Compact acoustic bandgap material based on a subwavelength collection of detuned Helmholtz resonators

This study explores the amplitude and phase transmission of audio-frequency sound through a waveguide side-loaded with a series of closely spaced and sequentially tuned Helmholtz resonators. This system exhibits a series of acoustic bandgaps due to the Helmholtz resonance and standing-wave cavity modes. The bandgaps are achieved in a physically compact manner in that the resonators are spaced by much less than a wavelength. The response of the Helmholtz resonator-loaded waveguide is described by a simple adaptation of an existing theory. Finally, within the forbidden transmission regions the system exhibits narrow bands of negative group delay.