# Power Spectral Density

June 25, 2019

Back to: Random Testing

### Why Power Spectral Density (PSD) is Important

A random process, or waveform, is inherently irregular, non-deterministic, and non-repetitive. An observer cannot use one measured point in time to predict any point before or after. They can be analyzed and understood from a statistical standpoint, but any specific point, or point in time, is unknowable until it occurs.

Random waveforms are a frequently used test mode in vibration testing because they directly correlate to real world vibration. Events and behaviors in the world around us are random and it is important to realistically test the products that are going to be used in the real world.

The most common tool used to analyze random vibration is Power Spectral Density (PSD). Resonances and harmonics, hidden in a time history graph, become clearly visible in a PSD graph. In practice, generating a PSD is usually the first step in examining and analyzing a random waveform.

However, generating a useful PSD is not as simple as pushing a button. Various parameters give test engineers and technicians the ability to tailor PSD calculations to fit a specific test situation. For the parameter settings to be effective, those engineers and technicians need a clear grasp of what the PSD is.

### The Next Two Lessons

The next two lessons provide a text-based description, enhanced by example graphs, of the multiple steps involved in calculating a PSD. Each step is a mathematical process, but these descriptions avoid equations while still communicating the critical concepts. For a mathematically rigorous description of the PSD, and related calculations, navigate to *course* **Random** > *section* **Converting Recorded Data**.