On Wed, 20 Sep 2006, Ashifi wrote:
could you kindly tell me what the argument "slowness" in the
continuous_src_time function definition in the meep.hpp source file
means? If I've been following the archives correctly, the "double w"
parameter would correspond to the width, which makes the source turn on
with a ramp (as opposed to instantaneous, which excites modes, bad, etc).
The comment right above the continuous_src_time definition says there's
optional source turn on and turn off. Does the width (w) control both?
The slowness is preset to 3.0.
If you have a width of 0 (the default), then the source turns on sharply.
Otherwise, the source turns on with a (1 + tanh(t/width - slowness))/2
shape. That is, the width parameter controls the width of the turn-on.
The slowness parameter controls how far into the exponential tail of the
tanh function the source turns on.
The default slowness of 3.0 means that the source turns on at (1 +
tanh(-3))/2 = 0.00247 of its maximum amplitude. A larger slowness
parameter means that the source turns on even more gradually to start with
(farther in the exponential tail).
Steven
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