[LAD] A Cathedral in your backyard

Joern Nettingsmeier nettings at folkwang-hochschule.de
Wed Apr 25 09:33:28 UTC 2007


Jens M Andreasen wrote:
> Uhmm ...
> 
> You say that one should antiphase the signal from the speakers in the
> opposite direction?  

no. i said that "classic" ambisonics decoders do it, and it leads to 
problems in large auditoriums with listeners outside the sweet spot.

take a stereo rig: you can put a source "outside" the speakers by adding 
antiphase sound to the opposite one. impressive, but works only if 
you're in the middle. close to the antiphase speaker, the image 
collapses into that speaker.

> Since this is a signal supposed to emerge from the
> center and the speakers are arranged in a circle, then either there are
> no speakers in the opposite direction or all speakers can be considered
> to be opposite.

imagine you are standing off-center. all speakers start playing. two 
problems: one, if the auditorium is large, the speakers close to you 
might hit a lot earlier than those on the opposite side. if they are 
more than about 30ms apart, the haas effect kicks in and you will 
localize the sound as coming from the closer speakers.
two, if the levels of the far and close speakers differ by more than 6-8 
db, you will localize the sound at the louder source even without the 
haas effect. not so much an issue, because such arrays carry a long way 
and the level does not drop much.


> Is origo a special case that doesn't work? 

origo?

 > I would have thought it to be
> the simple base-case, even simpler than the "dropping the soap in a
> bathtub, but reversed" wawe propagation I suggested earlier.

yes. reversed. i'm not an expert on this, but my guess is that when you 
try such things, you have to take into account that it is *not* a 
standing wave field. it travels, but in the wrong direction. that means 
your ITD cues (interaural timing difference) will be wrong.

i guess projecting virtual sound sources inside an auditorium is akin to 
making things invisible according to douglas adams: in almost all cases, 
it's more convenient to actually put a real sound source there :)

-- 
jörn nettingsmeier

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Kurt is up in Heaven now.



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