Sonic crystal composites for selective noise reduction

Commercially available sound absorbing materials attenuate sound over a broad frequency band. We are fabricating and characterizing geometrically compact composites called sonic crystals for shielding against noise generally limited to a particular frequency band. These sonic crystals have periodic arrays of macroscopic acoustic scatterers arranged in a pattern analogous to a single crystal lattice in a host matrix material. Most of the work done on sonic crystals so far has been using rod scatterers arranged in a 2-D lattice in air. Such sonic crystals have very little use as a noise reducing composites material for structures. Instead, we fabricated sonic crystals using solid metal spheres, or hollow metal beads arranged in a lattice pattern and embedded in an unsaturated polyester resin matrix. We demonstrate that in these composites, the sound transmission coefficient for certain frequencies drops abruptly, giving rise to frequency band gaps. In order to measure acoustic band gaps in these damping materials, an audio loudspeaker directs broadband sound at the specimen placed in a reflectionless acoustic enclosure. The transmitted sound is measured by a wide band microphone; fast Fourier transformed and normalized to obtain transmission coefficient spectra. Data was obtained, for example, on a sonic crystal made of 0.125-inch diameter metallic beads in a simple cubic lattice (13×13×13 array, with spacing of 0.16 inch between the beads) and cast in an unsaturated polyester resin matrix. We measured a band gap of ∼3.8 kHz located at ∼3.6 kHz where the drop in the transmission coefficient was ∼11 dB. The reduction in transmission coefficient in the band gaps depends upon the material properties of scatterers and host, fill ratio and lattice configuration. We will present detailed results for acoustic band gaps in the propagation of sound waves through several macroscopic sonic crystal composites with various types of scatterers arranged in three dimensions and explore their industrial uses for noise reduction.