The **power spectrum** is a plot of the portion of a signal's power (energy per unit time) falling within given frequency bins. As opposed to the frequency spectrum, the power spectrum does not show spatial or phase angle information. The most common way of generating a power spectrum is by using a Fourier transform and taking the square of the magnitude of the complex coefficients. Other techniques such as the maximum entropy method can also be used to estimate the power spectrum. ## Existence of the power spectrum The power spectrum of a signal exists if and only if the signal is stationary. If the signal is not stationary then the same methods used to calculate the power spectrum can still be used, but the result cannot be called the power spectrum.
## Spectral centroid In acoustics, the **spectral centroid** of a sound is the midpoint of its spectral energy distribution, i.e. the frequency that divides the distribution into two parts of equal energy.
## Spectrogram If the power spectrum is plotted in time then the graph is called the spectrogram.
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