Target Tonal Presentation did you first hear a change in the second tone. That is, when did the second of the three tones appears to move within your head (i.e., moves from near the center of your head, out away from the center, and then back to the center). The Target Tonal Presentation at which you first heard a change multiplied times 30 microseconds is an estimate of your threshold for detecting an interaural time difference. For instance, if you heard a change for Target Tonal Presentation T:3, then your interaural time diference threshold is about 90 microseconds (30 X 3 = 90).
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For the eight Target plus Distractor Tonal Presentations (TD:1 through TD:8). A 750-Hz tone is added to the same eight 450-Hz tones that were used above for the Target Tonal Presentations. The 750-Hz tone is always presented without any interaural differences (diotically). Again decide for which Target plus Distractor Tonal Presentation did you first hear a change in the middle tone. The Target plus Distractor Tonal Presentation at which you first heard a change multiplied times 30 microseconds is an estimate of your threshold for detecting an interaural time difference when the 750-Hz distractor is present. The difference between this threshold and that obtained for the Target Tonal Presentation (above) is an estimate of binaural interference. For instance if 90 microseconds was your Target Tonal Presentation threshold obtained above and 210 microseconds (TD:7) was your Target plus Distractor Tonal threshold, then there was 120 microseconds of binaural interference (210-90=120).
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Suggested References:
Dye, R. H. (1990). "The Combination of Interaural Information across Frequencies: Lateralization on the Basis of Interaural Delay," Journal of the Acoustical Society of America 88, 2172-2184.
Stellmack, M. A. and Dye, R. H. (1993). "The Combination of Interaural Information across Frequencies: The effects of number and spacing of components, onset asynchrony, and harmonicity" Journal of the Acoustical Society of America 93, 2933-2947.
Dye, R. H., Niemiec, A. J., and Stellmack, M. A. (1994). "Lateralization on the Basis of Interaural Envelope Delays: The Effect of Randomizing Component Starting Phase," Journal of the Acoustical Society of America 95.
Dye, R. H., Yost, W. A., Stellmack, M. A., and Sheft, S. (1994). "Stimulus Classification Procedure for Assessing the Extent to Which Binaural Processing Is Spectrally Analytic or Synthetic, Journal of the Acoustical Society of America 96, 2720-2730.
Dye, R. H. (1996). "The Relative Contributions of Targets and Distractors in Judgments of Laterality Based on Interaural Differences of Level," in Binaural and Spatial Hearing (eds. R. H. Gilkey and T. R. Anderson), Lawrence Erlbaum, Associates.