Chuang Shi

Grating lobe elimination in steerable parametric loudspeaker

In the past two decades, the majority of research on the parametric loudspeaker has concentrated on the nonlinear modeling of acoustic propagation and pre-processing techniques to reduce nonlinear distortion in sound reproduction. There are, however, very few studies on directivity control of the parametric loudspeaker. In this paper, we propose an equivalent circular Gaussian source array that approximates the directivity characteristics of the linear ultrasonic transducer array. By using this approximation, the directivity of the sound beam from the parametric loudspeaker can be predicted by the product directivity principle. New theoretical results, which are verified through measurements, are presented to show the effectiveness of the delay-and-sum beamsteering structure for the parametric loudspeaker. Unlike the conventional loudspeaker array, where the spacing between array elements must be less than half the wavelength to avoid spatial aliasing, the parametric loudspeaker can take advantage of grating lobe elimination to extend the spacing of ultrasonic transducer array to more than 1.5 wavelengths in a typical application.

Development of Parametric Loudspeaker

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.

Product directivity models for parametric loudspeakers

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.

A psychoacoustical preprocessing technique for virtual bass enhancement of the parametric loudspeaker

The parametric loudspeaker is a novel type of loudspeaker that can project a directional sound beam. It is commonly used in creating personal sound zone and projecting private messages to a targeted audience. However, the parametric loudspeaker possesses a very poor bass (or low-frequency) response due inherently to the nonlinear acoustic principle generating sound from ultrasound in air. A psychoacoustic signal processing method known as “virtual bass” has been successfully implemented in some consumer electronics with miniature or flat loudspeaker unit, aiming to enhance their bass performances. In this paper, we adapt this “virtual bass“ approach for parametric loudspeakers. Unlike conventional loudspeakers, the parametric loudspeaker brings in an added degree of complexity in “virtual bass” enhancement due to its inherent nonlinear acoustic property. Accordingly, a new preprocessing technique is proposed for the parametric loudspeaker to psychoacoustically reproduce the low-frequency components within an octave below its cut-off frequency.

Identification of the parametric array loudspeaker with a volterra filter using the sparse NLMS algorithm

Volterra filters can be applied to a wide range of nonlinear systems, keeping only the low order kernels to yield a good approximation. The parametric array loudspeaker (PAL), as a weak nonlinear acoustic system, is an attractive directional sound reproduction device. Volterra filters have been adopted in the linearization system of the PAL that efficiently reduces the nonlinear distortion with no need of solving the nonlinear acoustic equation. In this paper, the ultrasound-to-ultrasound Volterra filter is proposed, being inspired by the nonlinear acoustic principle, to provide a better systematic representation of the PAL. Experiment results are presented to prove the effectiveness of the proposed approach, where the sparse NLMS algorithm is carried out in the identification.

A convolution model for computing the far-field directivity of a parametric loudspeaker array

This paper describes a method to compute the far-field directivity of a parametric loudspeaker array (PLA), whereby the steerable parametric loudspeaker can be implemented when phased array techniques are applied. The convolution of the product directivity and the Westervelts directivity is suggested, substituting for the past practice of using the product directivity only. Computed directivity of a PLA using the proposed convolution model achieves significant improvement in agreement to measured directivity at a negligible computational cost.

A comparative study of preprocessing methods in the parametric loudspeaker

The parametric loudspeaker is a directional sound reproduction device making use of the parametric sound generation. A sound beam is formed as a result of nonlinear interactions between ultrasonic beams. The parametric loudspeaker is advantageous in transmitting an equally narrow sound beam from a smaller emitter as compared to the conventional loudspeaker. Due to this advantage, parametric loudspeakers are readily applied in a variety of sound field control applications, such as creation of personal listening spots, spatial audio reproduction, and active noise control. However, there is a long concerned drawback of the parametric loudspeaker, whereby harmonic and intermodulation distortions are byproducts of the parametric sound generation. Hence, a comparative study of six preprocessing methods, including two proposed methods from this paper, is carried out. Harmonic and intermodulation distortions are demonstrated by experiments.

A preprocessing method to increase high frequency response of a parametric loudspeaker

Unlike a conventional loudspeaker, a parametric loudspeaker whose principle is derived from nonlinear acoustic effects in air can create a controllable sound beam with a compact emitter. Therefore, the parametric loudspeaker is advantageous in deployments where personal audio zones are in demand. However, it is a dilemma that the sound generation principle of parametric loudspeaker leads to a narrow bandwidth of itself, which hinders its popularization in applications requiring high fidelity. In this paper, we propose a tentative treatment of harmonic distortions incurred in parametric loudspeakers, whereby the asymmetrical amplitude modulation (AAM) method is used to extend the high frequency response by doubling the higher cut-off frequency.

Hybrid immersive three-dimensional sound reproduction system with steerable parametric loudspeakers

A loudspeaker must be both dispersive and directive to accurately reproduce spatial audio from digital media. To address this problem, an audio system that has a unique combination of conventional and parametric loudspeakers has previously been proposed and proved to be effective in producing an immersive 3D soundscape. However, this system has two drawbacks: (1) There is only one fixed sweet spot, and (2) only one listener within the sweet spot can enjoy the complete experience. Therefore, a hybrid 3D sound reproduction system combining conventional loudspeakers with a pair of steerable parametric loudspeakers is proposed in this paper. By using this new combination of conventional and steerable parametric loudspeakers, the sweet spot can be steered towards the listeners head position. Thus, the listener no longer needs to keep his head stationary while watching movies or playing games, which resulting in a more relaxing and pleasant experience. Furthermore, a dual-beamsteering method is proposed ...

An overview of directivity control methods of the parametric array loudspeaker

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.