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The spectral density of a signal is a way of measuring the strength of the different frequencies that form the signal. For example, if we had a sound wave from a piano in which the keys of middle C and A were struck, then the pressure variations making up the sound wave would be the signal and "middle C and A" are in a sense the spectral density of the sound signal. Mathematically, if the notes were pure sinusoids (produced by something like a tuning fork; not a piano), the spectral density would be a function of sound frequency, with two spikes at 261.6 Hz and 440 Hz, corresponding to the frequencies of middle C and A.
The spectral density is a general concept applied to a signal which may have any physical dimensions or none at all. In physics, the signal is usually a wave, such as an electromagnetic wave, or an acoustic wave. The spectral density of the wave, when multiplied by an appropriate factor, will give the power carried by the wave, usually per unit frequency or per unit wavelength. This is then known as the power spectral density (PSD) or spectral power distribution (SPD) of the signal. The units of spectral power density are commonly expressed in watts per hertz (W/Hz) or watts per nanometer (W/nm) (for a measurement versus wavelength instead of frequency).
Although it is not necessary to assign physical dimensions to the signal or its argument, in the following discussion the terms used will assume that the signal varies in time
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