There are several methods for describing electrical or mechanical vibration phenomena. If the vibration is periodic all the information necessary for a complete description is contained in one period of the signal. However, if the signal is non-periodic an infinitely long record of the signal is theoretically necessary for complete descriptoin. This is, of course, an impossible requirement in practice. Furthermore, if the time record becomes too long it also becomes a very inconvenient signal description and other methods are commonly used.
One of the best known methods is the use of frequency analysis technique which describes the phenomenon not in the time domain, but in the frequency domainf) The result of a frequency analysis is a frequency spectrum which may be presented in various ways. Two of the most commonly used methods are l. An amplitude level versus frequency diagram and 2. power spectral density diagram The use of both these types of presentation is discussed in detail in the following with special emphasis on the power spectral density method