Consistent production of air-backing equivalent low impedance material using new materials

To realize air-equivalent backing material for high efficiency transducer, a light, stiff and strong and not-too-fragile, yet acoustically not-too-much reverberating material is required. Conventional syntactic foams are based on particle size regulated industrial origin silica or other glass microballoon with epoxy or urethane or other matrix resin, which are basically adhesive materials having relatively high viscosity. Higher filling factor of balloons are required to acoustically simulate the air, these points have been the limiting factor. In this study a ultra-low-viscosity resin (Metathene) is used for filling matrix together with natural origin, particle size distributed microballoon (Shirasu) as the filler. In laboratory environment, solid and rigid syntactic foams having 0.18 to 0.2 Mrayl acoustic impedance have been realized consistently, without using any vacuum injection or pressure-cooker process. To maximize the filling factor (>80%as estimated) and to reject solid inclusions, gravitational redistribution (floating up) of filler during solidifying process of matrix resin has been found very much effective. Metathene is an injection molding plastic material based on solidifying by ring-opening metathesis polymerization of dicyclopentadiene having so low viscosity as 3 to 5 mPaS (like water) before solidification starts (for example epoxies or silicones are in a range of 500 to 700 mPaS). Shirasu microballoon is yield of 2ndary popping expansion hot process (like 1000 deg. C) of volcanic ash sand particularly available in Kagoshima, Japan. The study has been supported partly by Kagoshima prefecture local government R&D fund, Kagoshima, Japan.