Acoustic mirrors can reflect and focus sound waves under water, but typical mirrors require very complex configurations or geometries. Sallam et al. designed, simulated, and experimentally verified a flat acoustic mirror for underwater applications.

Their mirror output an approximately 70-fold increase in acoustic pixels compared to conventional methods, suggesting the device can manipulate reflected sound waves more finely. The team showed the mirror could also focus these waves spatially, as evidenced by the mirror’s ability to induce higher pressure in complex patterns such as a flower shape and construct multifocal points onto a target area.

“We demonstrated, for the first time, a 3D-printed metallic holographic acoustic mirror for the manipulation of reflected ultrasonic waves in water,” co-author Shima Shahab said. “We envision that the mirroring concept has the potential to extend the realm of critical applications that employ manipulation of reflective acoustic waves.”

Since water can introduce unwanted uncertainties in experimental measurements, the authors first designed and analyzed the mirror in a physics simulation software to determine the mirror configuration and composition that would best optimize their results. After 3D printing their design, the authors verified their mirror’s performance by performing pressure scans in underwater experiments, finding the mirror could reflect waves up to the megahertz range in specific patterns.

The authors envision three future directions for their study: testing the mirror in contactless acoustic energy transfer applications, studying the effect of nonlinearities at higher sound-wave intensities of the sound field, and integrating the reflective mirror with transmission acoustic holograms to enable even more control over incoming sound waves.

Source: “Holographic mirrors for spatial ultrasound modulation in contactless acoustic energy transfer systems,” by Ahmed Sallam, Vamsi C. Meesala, Muhammad R. Hajj, and Shima Shahab, Applied Physics Letters (2021). The article can be accessed at