[LAU] ambisonics and number of speakers

[Jörn Nettingsmeier]

On 02/25/2010 11:01 PM, Arnold Krille wrote:
> With the currently finished 4 channels the surround is good. But it still has
> problems with imaging the sources. Originally I was thinking about six
> channels in the plane and two over-head to have a half-sphere. But I will
> definitely try eight-channels in the plane before. I think it will sound better
> then six channels...

it depends on the order of your source signal.
when i got the chance to play with a second order mix in the sala bianca 
in parma (a four-wall wavefield synthesis rig) with fons, our first test 
was to create six virtual speakers in a hexagon. sounded very nice, even 
better than discrete speakers because you can place them further away to 
better resemble a plane wave.
funny thing was when we went to eight speakers, the localisation 
deteriorated, very slightly but noticeably.

generally, it seems to be advisable not to use too many speakers for any 
given order. 8 is great for 3rd order horizontal, ok for 2nd and 
probably too many for first. there is a sound scientific explanation for 
this which i have forgotten... let me try (and i hope fons will jump in 
if i'm getting  stuff badly wrong):
i believe it has to do with the blurring of the energy vector at HF - 
ideally, you want just one speaker to play a discrete source (which 
means rE equals one, for a source coming from where the speaker is).
the higher the number of speakers that contribute (and if your signal is 
first order, all speakers will, to some extent), the lower the energy 
vector, which means you only get good directional cues in the LF band.

additionally, HF phasing effects become more pronounced since a number 
of more-or-less coherent sources interfere. you can work around that by 
"de-tuning" the delay compensation, but then you trade localisation 
precision for phasiness.

these issues also explain why a 3d rig gives worse 2d reproduction than 
a horizontal-only rig. it's all not as bad as it sounds, but your 
investment might not pay off the way you're hoping.

best,

jörn