We present a new simulation method for "accurately" determining the so-called long-wavelength effective dynamic bulk modulus of gasessuch as ambient air-saturating porous media of arbitrary microgeometries. The quotation marks at "accurately" remind us, firstly, of the limits of our definition of the effective modulus. These limits are those of the Biot model, often used in noise control applications, which assumes that the geometries remain simple so that the long-wavelength motions of the gas and the solid at the pore scale are those of quasi-incompressible phases. Secondly, they refer to the finite accuracy of the simulation method itself, which converges according to the classical stochastic law in 1/ √ N with N the number of trials. The simulation method is based on the known properties of Brownian motion of diffusing particles which are released in the saturating gas and undergo finite radioactive decay in the bulk and instantaneous absorption at the pore walls. The algorithm is illustrated by showing how the determination of the mean survival times of such particles leads to the calculation of the effective bulk modulus as it is defined in the framework of Biot theory; we also show how it leads to purely geometric constructions to determine a number of parameters such as the static thermal permeability and the static thermal tortuosity. We first validate our radioactive decay Brownian motion simulation technique in the simple case of cylindrical circular pores. The power of this method is further illustrated by rapidly deriving the effective