Bioacoustic Index from Boelman, et al. 2007.
Inspired on Matlab code courtesy of NT Boelman.
Several parts where changed, in particular log math, so this won't be
directly comparable to the original code in the paper.
The Bioacoustic Index is calculated as the "area under each curve included all frequency bands associated
with the dB value that was greater than the minimum dB value for each curve. The area values are thus a
function of both the sound level and the number of frequency bands used by the avifauna" (Boelman, et al. 2007).