FFT stands for Fast Fourier Transform. It is an algorithm that performs spectral analysis on audio (this algorithm can work on images as well, but that’s not the intention here). The result of this algorithm tells you the magnitude of individual frequencies in an audio. As such it is most often used in audio visualization.
In Lightact, FFT is done with Audio FFT node. Inputs are an Audio stream and a float Margin, which specifies whether we want to analyze a different time point than where we are at the moment in the audio stream. The output is an array of floats, where each float represents the magnitude of all the frequencies in a particular bin.
In the Number of bins dropdown, you can select 8,16, 32, 64 … number of bins. This number specifies into how many sections (aka bins) the frequencies will be split into. Usually, 16 or 32 works just fine.
If you want to grab the value of just one bin (say you are interested only in lower tones) then you might want to use Get Float Array Item node, which returns the value of an item at the Index.
If you, however, want to get the overall volume, then you might want to use Float Array Sum node, which sums up the whole array. In this case, it’s probably good not to use too many bins as this will affect performance.