Engineering Helmholtz resonators for directional low-frequency sound attenuation

Controlling the absorption and diffusion of sound in the audible range constitutes an exciting field of research. Acoustic absorbers and diffusers perform extraordinarily well at high frequencies with sizes comparable to the wavelength of the working frequency. On the other hand, efficient low-frequency attenuators demand large volumes leading to unpractical sizes and high manufacturing costs. However, can the size of the resonator be reduced while also decreasing the working frequency? The answer is, counterintuitively, yes. This work investigates this phenomenon by studying a series of 3-D-printed monoatomic metamaterials based on membrane-coupled Helmholtz resonators. The results reveal that these systems, apart from significantly decreasing the bandgap frequency by around 36%, produce a cardioid directional response that contrasts with the omnidirectional response from traditional Helmholtz resonators. The obtained results suggest that by following this approach, low-frequency attenuation can be achieved via miniaturised devices while additionally providing them with a sense of directionality. This combination of features makes these sensors the perfect candidates for the next generation of acoustic hearing devices and acoustic attenuators, since the freedom that 3-D-printing provides allows for fine control of wave manipulation.