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I don't know at the moment what the calibration code is doing. Since the variables "pper" and "p" are almost perfectly correlated, I'm assuming that the temperature correction, if it is being applied, is quite small (at least at these frequencies). In any case, since this "filtering" appears in "pper", I'm assuming that it hasn't been introduced by post-processing.
Did Did anyone else look at pressure spectra before I arrived? If so, did you see similar behavior??
I might try changing the sample-side tubing on one of the transducers, in a long-shot attempt to figure this out, but otherwise I'm stumped.