In physics, the spectral density, or more correctly the power spectral density (PSD) of a given bandwidth of electromagnetic radiation is the total power in this bandwidth divided by the specified bandwidth.

Spectral density is usually expressed in watts per hertz (W/Hz).

There exist both power and energy spectral densities. The energy spectral density is defined as

Note that the total energy in the frequency domain equals the total energy in the time domain:

This is a result of Parseval's theorem.

Practically, for discrete-time signals, the PSD is calculated using the FFT. One example is Welch's method.

See also

• Power spectrum
• Fourier transform
• Spectral leakage
• Window function

External links

• Power Spectral Density Function - from Wolfram Research's "Time Series Documentation"