Voxengo – Span

SPAN is a free real-time “fast Fourier transform” audio spectrum analyzer AU, AAX and VST plugin for professional music and audio production applications. For the most part it was derived from Voxengo GlissEQ dynamic parametric equalizer and reproduces its spectrum analysis functionality.

Fast Fourier transform spectrum
Surround spectrum analyzer
Stereo correlation meter
K level meter
Linear-phase spline equalizer