Advantages and Disadvantages of Intensity Measurements

Appr. 15 years ago very accurate sensors for sound waves became available. Together with modern signal processors they allow the measurment of the energy flow (intensity). Thus it is possible to determine directly sound powers and to separate uncorrelated sources. This makes intensity very useful for machine or product monitoring in a noisy environment. For the localisation of partial sound sources intensity measurements are used quite often although they may give erroneous results in the presence of correlated sources. For sound in structures intensity measurements are still in the laboratory stage — with a few exceptions. They are much more complicated to perform and up to now they can be made only on the surface of a structure. In the paper the basic relations for structural intensity and crude approximations (which generally are used) will be given. Apart from background noise suppression, measurements of structural intensity are used to visualise the energy flow on structures and to find faults such as so-called sound bridges in structures. Because of the approximations that are made in the measurements, the accuracy is not high especially in the low frequency ranges.