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Re: HRTF failure

Sorry if you receive multiple copies of this email - I accidentally sent
it to the wrong address yesterday!


You could try looking at papers such as this one to see some of the issues
involved in presentation of binaural stimuli via headphones:

Your problem MAY be because of the headphones. I don't know the detailed
specs, but I believe the DT-770 is a circumnaural diffuse-field design.
Diffuse-field headphones try to recreate the effect of an HRTF (very
loosely). This is intended to make audio recorded and mixed for loudspeaker
presentation sound realistic on headphones. If you play a binaural recording
through these headphones, the HRTF will have been applied twice - once
(accurately) during the Kemar recording, and then again, albeit
approximately, on playback.

For realistic playback of binaural recordings, you should really use a
free-field headphone (which doesn't try to mimic an HRTF). It might even be
worth investing in some good in-ear headphones to make sure there's no
residual effect due to the listeners' pinnae. If your budget won't stretch
that far, I believe the Beyerdynamic DT-660 is a free-field design, is
cheaper than the DT-770, has a much flatter bass response, but is otherwise
very similar. It's not as well-liked by the audiophile community, but I
think that's because they don't usually listen to binaural recordings, so
the DT-770's diffuse-field design might be more appropriate for them.

Incidentally, if you want to compare measured frequency responses,
impedances, etc. for the more popular headphones, there's a rather nice
"build-a-graph" feature at this web address:

I hope that helps!

Steve Beet
Principal R&D Engineer, Aculab plc, UK

-----Original Message-----
From: AUDITORY - Research in Auditory Perception
[mailto:AUDITORY@xxxxxxxxxxxxxxx] On Behalf Of Christian Kaernbach
Sent: Friday, November 14, 2008 10:44 AM
To: AUDITORY@xxxxxxxxxxxxxxx
Subject: HRTF failure

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 

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 

Thank you very much in advance,

Christian Kaernbach
Christian-Albrechts-Universität zu Kiel