lowpass filter. stumbled upon the formula by accident, when playing around with interpolation, and non-linear envelopes for the fx_kick. instead of using an incoming sample directly, we 'travel towards it', going just a small step towards it. kind of slowing down the fastest movements in the waveform. like a lowpass filter. quite simple to calculate (a plus, a minus, and a multply per sample, double for stereo), and a different way of looking at filtering.