The Effects of Coupled Source/cavity modes on the Acoustic Insertion Loss of Close-Fitting Enclosures

This paper reports a theoretical and experimental study of the prediction of the insertion loss for low frequency noise from close-fitting enclosures. Experimental work has been carried out for various small panel designs with several common structural materials using a special test rig. The theoretical model considers not only the modal response of the enclosure plate but also the 3 dimensional acoustic modes in the air gap and the modal component of the source plate. The theory and the experiments show two important points; 1) the contribution of the non-fundamental mode of the source plate is important for the insertion loss of the enclosure; and 2) the cavity modes have significant effects on insertion loss not just at the acoustic resonances but they also affect the coupling between the source and enclosure plates. The effects of stiffness and damping on the higher structural and acoustic modes are also studied in the experiments. Comparisons of various tested plates suggest that wood is potentially a good material for low frequency noise reduction due to the high stiffness-mass ratio. The attachment of stiffeners can improve the insertion loss at some frequencies, but may decrease the insertion loss at other frequencies.