Spectral rolloff

[MichaelSvejcar]

Hello, first of all, thank you for developing this useful library. One thing is unclear to me though. I want to calculate the spectral rolloff from audio samples represented as float[] with individual samples in range <-1; 1>. After calculating the MagnitudeSpectrum using Fft object, how do I know which frequencies to pass into the Rolloff method, as the same-named parameter? Is there any way to retrieve this information from the Fft object somehow? I was not able to find anything like it. This is the code i have so far:

var signal = new DiscreteSignal(sampleRate, samples);
var fftSize = MathUtils.NextPowerOfTwo(signal.Length);
var fft = new Fft(fftSize);
var spectrum = fft.MagnitudeSpectrum(signal);


var frequencies = ??
var rolloff = NWaves.Features.Spectral.Rolloff(spectrum.Samples, frequencies);

But what should I pass as the "frequencies" here?
Thank you for any help, appreciate it!