Modal analysis is a powerful tool which can be used 10 create a dynamic model of a structure, but for sonic problems the application of a dynamic model is not feasible. In some applications the operating dynamic excitation forces cannot be determined, and in others only a few of the modes may be excitedly in normal operation. For such cases, a complete dynamic model is not needed. This paper presents an alternative technique - the measurement of operating-modes (OM). The operating-mode shapes represent a set of shapes into which the forced vibrations can be decomposed: one shape for each significant modal frequency. The visualized shapes aid understanding of the dynamic behaviour of a structure and hence how improvements may be made to the design for optimum performance in a particular operating condition. The technique was applied in the development of a prototype loudspeaker cabinet. To simulate normal operation, the loudspeaker was driven using weighted random noise with a shaped auto-spectrum equivalent to a typical music spectrum. The operating modes arc identified from complex transmissibility functions measured between a reference accelerometer and a roving accelerometer on the cabinet. Further qualitative and quantitative applications of measured operating-modes are also suggested.
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