This paper considers the problem of modeling pinna-related transfer functions (PRTFs) for 3-D sound rendering. Following a structural modus operandi, we present an algorithm for the decomposition of PRTFs into ear resonances and frequency notches due to reflections over pinna cavities. Such an approach allows to control the evolution of each physical phenomenon separately through the design of two distinct filter blocks during PRTF synthesis. The resulting model is suitable for future integration into a structural head-related transfer function model, and for parametrization over anthropometrical measurements of a wide range of subjects.