Evaluating the Role of Unit Cell Multiplicity in the Acoustic Response of Phononic Crystals Using Laser-Plasma Sound Sources.

Acoustic metamaterials and phononic crystals are progressively consolidating as an important technology that is expected to significantly impact the science and industry of acoustics in the coming years. In this work, the impact of unit cell multiplicity on the spectral features of the acoustic response of phononic crystals is systematically studied using the recently demonstrated laser-plasma sound source characterization method. Specifically, by exploiting the advantages of this method, the impact of the number of repeated unit cells on the depth of the phononic band gaps and the passband spectral features across the entire audible range is demonstrated. These experimental findings are supported by specially developed computational simulations accounting for the precise structural characteristics of the studied phononic crystals and are analysed to provide a phenomenological understanding of the underlying physical mechanism. It is shown that by increasing the unit cell multiplicity, the bandgaps deepen and the number of resonant peaks in the crystal transmission zones increases. The resonant mode shapes are computationally investigated and interpreted in terms of spherical harmonics. This study highlights the tunability and design flexibility of acoustic components using phononic crystals, opening new paths towards applications in the fields of sound control and noise insulation.