Focusing of airborne ultrasound emitted by a flexurally vibrating plate using a transmission mask with spatially designed holes

We propose a focusing method of intense midair ultrasound out of ultrasonic emission from a single flexurally vibrating plate partially covered with a purposely designed amplitude mask. Many applications relying on nonlinear acoustic effects, such as radiation force employed in acoustic levitation, have been devised. For those applications, focused intense airborne ultrasound is conventionally formed using phased arrays of transducers or sound sources with specific fabricated shapes. However, the former strategies are costly, and the latter may require minute three-dimensional fabrication processes, which both hinder their utilization, especially in constructing a large ultrasound-emitting aperture. Our method offers a possible solution for this, where the amplitude masks are designed depending on the positions of nodes and antinodes of the vibrating plate. We experimentally demonstrate the successful formation of midair ultrasound focus at a desired position, which is localized in a region whose diameter is less than a few wavelengths. Our method only requires a monolithic plate, a driving transducer under the plate, and an amplitude mask fabricated out of laser machining of an acrylic plate. Magnification of the spatial scale of ultrasound apertures based on our method is much more readily and affordably achieved than conventional methods, leading to new midair ultrasound applications with the whole-room workspace.