# What the Three Words Mean

June 25, 2019

Each word in Power Spectral Density is chosen to represent an essential analysis component.

Power
Refers to the fact that the magnitude of the PSD is the mean-square value of the signal being analyzed. It does not refer to the physical quantity ‘power’ (as in watts or horsepower). But since power is proportional to the mean-square value of some quantity (such as the square of current or voltage in an electrical circuit), the mean-square value of any quantity has become known as the power of that quantity.

Spectral
Refers to the fact that the PSD is a function of frequency. The PSD represents the distribution of a signal over a spectrum of frequencies, just like a rainbow represents the distribution of light over a spectrum of wavelengths (or colors). The following lesson will discuss how a signal is converted from the time domain into the frequency domain during the PSD calculation.

Density
Refers to the fact that the magnitude of the PSD is normalized to a single hertz bandwidth. For example, with a signal measuring acceleration in units of G’s, the PSD has units of G2/Hz.

Since the name PSD does not include the quantity being measured, the word power is sometimes replaced by the name of the quantity being measured. For example, the PSD of an acceleration signal is sometimes referred to as Acceleration Spectral Density.