Hi,
Date: Mon, 25 Jul 2011 08:24:55 -0000
From: Richard Lee <rica...@justnet.com.au>
________________________________
Mr. Hunt, I hope Sampo & Fons have been sufficiently enlightening. A
Classic Ambi rig or soundfield mike has no concept of a "unit circle".
They record & present distance as presented to them. The mike cos
Helmholtz etc and the Classic Ambi rig with tricks like NFC. Even
simple
1st order Classic Ambi rigs with NFC do a good job at plane wave
reconstruction at LF.
Both have been exemplary in their clarity, unlike myself.
I was so involved in thinking about what I was trying to say, and the
relationship between Distance Compensation in a decoder and Near
Field Compensation in NFC. that I didn't re-read what I'd written
objectively enough before sending something with so many errors. I
apologise, though the result has been very interesting.
The Distance Compensation (aka NFC, and not the shelf filters)
attempts to correct for the loudspeakers not producing plane waves at
the listener.
True the "Classic Ambi rig or soundfield mike ..... record & present
distance as presented to them". The concept of a 'unit circle' only
appears in the encoding equations, which describe how to 'pan' mono
sources to produce B-Format signals. When you try to include distance
in these there is a different behaviour outside the radius of the
speakers, than inside. Direction is determined by coordinates limited
to being inside the unit circle, whereas distance (and its effects on
amplitude, time of arrival, and changes in reflections and
reverberation) must use unlimited coordinates. Then it is useful to
consider the radius of the speaker rig as unity, and all distances as
being relative to that.
It has ben suggested that W = S(1 - 0.293(sq(x) + sq(y) + sq(z)),
where S is source amplitude, be used to increase W to compensate for
X,Y & Z tending to zero and perceived loudness decreasing instead of
increasing with proximity. Again this is only applicable inside the
'unit circle'. This is possibly only to be used if there is no other
amplitude/distance law in operation.
The term NFC can be used in two different contexts: decoding, as in
your above paragraph, and encoding, as in NFC-HOA, though that would
also seem to include the decoding.
For a synthetic source to replicate this serendipitious situation,
you have
to
1) Add proximity for close sources or motorcyles as the Encoding
Eqns in
Appendix of "Is my Decoder Ambisonic?" This is the most important
(only)
cue available for close sources in anechoic conditions
Agreed, but quantifiable only relatively i.e. for moving sources. For
an unknown static source it tells you very little.
2) Add a suitable reverb pattern as MAG's "Distance Panners". You
need to
do this not cos 1) dun wuk but cos real life distance perception is
TERRIBLE under anechoic conditions. Ambisonics is probably the
best "I am
there" system cos it's isotropic nature reproduces reverb and other
diffuse
fields 'accurately'. This was one of MAG's obsessions, even with
stereo.
Anechoic conditions are fairly rare, and reverberant conditions
common, especially in urban societies. More reverberation, or rather
the variation in the ratio of direct to reverberant sound levels,
suggests greater distance, but again it is only relative, and the
nature and level of the reverberation is not simply related to source
distance.
Reverberation is also used compositionally, to suggest distance, for
effect, and to unite disparate sources in a common 'acoustic'.
3) For very far souces, you might want to add HF absorption etc but
this is
probably out of the realm of the sources you want to simulate.
As you say, HF absorption is only perceptible at large distances, and
again is only relative.
Another factor is time of arrival of sound from a distant source, not
directly perceptible unless accompanied by a visual event, or a
source which is travelling towards or away from you, the Doppler
effect. It can be almost deleterious to model this, though it is
commonly experienced.
Ciao,
Dave Hunt
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