Why is this generic BP filter seem to magnifying amplitudes when I run it at lower sample rates? The bass range seems to 32 times louder at 1275 Hz (44.1 kHz / 32) than at 44.1kHz?

Here is a graph of a birdsong which has been passed through 128 BandPass filters on MIDI notes 0 to 128.

To make the graph draw faster, I have attempted an experiment: skip 32 audio samples for frequencies lower than 150HZ.

The filter-bank draws 15 times faster, but the results are wrong: the frequencies lower than 150Hz seem to have been multiplied by an amplitude of 32? What is happening?

enter image description here correct graph: enter image description here The same downsampling method doesn't amplify a sine sweep, so I think it will not be possible to use this computation optimizing trick for a high quality spectrogram? enter image description here


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