The use of multilayer absorbers has become increasingly important in noise reduc — tion.74 It is possible to tailor the materials for maximum absorption for the broadest frequency range by layering them.2

NAC values for various frequency ranges can be enhanced by changing the den­sity and composition of the fibrous structure. This area needs more investigation on the inner structure of multilayer absorbers.74 Multi-layer absorbers achieve higher sound absorption than the mono-layer absorbers with the same thickness.17 Ingard35
reported a significant drop in the critical frequency above which maximum absorp­tion was achieved when the fibrous absorber consisted of layers with different flow resistivities. The critical frequency was significantly higher for a single-layer mate­rial with the same total thickness and flow resistance.

Ingard35 reported that the sound absorption is greater when the flow resistivity, r0, increases from the surface toward the rigid backing except for low frequencies below 150 Hz. This is in agreement with the statement that the sound waves should be able to penetrate the porous absorber in order for sound attenuation to occur. Ackermann et al.75 reported that the smoothness and evenness of the surface of the absorbers facing the flow keeps frictional losses low and this allows higher sound absorption.

Yilmaz et al.4 investigated the effect of sequence of two layers of poly (lactic acid) (PLA) and one layer of hemp fibrous webs on NAC values. They found that the biocomposite with the hemp layer facing the sound source higher sound absorp­tion than the fabric with the hemp layer facing back plate or the hemp layer between the other two PLA layers. In fact, the composites including hemp layers facing the sound source or the back plate were actually the same materials, but just reversed. Hemp layer had greater pore sizes due to the coarser hemp fibers compared to man­made PLA fiber layer, so they allowed more sound waves to penetrate into the mate­rial. Consequently, it is possible to achieve better sound absorption only by chang­ing the direction of the same composite absorber.

Using a different approach, Atalla et al.76 produced nonhomogeneous absorbers from rock wool and glass fiber which include patches with different flow resistiv­ity values in the same single layers. They found that surrounding patches interact together and better performances are obtained compared to homogeneous materials. This approach may also be applied to biocomposites.