A intuition for creating a lowpass filter ... audio happens in the time domain ... Fourier transform outputs the frequency domain representation of the audio you feed it ... number of audio samples you feed into the FFT call determines the element count of the array returned ... now you have an array of your audio in the frequency domain ... for lowpass crank down to zero array elements above the first quarter of them ... this silences those frequencies ... to render the audio ( generate sound ) feed your edited frequency domain array into an IFFT ( inverse Fourier transform )