The width of the window is inversely proportional to the parameter a.
Use larger a for a narrower window. Use larger m for longer
tails.
$$w = e^{(-(a*x)^{2}/2 )}$$
for x <- seq(-(n - 1) / 2, (n - 1) / 2, by = n).
Width a is measured in frequency units (sample rate/num samples). It should
be f when multiplying in the time domain, but 1/f when multiplying in the
frequency domain (for use in convolutions).