During the 1980s the technology of modal testing became very widely practised in all those engineering disciplines where vibration and other dynamic phenomena affect the behaviour and performance of structures and machines. The techniques involved in carrying out a modal test were developed to a high degree of sophistication while the applications to which the results of these tests could be put became more numerous and more powerful. At the same time as the advantages of modal testing were being enjoyed by an increasing audience, some of the drawbacks of inexpert use of the technology were...