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This might be useless (it's not really my area), but it would not be too hard to compare the whole spectra of all individuals just checking for e.g. the R²-value for the goodness of fit. I'd suggest smoothing the spectra first. You "split"-criteria could be when R² drops below a certain value between the two sub-species (?) but check that the R² within the sub-species remains higher.
The ISO standard has all the relevant info in the intro, and the intro is free :). I can edit the formula to include the integral, but as the answer is using general language, cumulative energy is calculated as the integral of the squared pressure, so that becomes dimensionsless, unless I'm mistaken?
I think that I don't know enough about the system to definitively give you an answer. But I interpret the animations as pressure being proportional to the density of "particles" irrespective of the direction of compression (as you write). I can however imaging that the pressure under a wave is less than if the same depth had been in still water as I think the vertical motion affects the pressure, so that the pressure fluctuations are out of phase with the wave height oscillations. We'll have to get a physicist on the case!
