Hi all
For those inclined to have an interest for some of the signal processing I have involved
myself with (the averaging ideas), and for general interest, I've worked a bit with the
(demo version) Comsol 5.1 physics simulation software and in particular one example: the
Standing Wave Computations.
Here's a picture showing "standing waves" at a frequency of 90.6 Hz (the lowest mode in
this example was 74 Hz) in a room with some furniture of 4x5x2.5 meter, and the sound
pressure level in dBs on the walls, floor and furniture with a color legend:
http://www.theover.org/Musicdspexas/comsol5cm.jpg (59 kilo Byte)
http://www.theover.org/Musicdspexas/comsol5c.png (217 kB, original size)
What's the connection with DSP ? Well, if you take it you work at CD sample rate, the
corresponding "size" of a sample is appr. 340m/s / 44100 Hz ~~ 7.7 millimeter. Suppose we
do some form of filtering with a FIR length of 256 samples (a small FFT, a tabled
convolution FIR, or a sample reconstruction reconstruction filter), the associated
"length" of that sample train of 256 samples becomes an acoustic wave of about 2 meters.
The shown computation of the scientific solver for the eigen-modes shows little errors in
the low pressure parts of the standing waves, where the "S" figures on the wall indicate
knots of the self-resonance of the room, around a band of say 30 cm.
What are the connections ? Well driving the eigen-modes isn't usually what people want to
do, but it happens automatically, and everything that comes out of your speakers will
excite a resonance in the room. Of course, you may also want to compute early reflections,
for instance at the listener "sweet spot", or the whole sound transient that comes from an
excitation like a balloon pop, etc., but this is still interesting to think about, at the
very least concerning where to put damping for monitoring DSP experiments, and what the
combined error of a digital processing and D to A chain does in a listening space!
T.V.
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