SPLIT VIBRATION MODES IN ACOUSTICALLY-COUPLED DISK STACKS

Vibration of an array of stacked annular plates, in which adjacent pla tes couple weakly through an acoustic layer, is investigated through e xperimental and theoretical methods. Such acoustic coupling manifests itself through split natural frequencies, beating in the time response s of adjacent or separated plates, and system-level modes in which pla tes in the array vibrate in- or out-of-phase at closely-spaced frequen cies. Laboratory measurements, including a technique in which the freq uency response function of all in-phase modes but no out-of-phase mode s, or visa versa, is measured, demonstrate the contribution of couplin g to the natural frequency spectrum, and identify the combinations of design parameters for which it is important. For the lower modes of pr imary interest here, the natural frequencies of the out-of-phase syste m modes decrease as the air layer becomes thinner while those of the i n-phase mode remain sensibly constant at the in vacuo values. A vibrat ion model comprising N classical thin plates that couple through the t hree-dimensional acoustic fields established in the annular cavities b etween plates is developed, and its results are compared with measurem ents of the natural frequencies and mode shapes.