Abstract:
In noise control foam both the bulk solid and fluid phases participate significantly in the wave propagation process. Here it is suggested that resonances of the foam's solid phase can be tuned to produce significant narrow-band absorption superimposed upon the broadband attenuation characteristic typically offered by the fluid phase of an elastic porous material. Two approaches have been considered. In the first, aluminum masses are applied to small pieces of foam (say, 25 mm square and 6 mm deep) that are then attached to a panel. Each piece of foam then comprises a small resonator whose natural frequency is essentially determined by the bulk stiffness of the solid phase of the foam together with the added mass. It has been found that the application of an array of foam resonators causes the transmission loss of the base panel to be increased over approximately an octave. In a second approach, a continuous foam lining is segmented, and a piece of perforated metal is applied to each segment. Again, a narrow-band increase in transmission loss is observed, in this case combined with the broadband transmission loss increase normally produced by a layer of porous material.