Nonfibrous sound-absorbing materials

Most conventional sound absorbing materials are fiber-like in their construction. Their behavior has been studied extensively and is now well understood. However, it is sometimes desirable to avoid the use of fibrous materials for environmental and other reasons. Consequently, it is of interest to explore the use of nonfibrous materials in the context of sound absorption. Here, the sound absorption offered by tensioned, circular membranes is considered. Measurements of the dissipation within such membranes in a standing wave tube show peaks at the fluid-loaded membrane resonance frequencies. It is suggested that the energy dissipation at those frequencies arises from a hysteretic loss mechanism in the membrane. To represent this situation, a modal membrane model was coupled to a modal solution in the cylindrical space on both sides of the membrane. Both infinite and finite-depth, rigidly terminated backing spaces have been considered. Laser-Doppler velocimetry measurements have been performed to confirm the modal response of the membranes. Good agreement was found between measured and predicted acoustical properties for a number of tensioned membranes. It was shown that a tensioned membrane may offer significant absorption at particular frequencies. In particular, it was found that a family of sound absorption peaks resulted from losses due to local flexing of the membrane stiffened by the tension applied to the membrane. It was also found that other stiffness mechanisms, such as flexural stiffness and stiffening by curvature, can generate similar families of resonances, and thus could also account for sound dissipation in membrane systems. The sound energy dissipation characteristics of tensioned, permeable membrane were also studied, and it was found that the flow resistive mechanism results in nearly-uniform dissipation over a broad range of frequencies. For the purpose of applying nonfibrous sound absorbing systems in practice, a new type of membrane-based foam has been being studied. Two types of foam models have been developed based on the dynamics of a single, tensioned, permeable, lossy membrane: i.e., rigid and flexible frame models. It has been found that those models accurately reproduce the acoustical behavior of the particular type of foam being studied here. Finally, foam designs having improved sound absorption characteristics were identified by using an optimization procedure