Modal analysis is a powerful tool which can be used to create a dynamic model of a structure, but for some 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 excited 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 and understanding of the dynamic behavior of a structure and hence how improvements may be made to the design for optimum performance. 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 shaped auto-spectrum equivalent to a typical music spectrum. The operating modes are 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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