Audible bandgaps in phononic sonic crystals for educational demonstration

This experiment explored the use of phononic sonic crystals (PSCs) as an educational tool to demonstrate basic principles of wave propagation and phononics. Phononic crystals are periodic structures that attenuate a range of frequencies—called a bandgap—where the periodicity is on the order of the wavelength. Two PSCs were designed, constructed, and tested, to demonstrate the relationship between periodicity and bandgap frequency. The band structure of a 2D array of cylindrical rods was numerically constructed using eigenmode analysis in a finite element simulation. Based on the numerical simulations, two PSC spacings were selected for fabrication with bandgaps in the audible range of human hearing: a 3.5 cm spacing corresponding to a bandgap centered around 3200 Hz and a 6 cm spacing corresponding to a bandgap centered around 5600 Hz. A swept tone was transmitted through both PSCs and the resulting transmission curves were analyzed. The smaller spaced PSC was found to produce a bandgap from 5350z to 6000 Hz, with a 95% transmission drop; and the large PSC was found to produce a bandgap from 2800z to 3750 Hz with an 84% transmission drop. Both crystals were presented at the University of Illinois Urbana-Champaign’s Engineering Open House in April 2024, and their attenuations in the bandgap frequencies were perceived audibly.