Functionally graded cellular microstructures whose porosity (i.e. volume fraction of void to solid) is engineered to meet specific requirements are increasingly demanded by bio-engineers, who wish to exploit their synergistic mechanical, chemical and thermal properties. Because traditionally polymeric foams have been manufactured with homogeneous porosity, established processes cannot control the distribution of porosity throughout the resulting matrix. Motivated by the creation of a flexible process for engineering heterogeneous foams, this paper reports how the manufacture of polymeric foams with a variable porosity distribution can be achieved by ultrasound irradiation during the 'sensitive' stages of the polymerization reaction. This paper reports how for each of the five distinctive stages of polymerization (i.e. cream, rising, packing, gelation and solidification) the energy and mass balances were studied in order to determine the underlying mechanisms that ultrasound employs to affect the reaction. It was concluded that controlled ultrasonic irradiation affects convective mass transfer during foaming, especially during 'rising' and 'packing' stages, and enhances the diffusion of the blowing agent (i.e. CO2(g)) from bubble to bubble in the 'packing' and 'gelation' stages. The mechanical work put into the system by ultrasound assists both the convection and diffusion by increasing the rate of mass flux. The paper concludes with some experimental results that support the above hypotheses.