Continuous-wave amplitude control via the interference phenomenon in acoustic structures

We propose a strategy to continuously tune the amplitude of acoustic waves based on the interference between two mode-conversion paths in passive acoustic structures. The interference phenomenon is attributed to two conjugate acoustic geometric phases obtained with two mode-conversion processes in a hybrid-type geometric-phase meta-atom (HGPM) pair. Notably, 100% modulation depth of the wave amplitude is achievable by simply varying the local orientation angle of the meta-atom. We utilize an acoustic structure made of two cylindrical resonators to construct a deep-subwavelength secondary source with designated initial phase delay, and the HGPM supporting the desired mode-conversion functionality is accordingly fabricated with four secondary sources. Both theory and experiment consistently verify the continuous amplitude modulation function of the HGPM pair, which showcases a general scheme for reconfigurable amplitude-type acoustic metadevices, i.e., those that require grayscale amplitude modulation for acoustic field engineering.