PARSEVAL’S THEOREM
Think of f(t) as a voltage applied to a one Ohm resistor
By definition, the left hand side is the energy of the signal
Intuitively, if the Fourier transform has a large magnitude over a frequency range
then the signal has significant energy over that range
And if the magnitude of the Fourier transform is zero (or  very small) then the
signal has no significant energy in that range
Parseval’s theorem permits the determination of the energy of a signal in a
given frequency range