Internal soundgen function
getAM(m, amRange = c(10, 100), amRes = NULL)numeric matrix of non-negative values with colnames giving temporal modulation frequency
the range of temporal modulation frequencies that we are interested in as "amplitude modulation" (AM), Hz
controls the width of window over which we look for local maxima
Returns a list with the frequency (Hz) and depth of amplitude modulation (dB relative to global max, normally at 0 Hz).
Helper function for calculating amplitude modulation based on the modulation spectrum. Algorithm: averages AM across all FM bins in the positive half of the modulation spectrum and looks for a peak in the specified AM frequency range.