Re: [LAD] A Cathedral in your backyard

Uhmm ...

You say that one should antiphase the signal from the speakers in the
opposite direction? 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.

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

I understand that standing outside the ring of more or less
omni-directonal speakers will sound in unpredictional ways, but this is
not the subject. Only a few of those (say 200) involved would be allowed
at any one time to enter the ring to experience the effect, otherwise
the mass of the crowd would get in the way.

/jens

On Wed, 2007-04-25 at 09:51 +0200, Joern Nettingsmeier wrote:
> Jens M Andreasen wrote:
> > What I think would be possible as an experiment though (without
> > involving a budget the size of the Pope or TU-Berlin), is to place a
> > large amount of uniform speakers in a circle and then feed them with the
> > same monophonic signal. If everything works as expected, it should then
> > appear like the sound emerges from the center of the circular array of
> > speakers.
> >
> > * The speakers could be a collection of ghettoblasters and kitchen
> > radioes with fresh batteries.
> > * The signal could be a local radiostation (hey, this
> > is /wireless/ :-).
> > * The location could be a park or a stadium in your city.
> >
> > Would that work?
>
> no :-D
>
> you might indeed get a "standing" wavefield that looks the same, but the
> problem is it's not a standing wave. :) listeners outside of the focal
> point would hear the speakers nearer to them first, leading to phantom
> images. generally, in large auditoriums, you have to be very careful
> with the signals coming from the opposite direction... for instance,
> "proper" ambisonics decoding calls for antiphase signals from opposite
> speakers, but that falls apart outside the "sweet spot". that's why
> malham et al. came up with their "controlled opposite" rendering, which
> basically means that the speakers opposite a sound source are quiet.
>
> i got a chance to listen to the wavefield synthesis system at tu berlin
> for several hours (thanks torben for a late-night session!), and i found
> that virtual sound sources inside the speaker array work very well for
> low and mid frequencies (torben had me step inside a techno bass drum,
> and it was quite astonishing), but it only works for higher frequencies
> as long as you're not in between the speakers and the virtual sound
> source (due to dampening i guess, and because the virtual source has not
> converged correctly yet).
>
>
>

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