Yang-Hann Kim

Generation of an acoustically bright zone with an illuminated region using multiple sources

This article addresses the way in which we can generate an acoustically bright zone in a space. The bright zone is defined as the volume where we can have higher acoustic energy than in other space. A method is proposed to generate the bright zone by controlling multiple monopole sources. Two kinds of cost functions involved with acoustic brightness are defined. One is the ratio of the brightness of a zone to the input power, and the other expresses the contrast between the bright zone and the other space. Through eigenvalue analysis, the optimal volume velocity distribution of the monopoles has been obtained.

A realization of sound focused personal audio system using acoustic contrast control

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.

Scattering effect on the sound focused personal audio system

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 users 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 users 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 users head in the focused region. In this paper, the scattering effect is shown computationally on the simple assumption that the users 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.

Manipulation of sound intensity within a selected region using multiple sources

In this paper, the authors propose a method of enhancing sound in a selected region by controlling multiple sources. The physical variables of enhancing sound have not been well defined, but we may consider basic acoustic variables such as acoustic potential energy, sound power or intensity. A method of maximizing sound potential energy was found to be very straightforward [J.-W. Choi and Y.-H. Kim, J. Acoust. Soc. Am. 111, 1695 (2002)]. In this paper, the authors attempt to control the sound power or intensity of a zone in a desired direction. It is noteworthy that control of the direction and magnitude is needed to enhance the sound intensity. This control requires a new definition of the direction and magnitude of spatially distributed intensity. For this purpose, the authors introduce two different kinds of cost functions, and the theoretical formulation based on the new definitions show the possibility of maximizing the sound intensity of a selected zone in a desired direction.

A theoretical model to predict the low-frequency sound absorption of a helmholtz resonator array.

A theoretical method based on mutual radiation impedance is proposed to compute the sound absorption performance of a Helmholtz resonator array in the low-frequency range. Any configuration of resonator arrangement can be allowed in the method, while all the resonators may or may not be identical. Comparisons of the theoretical predictions with those done by the past studies or experiments show that the present method can accurately predict the absorption performance in more general cases.

Wavefront control by wave number domain focusing

This study introduces a novel method that can manipulate propagating direction of wavefronts within a selected region using source array. The idea of the proposed method stems from a method of acoustic contrast maximization, which has been used to focus sound energy within a zone of interest. This paper attempts to focus sound energy in the wave number domain, so that the sound energy of a selected region is concentrated on a desired wave number area. This makes it possible to generate a plane wave that propagates to a desired direction. The simple pure‐tone case is considered to express the idea in the wave number domain; then, the method is extended to more general case, where an excitation signal has a broadband spectrum. Numerical and experimental results obtained in various conditions certainly validate that the direction of the wavefront can be manipulated for some finite region in space. [Work supported by the BK21 project initiated by Ministry of Education and Human Resources Development of Korea.]

Helmholtz resonator array for low‐frequency sound absorption

An array of Helmholtz resonators is often used for reducing low‐frequency noise because of the high performance at its resonance frequency. One of the very attractive characteristics of the resonator array is that its effective frequency band is much wider than what can be obtained by a single resonator. This paper discusses a method of designing a Helmholtz resonator array panel for low‐frequency sound absorption. First, various experimental results are introduced and studied. Second, based on the experimental results, a theoretical method is presented that can predict reasonably well the panel’s absorption characteristics. Any configuration of resonator arrangement can be treated in the method, while all the resonators may or may not be identical. Comparisons between the predicted and experimental results reveal the accuracy of the proposed method. Finally, a numerical optimization is performed to design the Helmholtz resonator array panel. Some examples of optimal designs under some restrictions are al...

Wave front design by acoustic contrast control

A way to design and generate desired wave fronts using multiple sound sources is proposed. In principle, any kind of wave front can be described as a sum of orthogonal basis functions, and the wave front of a specific shape can be generated by focusing multiple sources’ energy on a single basis function. Once the sound field is decomposed into a set of orthogonal functions, we can apply a conventional focusing algorithm to generate a desired wave front. Extending the concept of energy focusing, we can also generate a wave front packet, whose energy is concentrated on a group of orthogonal functions rather than single component. This enables us to generate a wave front within a desired bound. For this purpose, we employ acoustic contrast control, which focuses sound energy on a selected group of orthogonal functions by enhancing the energy difference between the focal region and others. Various example cases demonstrate how the proposed method can generate a group of planar and spherical wave fronts propag...

Spatial Manipulation of Sound Using Multiple Sources

Spatial control of sound is essential to deliver better sound to the listener`s position in space. As it can be experienced in many listening environments. the quality of sound can not be manifested over every Position in a hall. This motivates us to control sound in a region we select. The primary focus of the developed method has to do with the brightness and contrast of acoustic image in space. In particular, the acoustic brightness control seeks a way to increase loudness of sound over a chosen area, and the contrast control aims to enhance loudness difference between two neighboring regions. This enables us to make two different kinds of zone - the zone of quiet and the zone of loud sound - at the same time. The other perspective of this study is on the direction of sound. It is shown that we can control the direction of perceived sound source by focusing acoustic energy in wavenumber domain. To begin with, the proposed approaches are formulated for pure-tone case. Then the control methods are extended to a more general case, where the excitation signal has broadband spectrum. In order to control the broadband signal in time domain, an inverse filter design problem is defined and solved in frequency domain. Numerical and experimental results obtained in various conditions certainly validate that the acoustic brightness, acoustic contrast, direction of wave front can be manipulated for some finite region in space and time.

A unified approach for the spatial enhancement of sound

This paper aims to control the sound field spatially, so that the desired or target acoustic variable is enhanced within a zone where a listener is located. This is somewhat analogous to having manipulators that can draw sounds in any place. This also means that one can somehow see the controlled shape of sound in frequency or in real time. The former assures its practical applicability, for example, listening zone control for music. The latter provides a mean of analyzing sound field. With all these regards, a unified approach is proposed that can enhance selected acoustic variables using multiple sources. Three kinds of acoustic variables that have to do with magnitude and direction of sound field are formulated and enhanced. The first one, which has to do with the spatial control of acoustic potential energy, enables one to make a zone of loud sound over an area. Otherwise, one can control directional characteristic of sound field by controlling directional energy density, or one can enhance the magnit...