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.
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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