Editing Spectral density (section)

== Applications ==
{{further|Spectrum}}
Any signal that can be represented as a variable that varies in time has a corresponding frequency spectrum. This includes familiar entities such as [[visible light]] (perceived as [[color]]), musical notes (perceived as [[Pitch (music)|pitch]]), [[radio frequency|radio/TV]] (specified by their frequency, or sometimes [[wavelength]]) and even the regular rotation of the earth. When these signals are viewed in the form of a frequency spectrum, certain aspects of the received signals or the underlying processes producing them are revealed. In some cases the frequency spectrum may include a distinct peak corresponding to a [[sine wave]] component. And additionally there may be peaks corresponding to [[harmonics]] of a fundamental peak, indicating a periodic signal which is ''not'' simply sinusoidal. Or a continuous spectrum may show narrow frequency intervals which are strongly enhanced corresponding to resonances, or frequency intervals containing almost zero power as would be produced by a [[notch filter]].

=== Electrical engineering ===

[[File:Spectrogram-fm-radio.png|thumb|right|Spectrogram of an [[FM radio]] signal with frequency on the horizontal axis and time increasing upwards on the vertical axis.]]
The concept and use of the power spectrum of a signal is fundamental in [[electrical engineering]], especially in [[communication systems|electronic communication system]]s, including [[radio communication]]s, [[radar]]s, and related systems, plus passive [[remote sensing]] technology. Electronic instruments called [[spectrum analyzer]]s are used to observe and measure the '''''power spectra''''' of signals.

The spectrum analyzer measures the magnitude of the [[short-time Fourier transform]] (STFT) of an input signal. If the signal being analyzed can be considered a stationary process, the STFT is a good smoothed estimate of its power spectral density.

=== Cosmology ===
[[Primordial fluctuations]], density variations in the early universe, are quantified by a power spectrum which gives the power of the variations as a function of spatial scale.