# What the Three Words Mean

June 25, 2019

Back to: Random Testing

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 G^{2}/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**.