HRTF failure (Christian Kaernbach )

Subject: HRTF failure
From:    Christian Kaernbach  <auditorylist@xxxxxxxx>
Date:    Fri, 14 Nov 2008 11:43:50 +0100

Dear List, We encounter a problem when trying to place a sound at a virtual position in space by means of head related transfer functions (HRTF). We use sounds from the IAPS database (International Affective Digitized Sounds System, Bradley & Lang) as well as simple white noise of six seconds duration. We use the Kemar HRTF, the "compact data" with zero elevation. We convolve the original sound data with the HRTF data as suggested in the documentation. The final sounds are presented using Beyer Dynamic DT770 headphones. We have tested the precision with which our sounds are placed in virtual space, by presenting them to eight listeners. The listeners had a touchscreen lying on their lap, with a circle plotted on it, and they could indicated the direction where they perceived that the sound came from. We presented to them in total 144 sounds, 72 noises and 72 IAPS sounds, coming from 36 virtual directions (0°, 10°, 20°...) in randomized order. The results are shown in a figure that I put in the internet: The red dots are from IAPS sounds, the yellow dots are from the noises. The x-axis shows the "true" (virtual) angle, the y-axis shows the estimated angle. As can be seen in this figure, listeners could well discriminate between sounds from the left and sounds from the right. But not more than that. There is a certain reduction of variance for sounds coming from 90° and from 270°, but there is no correlation with angle within one hemifield. Now we are eager to learn from you: What could be the cause for this failure? A) HRTFs are not better than that. B) The headphones are inadequate. C) It must be a programming error (we don't think so) D) .... We are grateful for any help in interpreting the possible cause for this failure. Thank you very much in advance, Chris -- Christian Kaernbach Christian-Albrechts-Universität zu Kiel Germany

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