The power spectrum of a signal is the power or more simply the energy of the signal at each frequency that it contains. It can also be considered as the range or spectra of energy or power of the given signal derived from the signals' range of frequencies. For example, white noise, which contains all frequencies at the same power, has a flat power spectrum. The power spectrum of the sound of the bottom key on a piano would have a high value at the frequency corresponding to its note, and low values elsewhere.