Measurement of phantom source localisation

[jlo]

Hi,

you all know that sound localisation relates mainly on interaural differences : level (ILD) and time (ITD).
Those localisation cues apply for real sources but also for phantom sources generated by two loudspeakers in stereo configuration.

To "improve" monophonic sound, some researchers proposed pseudostereophonic systems, mainly with assymetrical all-pass filters on left and right, widening the stereo image and increase of apparent source width.
On the opposite, better loudspeaker and room symmetry (phase, level and frequency matching, toe-in,....), should improve perceived localisation : better source accuracy and stability, less blur.

Here comes my question : how can we objectively measure subjective localisation ?
I'm not searching for a complete auditory model to estimate true source position but rather for an indicator to compare localisation from various loudspeakers in rooms.

In the huge litterature on auditory localisation, I found only a few showing objective measurement of a phantom source :
- Perceptual differences between wavefield synthesis and stereophony by Helmut Wittek, see page 42 : frequency dependant IACC measurement http://www.google.fr/url?sa=t&rct=j&...44697112,d.ZWU
- Moglichkeiten und Grenzen der elektronishen Raumkorrektur bei Lautsprecherwiedergabe, Sebastian Goossens und Christian Gutmann, see page 15 : measure of groupdelay in lower frequencies, level differences at higher frequencies http://www.tonmeister.de/vdt/webdown...nberg_2011.pdf



From measured impulse responses, we can compute frequency related ILD and ITD and estimate perception of a "theoretically central" phantom source :
- azimuth angle : position, accuracy, stability
- source focus (no localization blur)
So I did a simple software to estimate localisation position computing ILD and ITD from IR measurements.
This soft will be free (but works on windows only) and is not yet finished (need just a few days more). So I'm happy if I can get some good suggestions here (and if some want to debug it, tell me)
This soft has a nice feature, the calculation is open : a GNU Octave script that everybody can modify and improve.
For now, the script does the following :
- frequency dependant windowing of the IR
- separate spectrum in 30 bands with a FIR filter
- calculate L/R IACC and level difference in each band
- compute localisation from both IACC and level (time intensity trading/adding)
First look of results :

A few remaining questions :
- can we really avoid measuring with an artificial head and simply work with L and R impulse responses ?
- should we use a frequency dependant time widow ? a longer window ?
- does the calculated IACC represent the real ITD over the whole spectrum ?
- is IACC + level enough or are there other parameters to add such as HF signal enveloppe, HRTF, binaural crosstalk,... ?
- what frequency filtering should be used : gammatone, 1/6 octave,...
- how should ITD be calculated ? IACC, direct phase, groupdelay, envelope of signal ?
- time/angle and level/angle are now linearly approximated : should this be changed ?
- should we measure over a whole listening area to check stability of localisation ?
- should we also measure with non central phantom sources ?
- and the most important, does this analysis correlate to perception ? how to precisely check it ?

[john k...]

Can you measure the length of an imaginary string?

[Joachim Gerhard]

Yes, it is any size.

[john k...]

Originally Posted by Joachim Gerhard

Yes, it is any size.

[jlo]

Can you measure the length of an imaginary string?

With the help of imaginary books
Spatial Hearing - The Psychophysics of Human Sound Localization: Jens Blauert

Anyway, here is an imaginary download page : blogohl: POPS Position Of Phantom Source
First results are encouraging (well, only for me).

[Pano]

Originally Posted by jlo

- and the most important, does this analysis correlate to perception ? how to precisely check it ?

That's the most interesting question for me. Can we make your software closely relate to what people hear?

I'm game to test it, I have access to many speakers and many rooms. We just need to figure out some test procedures. I'll leave the heavy lifting (math and code) to you. I'll just test, measure and report.

[dewardh]

Originally Posted by john k...

Can you measure the length of an imaginary string?

Yes. Send me twenty imaginary dollars and I'll send you a 25' imaginary tape measure.

But it is, I think, a serious question that deserves more than a flippant response. We all talk about "imaging" and "localization" and even (some of us) "auditory scene" . . . but always in the most subjective of terms. There is no metric (that I know of) to quantify the "experience" in a way that would enable reasonable comparisons . . . not even a simple "81% of test subjects correctly placed the flute to the left of the oboe and both left of center". And we rarely distinguish, in our discussions, between the accurate reproduction/representation of a real scene and the synthetic creations of studio recordings or movie sound tracks.

Perhaps it's just hopeless . . . not enough people seem to care.

[jlo]

Originally Posted by Pano

I have access to many speakers and many rooms. I'll just test, measure and report.

Thanks, I would be especially interested about results for high directivity speakers : does the phantom image stay more central than with low directivity speakers ?

[Rudolf]

Originally Posted by dewardh

We all talk about "imaging" and "localization" and even (some of us) "auditory scene" . . . but always in the most subjective of terms. There is no metric (that I know of) to quantify the "experience" in a way that would enable reasonable comparisons . . . not even a simple "81% of test subjects correctly placed the flute to the left of the oboe and both left of center".

Here I am all with dewardh.
I believe we must start with a lower level of precision than localization angles. In my place I can deliberately change those angles by changing the proportions of the stereo triangle or the degree of wall absorption. We would have to agree on exactly the same room situations to come to equal/comparable terms.

How about instead testing the degree of resolution first? Which different instruments/sources can we separate spatially in an "auditory scene" (Yes, I'm one of those using that term )?
My experience is, that percussion instruments are quite easy to keep apart. For a first and very basic start and to keep us from infringing copyrights on musical content we could start at this adress:
video

We would start "Video 3" on that page, move forward to 2:55 min and play the "Black kit". And after listening we tell us, how many different locations we can identify for the parts of the drum kit and how they are distributed.

Would this be a feasible way to compare?

Rudolf

[markus76]

Originally Posted by Rudolf

Would this be a feasible way to compare?

Don't think this would work well. Once you know how something should sound, it will. I've tricked myself many times with the barrier. When the barrier was removed, spatial quality didn't degrade. But when I listened the next day, starting without the barrier, spatial quality was clearly degraded. It seems the brain holds on to a certain "processing" for some time.