The field of transformation optics has flourished over the past decade, allowing scientists to design metamaterial-based structures that shape and guide the flow of light. One of the most dazzling inventions potentially unlocked by transformation optics is the invisibility cloak—a theoretical fabric that bends incoming light away from the wearer, rendering them invisible. Interestingly, such illusions are not restricted to the manipulations of light alone.
Transformation optics, a field that manipulates light to create optical illusions, has found its parallel in transformation acoustics, where similar techniques are applied to sound waves. Researchers have achieved significant progress in transformation acoustics, including the development of an “acoustic cloak” that can render objects invisible to sound. However, little attention has been given to the problem of location camouflaging.
Addressing this gap, Professor Garuda Fujii from Shinshu University in Japan has made strides in creating high-performance “source-shifters” that can alter the perceived location of sound sources. In a recent study published in the Journal of Sound and Vibration, Prof. Fujii introduces a novel approach to designing source-shifter structures using acrylonitrile butadiene styrene (ABS), an elastic polymer commonly employed in 3D printing.
Prof. Fujii’s approach revolves around inverse design based on topology optimization. The numerical method involves reproducing the pressure fields emitted by a virtual source, which would be misperceived as the actual sound source by nearby listeners. These pressure fields emitted by the real source are then manipulated to camouflage the source’s location, creating the illusion that the sound is originating from a different space. This is accomplished by designing an optimized metastructure that minimizes the discrepancy between the pressure fields of the actual and virtual sources through its geometry and elastic properties.
To determine the optimal design of ABS resin source-shifters based on various design criteria, Prof. Fujii employs an iterative algorithm within his approach. The simulations consider acoustic-elastic interactions between air and solid elastic structures, as well as the constraints imposed by current manufacturing technology.
Simulation results demonstrate that the optimized structures can minimize the difference between the pressure fields emitted by the masked source and those of an uncamouflaged source at the virtual location to as low as 0.6%. Prof. Fujii highlights the noteworthy performance of the ABS structures designed via topology optimization, especially considering their simple composition without complex acoustic metamaterials.
Prof. Fujii further investigates the camouflaging mechanisms by analyzing the significance of the distance between the virtual and actual sources. Surprisingly, he finds that a greater distance does not necessarily diminish the source-shifter’s performance. Additionally, he explores the impact of altering the sound frequency on the source-shifter’s efficacy, noting that the optimization process was carried out for a single target frequency. Lastly, he examines the possibility of topologically optimizing a source-shifter to operate at multiple sound frequencies.
While further refinements are required, Prof. Fujii’s study provides valuable insights into the development of illusion acoustics. His optimization method for designing high-performance source-shifters holds promise for advancing acoustic location camouflage and holography technology.
Source: Shinshu University