POPS Position Of Phantom Source

Summing localisation of sterophonic phantom sources depends mainly on interaural differences : level (ILD) and time (ITD).

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 position blur.

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 or configurations in rooms.

In the huge litterature on auditory localisation, I found only a few studies showing objective measurement of a phantom source :

Click the image to open in full size.
Click the image to open in full size.

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 impulse response measurements.
This soft is free (but works on windows only). It has a nice feature, the calculation is open : a GNU Octave script that everybody can modify and improve the pops.m octave file.

For now, the script does the following :

  • frequency dependant windowing of the IR
  • separate spectrum in third octave 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)

Here microphone is moved to the right and the left of the central listening position :






Here mic is at the right and loudspeakers are not toe-in and toe-in (second image). The central phantom stays more central when speakers are crossed in front. 






Left level decreased by 15dB and then by 8dB :





Mic at center position, L and R at normal distance and then L moved to front by 60cm :





Many remaining questions :

  • can we really avoid measuring with an artificial head and simply work with L and R impulse responses ?
  • is IACC + level enough or are there other parameters to add such as HF group delay, HRTF, binaural crosstalk,… ?
  • what frequency filtering should better be used : gammatone, 1/6 octave,…
  • how should ITD be calculated ? IACC, direct phase, groupdelay ?
  • time/angle and level/angle are now linearly approximated : should this be changed ?
  • should we also measure with non central phantom sources ?
  • and the most important, does this analysis correlate to perception ? how to precisely check it ?

 Download POPS now