12 Feb
2009
12 Feb
'09
6:06 a.m.
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.
Of course, I'm fully agree that the model we speak about is an
over-simplification of the reality. However, should we do simplification,
shouldn't we try to have the most correct over-simplified model? This is a
real question, I don't know whether the simplification is just too important
or if by computing the "correct" early reflections on the sides, the ceiling,
the back of the stage, and the back of the room leads to a better space
feeling.
I was considering ceiling as well when speaking about 2D geometry, but if the
ceiling is not flat you are right, it becomes 3D ; however if we do a single
reflection this is still easy. I am not suggesting to build a general purpose
ray-tracer ;-)
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
affair.
Do you have any experience about whether a simplified but realistic model is
worth its development time?
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.
Ok, so should we use an allpass filter, we could as well tune it by ear?
You normally wouldn't use stereo input to such a
spatialiser, but position each source independently.
So if one has a stereo input of a violin ensemble that spans 3 meters in
width, one should use two independent sources to compute the reflections?
What about depth, should one use two sources, one in front and one in back,
and compute early reflections for both?
Thank you, have a nice day,
Stéphane
--
http://stephane.magnenat.net