# Signals

- There are many different types of signals. The two main classification of signals are those that are deterministic and those that are nondeterministic. These may be subdivided as shown below:

**Deterministic**- For a deterministic signal the physical phenomenon can be represented by a mathematical relationship to a reasonable degree of accuracy.

**Periodic**- pure sine wave

**Complex periodic**- vibration signal from a petrol engine at idle.

**Almost periodic**- microphone measurement close to diesel engine at idle.

**Transient**- a swept sine wave, vibration signal from an engine that is increasing in speed, hammer force input modal excitation.

- For a nondeterministic signal the physical phenomenon cannot be represented mathematically.

**Stationary**- average statistical properties over a time interval of interest are constant e.g. interior noise of a vehicle driving at constant speed.

**Nonstationary**- characteristic of a time series for which the distribution changes over time e.g. interios noise of a vehicle that is coasting down (decreasing in speed).

**Nondeterministic - random**

**See also: **Fast Fourier Transform, Power Spectral Density, Settling Time.