Based on nonlinear acoustic and acoustic signal processing theoretical analyses, we designed and implemented a directional sound projection (or parametric loudspeaker) system, known as the audio beam system (ABS). The prototyping of the parametric loudspeaker consists of three main components: a digital signal processor (DSP), amplifier, and ultrasound emitters. The programmable DSP is the main processing block of the parametric array that performs preprocessing, such as equalization, dynamic range control, carrier control, and our proposed modified amplitude modulation techniques. The special-designed class-D amplifier is used to adjust the voltage gain to drive an array of ultrasound emitters, which are made of lead zirconate titanate or polyvinylidene fluoride materials. Due to the nonlinear interaction of the air, directional audible sound will be generated at a targeted sweet spot. The ABS was recently installed in a gaming booth in the Fusionopolis, a research and development complex located at the One-North business park in Singapore. We used two ABS speakers (ultrasound emitters) to project directional binaural sound from a gaining console to the gamer standing in front of the gaming booth. This setup confines sound to a predetermined sound zone. A laser or LED device fitted to the ABS speakers can project a light beam with the same direction as the sound beam to indicate the sweet spot on the floor. The gamer within the zone can enjoy playing the game without disturbing the other visitors beyond the defined "tune-in" zone.

A sound reproduction system usually consists of several types of loudspeakers to cater to sophisticated applications. The directivity of a loudspeaker is a measure of its efficiency in sending sounds to a particular direction instead of all directions. Demand to control the directivity of a sound reproduction system is gaining momentum with many new designs of directional loudspeakers, including the acoustic dome, horn loudspeaker, loudspeaker array, and parametric array loudspeaker (PAL). The PAL is an application of the parametric acoustic array in air, which generates a sound beam from the interaction of ultrasonic beams. The PAL has several desired features, such as its narrow beamwidth over a wide frequency range, low sound attenuation over a long distance, and ability to reproduce perceptually near sound images. The PAL is also advantageous in using a smaller driving unit to transmit an equally narrow sound beam as compared to the conventional loudspeaker and broadside loudspeaker array. An overview of directivity control methods of the PAL is presented in this paper. In particular, acoustic models and signal processing techniques in controlling the directivity of the PAL are emphasized.

In a recent work, the beamsteering characteristics of parametric loudspeakers were validated in an experiment. It was shown that based on the product directivity model, the locations and amplitudes of the mainlobe and grating lobes could be predicted within acceptable errors. However, the measured amplitudes of sidelobes have not been able to match the theoretical results accurately. In this paper, the original theories behind the product directivity model are revisited, and three modified product directivity models are proposed: (i) the advanced product directivity model, (ii) the exponential product directivity model, and (iii) the combined product directivity model. The proposed product directivity models take the radii of equivalent Gaussian sources into account and obtain better predictions of sidelobes for the difference frequency waves. From the comparison between measurement results and numerical solutions, all the proposed models outperform the original product directivity model in terms of selected sidelobe predictions by about 10 dB.

The proper axial length of the eye is crucial for achieving emmetropia. In this study, we present a wireless battery-free eye modulation patch designed to correct high myopia and prevent relapse. The patch consists of piezoelectric transducers, an electrochemical micro-actuator, a drug microneedle array, μ-LEDs, a flexible circuit, and biocompatible encapsulation. The system can be wirelessly powered and controlled using external ultrasound. The electrochemical micro-actuator plays a key role in precisely shortening the axial length by driving the posterior sclera inward. This ensures accurate scene imaging on the retina for myopia eye. The drug microneedle array delivers riboflavin to the posterior sclera, and μ-LEDs’ blue light induces collagen cross-linking, reinforcing sclera strength. In vivo experiments demonstrate that the patch successfully reduces the rabbit eye’s axial length by ~1217 μm and increases sclera strength by 387%. The system operates effectively within the body without the need for batteries. Here, we show that the patch offers a promising avenue for clinically treating high myopia.