[LAU] orchestra mixing, reverb, and spatialization

Fons Adriaensen fons at kokkinizita.net
Wed Feb 11 16:31:27 EST 2009

On Wed, Feb 11, 2009 at 08:47:31AM +0100, Stéphane Magnenat wrote:

> ... It would be much more convenient to be able to specify 
> the size of the room, the position and size of the instrument, the position 
> of the listener, and some parameters for the walls ; and let the program 
> compute the corresponding early reflections patterns. It is probably not that 
> difficult to do, as it only requires basic 2D geometry.

It's not so simple as it looks. The best spaces (for
orchestral music) do indeed have a simple 'shoebox'
shape rather than an 'auditorium' one that widens
towards the back, has a sloped ceiling, etc. But
they also have quite complex walls, with lots of
decoration, balconies, curved surfaces, etc.
Reflections from such structures are not so simple,
they are almost never specular (as in a straight
mirror) but include a lot of (frequency dependent)
scattering as well. Also 2D geometry is a bit too
simple - ceiling reflections are important as well.

> The main issues that I see are:
> - How many early reflections to take into account?

Unless you have a very weird shape (e.g. very long
and narrow), the first 4 or 5, or up to about 80ms.
But see above - what is heard (and measured) as a
single reflection can already be quite a complex

> - What is the relation between the room dimensions
> and the allpass impulses?

None AFAIK. A reflection in a real room is almost
never allpass - some bandpass would be a better
approximation. The allpass filters are used to
add artificial 'complexity' to the all too simple
models used by most reverberators, they don't really
correspond to anything physical. They reduce obvious
comb filter effects that could result from too simple
reflection models (e.g. just delay), and also improve
the reverb tail by 'randomising' it.
Allpass filters could be used to synthesise the 
complexity of any single reflection, but not in
the way it's done in most reverberators.

> - How to make nice use of stereo input; for instance, should the left wall 
> early reflection use the left channel source positioned at the most-left 
> point of the instrument area, and the corresponding layout for the right one?

You normally wouldn't use stereo input to such a
spatialiser, but position each source independently.



Laboratorio di Acustica ed Elettroacustica
Parma, Italia

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