We demonstrate strongly collimated beam formation, at audible frequencies, in a
three-dimensional acoustic phononic crystal where the wavelength is commensurate with the
crystal elements; the crystal is a seemingly simple rectangular cuboid constructed from
closely-spaced spheres, and yet demonstrates rich wave phenomena acting as a canonical
three-dimensional metamaterial. We employ theory, numerical simulation and experiments to
design and interpret this collimated beam phenomenon and use a crystal consisting of a finite
rectangular cuboid array of 4 × 10 × 10 polymer spheres 1.38 cm in diameter in air, arranged in a
primitive cubic cell with the centre-to-centre spacing of the spheres, i.e. the pitch, as 1.5 cm.
Collimation effects are observed in the time domain for chirps with central frequencies at 14.2 kHz
and 18 kHz, and we deployed a laser feedback interferometer or Self-Mixing Interferometer – a
recently proposed technique to observe complex acoustic fields—that enables experimental
visualisation of the pressure field both within the crystal and outside of the crystal. Numerical
exploration using a higher-order multi-scale finite element method designed for the rapid and
detailed simulation of 3D wave physics further confirms these collimation effects and
cross-validates with the experiments. Interpretation follows using High Frequency
Homogenization and Bloch analysis whereby the different origin of the collimation at these two
frequencies is revealed by markedly different isofrequency surfaces of the sonic crystal.