[dB]: 10log_{10}( |h|^{2} ) = 20log_{10}( |h| )

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y = fft(x);

f = (0:length(y)-1)*(Fs-1)/length(y); % frequency vector

Pf = (abs(y).^2)/length(y);

Pf = (y.*conj(y))/length(y);

Absolute value dB 100 40 10 20 1 0 0.1 -20 0.01 -40

octave - factor of two

decade - factor of ten

spatial frequency bandwidth:

(logF_{h} - logF_{l}) / log2

where F_{h} and F_{l} are the high- and low-frequency half-amplitude points, respectively.