| the distribution as a function of frequency of the energy of a signal, or a noise, having a continuous spectrum and finite total energy
NOTE 1 – The total energy of a signal or noise is by convention proportional to the time integral of the square of its instantaneous value. This integral is proportional to a physical energy if the characteristic quantity is a field quantity.
NOTE 2 – The energy spectral density of a deterministic signal exists if its representative time function is integrable square. It is equal to the squared modulus of the Fourier transform of a signal and also equals the Fourier transform of the autocorrelation function of the signal.