White Noise

The power spectral density of white noise is independent of frequency. Since there is essentially the same energy between any two identical frequency intervals (for example 84-86Hz and 543-545Hz), white noise narrow band FFT analysis will show as flat. However octave band analysis will show the level to rise by 3dB per octave because each band has twice the frequency range of the preceeding octave.

See also: Pink Noise.