This paper describes a method of generating a controlled sound field for listeners inside a circular array of loudspeakers without disturbing people outside the array appreciably. To achieve this objective, a double-layer array of loudspeakers is used. Several solution methods are suggested, and their performance is examined using computer simulations. Two performance indices are used in this work, (a) the level difference between the average sound energy density in the listening zone and that in the quiet zone (sometimes called “the acoustic contrast”), and (b) a normalized measure of the deviations between the desired and the generated sound field in the listening zone. It is concluded that the best compromise is obtained with a method that combines pure contrast maximization with a pressure matching technique.

A personal audio system that does not use earphone or any wire would have great interest and potential impact on the audio industries. In this study, a line array speaker system is used to localize sound in the listening zone. The contrast control [Choi, J.-W. and Kim, Y.-H. (2002). J. Acoust. Soc. Am. 111, 1695–1700] is applied, which is a method to make acoustically bright zone around the user and acoustically dark zone in other regions by maximizing the ratio of acoustic potential energy density between the bright and the dark zone. This ratio is regarded as acoustic contrast, analogous with what is used for optical devices. For the evaluation of the performance of acoustic contrast control, experiments are performed and the results are compared with those of uncontrolled case and time reversal array.

While sound source localization (SSL) using a spherical microphone array system can be applied to obtain visual beam patterns of source distribution maps in a range of omnidirectional acoustic applications, the present challenges of the spherical measurement system on the valid frequency ranges and the spatial distortion as well as the grid-related limitations of data-driven SSL approaches raise the need to develop an appropriate method. Imbued by these challenges, this study proposes a deep learning (DL) approach to achieve the high-resolution performance of localizing multiple sound sources tailored for omnidirectional acoustic applications. First, we present a spherical target map representation that can panoramically pinpoint the position and strength information of multiple sound sources without any grid-related constraints. Then, a dual-branched spherical convolutional autoencoder is proposed to obtain high-resolution localization results from the conventional spherical beamforming maps while incorporating frequency-variant and distortion-invariant strategies to address the inherent challenges. We quantitatively and qualitatively assess our proposed method’s localization capability for multiple sound sources and validate that the proposed method can achieve far more precise and computationally efficient results than the existing approaches. By extension, we newly present the experimental setup that can create omnidirectional acoustic scenarios for the multiple SSL. By evaluating our proposed method in this experimental setup, we demonstrate the effectiveness and applicability of the proposed method with the experimental data. Our study delivers the proposed approach’s potential of being utilized in various SSL applications.

The design, fabrication, and analysis of omnidirectional gradient-index (GRIN) phononic crystals (PnCs) for acoustic wave focusing and energy harvesting have been demonstrated both numerically and experimentally. Despite that omnidirectional functionality is a key factor to alleviate the directivity dependence issues, the concept has not yet been incorporated into acoustic energy harvesting. In this work, a symmetrical GRIN PnC structure consisting of cylinders with variation in filling fractions has been presented to tailor the spatial acoustic refractive index, thus enforcing the acoustic waves in any direction toward the targeted center area for focusing purposes. Both a numerical simulation and experimental validation confirm substantial sound energy amplification of the designed GRIN PnC over a broad frequency range from 250 Hz to 1 kHz. Notably, the maximum sound amplification occurs at the hybrid resonant frequency of the GRIN PnC structure and the acoustic duct system used to generate incident plane waves. Numerical simulation reveals that the cavity resonance and the refraction of the GRIN PnC mainly contribute to enhanced sound amplification in addition to the reflection from the acoustic duct. The GRIN PnC structure coupled with the acoustic duct system leads to enhanced harvesting output performance when integrated with a piezoelectric energy harvesting device.

Recently, a personal audio system has been studied that uses an array of loudspeakers to localize sound to only the area around a user. To realize this system, beamforming or acoustic contrast control has been applied on the assumption that sources radiate sound in a free-field. This means that not only reflection by walls, but also the scattering effect by the user’s head is neglected. Reflection by walls is negligible because personal devices are usually used in a short distance so that direct sound is dominant over reverberant sound. However, the scattering effect by the user’s head has a considerable effect on the focused sound field. For example, the region where sound energy is not focused becomes louder when a user is actually in the focused region due to the scattered sound by the user’s head in the focused region. In this paper, the scattering effect is shown computationally on the simple assumption that the user’s head is a rigid sphere. Then, an improving control method, which overcomes this effect, is proposed. The method is shown to outperform the previous method in terms of lowering the sound level in the side regions when a user is in the bright zone.

This paper is concerned with experimental validation of a recently proposed method of controlling sound fields with a circular double-layer array of loudspeakers [Chang and Jacobsen, J. Acoust. Soc. Am. 131(6), 4518-4525 (2012)]. The double-layer of loudspeakers is realized with 20 pairs of closed-box loudspeakers mounted back-to-back. Source strengths are obtained with several solution methods by modeling loudspeakers as a weighted combination of monopoles and dipoles. Sound pressure levels of the controlled sound fields are measured inside and outside the array in an anechoic room, and performance indices are calculated. The experimental results show that a method of combining pure contrast maximization with a pressure matching technique provides only a small error in the listening zone between the desired and the reproduced fields, and at the same time reduces the sound level in the quiet zone as expected in the simulation studies well above the spatial Nyquist frequency except at a few frequencies. It is also shown that errors in the positions of the loudspeakers can be critical to the results at frequencies where the distance between the inner and the outer array is close to half a wavelength.