Composite aerogels (with varying concentrations of silica and poly-dimethylsiloxane) were developed and their acoustic absorption coefficient as a function of composition and average pores size have been measured. The polydimethylsiloxane modified the ceramic structure of the silica aerogels, decreasing the material's rigidity while maintaining the high porosity of the aerogel structure. The composite aerogels were found to exhibit different modes of acoustic absorption than that of typical porous absorbers such as fiberglass. At some frequencies, the composite aerogels had 40% higher absorption than that of commercial fiberglass. Physical data show that these materials have a large surface area (> 400 m2/g) and varying pore sizes (d ~ 5 -20 nm).

Nonlinear interaction of two plane acoustic waves in the (0,0) mode of a square duct is investigated from the viewpoint of chaotic dynamics. Phase-space portraits are reconstructed from time series obtained in an experiment. It is demonstrated that limit sets formed by the phase space trajectories are “attractors.” The largest Lyapunov exponent and correlation dimension of these attractors are calculated, and the results indicate that these attractors are chaotic.

The experimentally determined normal incident mass-air-mass resonance frequency for a double leaf cavity stud building element is significantly greater than the theoretically predicted frequency for wood studs and steel studs manufactured from thicker sheet steel. This paper gives a method for calculating the effective mass-air-mass resonance frequency as the root mean square sum of the mass-air-mass resonance frequency and the resonance frequency of the first bending wave mode of the leaves between the studs. This calculation should use the isothermal mass-air-mass resonance frequency if the building element cavity contains porous sound absorbing material. If the cavity does not contain porous sound absorbing material, the usual adiabatic mass-air-mass resonance frequency should be used in the calculation. Because the exact boundary conditions of the building element leaves at the studs and the effective in situ damping are unknown, the paper gives empirical correction factors to determine the actual resonance frequency and the depth of the dip in the predicted sound insulation. This paper also gives empirically derived formulae for the line and point equivalent translational compliances of steel studs manufactured from different sheet steel gauges and compares them with formulae derived by other authors for the case of 25 gauge steel studs.

Various mitigation systems have been utilized for addressing heavy-weight impacts within the built environment. There is no standardized measurement system for assessing heavy-weight impacts and no metric for describing the performance of a building assembly, material, or sound level for this type of impact. The authors have continued their investigation into measurement methods for hard and soft heavy-weight impacts on a variety of structures and fitness flooring materials. This paper presents recent results.

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