Function rolloff 

pub fn rolloff(
    frequency: &[f64],
    magnitude: &[f64],
    low: f64,
    high: f64,
    fraction: f64,
) -> Option<f64>

The frequency beneath which fraction of the band’s energy lies, in Hz.

The roll-off point: 0.95 answers where ninety-five percent of the energy has accumulated, walking upwards from the bottom of the band.