- Wave propagation in periodic heterogeneous media is characterized by Brillouin zones (elastic waves in composites; and electro-magnetic waves in single crystals). The central objective of this experimental investigation is to demonstrate the existence of the Brillouin zones in two periodic particulate composites. In the past these effects have been observed in the one-dimensional (or layered) composites and in the two-dimensional (or fibrous) composites. This is the first experimental observation of Brillouin zones in the three-dimensional or particulate composites. Two types of through-transmission ultrasonic apparatuses were used: water immersion and direct contact. The heart of the system is a pair of accurately matched, broad-band, X-cut, piezoelectric transducers. At higher frequencies the Brillouin zones are not observed. The following is offered as a plausible explanation: at higher frequencies the particle resonances are excited and these tend to dominate the lattice resonances, thereby destroying the banded structure of the dispersion curve.