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:

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.
Nondeterministic - random
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).

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

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