Masahiro Toyoda

Sound transmission through a microperforated-panel structure with subdivided air cavities.

The absorption characteristics of a microperforated-panel (MPP) absorber have been widely investigated, and MPPs are recognized as a next-generation absorbing material due to their fiber-free nature and attractive appearance. Herein, further possibilities of MPPs are investigated theoretically from a sound transmission viewpoint. Employing an analytical model composed of a typical MPP and a back wall with an infinite extent, transmission loss through the structure is obtained. Although MPP structures generally have great potential for sound absorption, an improvement in the transmission loss at midfrequencies, which is important for architectural sound insulation, is not sufficient when using a backing cavity alone. Hence, to improve transmission loss at midfrequencies, an air-cavity-subdivision technique is applied to MPP structures. By subdividing the air cavity with partitions, each cell can create a local one-dimensional sound field as well as lead to a normal incidence into the apertures, which is the most effective condition for Helmholtz-type resonance absorption. Moreover, by providing the same motion as the back wall to the MPP, the sound-insulation performance can be further improved at midfrequencies.

Prediction of low-frequency structure-borne sound in concrete structures using the finite-difference time-domain method

Due to limitations of computers, prediction of structure-borne sound remains difficult for large-scale problems. Herein a prediction method for low-frequency structure-borne sound transmissions on concrete structures using the finite-difference time-domain scheme is proposed. The target structure is modeled as a composition of multiple plate elements to reduce the dimensions of the simulated vibration field from three-dimensional discretization by solid elements to two-dimensional discretization. This scheme reduces both the calculation time and the amount of required memory. To validate the proposed method, the vibration characteristics using the numerical results of the proposed scheme are compared to those measured for a two-level concrete structure. Comparison of the measured and simulated results suggests that the proposed method can be used to simulate real-scale structures.

Effects of an air-layer-subdivision technique on the sound transmission through a single plate

Many studies on the sound transmission through a single plate have been carried out theoretically and experimentally. The transmission-loss characteristics, in general, follow mass law. Therefore, increasing mass of a plate is a fundamental measure to improve the insulation performance. This method, however, has limitations and might not be a reasonable alternative in current standards. Furthermore, the transmission loss at the critical frequency of coincidence is deteriorated significantly even if the mass is rather large. In this paper, the effect of the air-layer-subdivision technique is studied in detail from the viewpoint of the sound transmission problem of a single plate. An analytical model of an infinite single plate with a subdivided layer is considered and the improvement of the transmission loss is estimated. The limitations of the technique are clarified with some parametric studies. In order to validate the predictions, an experiment was carried out. The transmission loss of a glass board with the air layer subdivided by acryl partitions was measured in the experiment. They were in good agreement with the theoretical ones near and above the coincidence.

Effects of Air-Layer Subdivision: A New Method of Improving Sound Insulation:

In the impact of floors, sound transmission through walls, and structure-borne sound transmission in buildings, there is a possibility of improving the insulation by restricting the air particle motion at the interface of the vibrating surface and the air. One of the practical ways for realizing this condition is by subdividing the air layer at the interface in the direction parallel to the radiating surface. This novel method for noise control has an attractive simplicity and a real practical benefit. In this paper, the mechanism, causing this effect, is discussed using analytical models of acoustic radiation and sound transmission. As a result of this investigation, it is seen that the effect of this method is characterized by reduction of acoustic radiation at low frequencies near and below the critical frequency of coincidence. The method is validated experimentally.

An experimental study on the absorption characteristics of a three-dimensional permeable membrane space sound absorber

In this study, we propose a rectangular and cylindrical three-dimensional space sound absorber using a permeable membrane and the absorption characteristics which are examined experimentally by reverberation room method. As a pilot study, a two-dimensional boundary element (2-D BEM) analysis is also conducted to predict the absorption characteristics of the absorbers. The experimental study revealed that the absorption coefficient is low at low frequencies and gradually increases with frequency. The absorption coefficient converges to 0.5 at the maximum which is similar to a single-leaf permeable membrane. The flow resistance and the surface density of the permeable membrane mainly affect the absorption characteristics at middle to high frequencies. At low frequencies, the heavy membrane contributes to the higher absorption performance. In the experiment specimens with high flow resistance produce higher absorptivity. Also, the cylindrical absorber shows slightly higher absorption coefficient than the rectangular absorber mainly at low frequencies. The 2-D BEM results show similar frequency characteristics as the measured values when the membranes flow resistance and surface density are low, but the numerical values overestimate overall the absorptivity of the absorbers.

Numerical analyses of the sound absorption of cylindrical microperforated panel space absorbers with cores

Microperforated panels (MPPs) are next-generation absorption materials because they can provide wideband sound absorption without fibrous materials and can be composed of diverse materials to meet global environmental demands. The fundamental absorbing mechanism is Helmholtz-resonance absorption due to perforations and an air cavity. MPPs are typically backed by rigid flat walls, but to reduce the restrictions on the MPP absorber properties, one of the authors has proposed MPP space sound absorbers without backing structures, including three-dimensional cylindrical microperforated panel space absorbers (CMSAs). Advantages of MPPs without backing structures are design flexibility and ease of use. Besides, the absorption characteristics of a CMSA with a core, which has a rigid cylindrical core inside the CMSA, have been experimentally tested, but a method to predict the absorption characteristics is necessary to design CMSAs with cores. Herein the two-dimensional combined Helmholtz integral formulation method is employed, and its prediction accuracy is evaluated by comparing the measured and predicted absorption characteristics of a CMSA with a core. Furthermore, a parametric study with regard to the core size is carried out to investigate the transition of the absorbing mechanism.

Study on coupled analysis for compact acoustic systems using finite-difference time-domain method based on Kirchhoff-Love plate theory and elastic wave

In this paper, we propose a coupled analysis method for compact acoustic systems (e.g., micro speakers, electret condenser microphones) using finite-difference time-domain (FDTD) method based on the Kirchhoff-Love plate theory and elastic wave. Acoustic systems are usually analyzed through any numerical analysis technique based on wave propagation. However, an exact frequency response is rarely obtained for compact acoustic systems. This is because the standard FDTD method cannot treat the change of the frequency response due to the air viscosity and cannot also treat the wave propagation in a solid. Thus, we cannot obtain exact frequency responses for compact acoustic systems through the analysis of coupled fluid-solid model based on standard FDTD method. In order to solve these problems, we utilize a coupled analysis method based on Kirchhoff-Love plate theory and elastic wave FDTD method considering air viscosity. The analysis method models diaphragm and voice coil as a two-dimensional plate model base...

Finite-difference time-domain analysis of the vibration characteristics of building structures using a dimension-reduced model

In order to accurately predict the vibration characteristics of buildings, wave-based numerical methods are effective from the viewpoint of the modeling accuracy of the physical mechanism and the detailed geometries of the simulated field. However, because of the performance of current PCs, the prediction of real-scale problems remains difficult. In order to address such problems, we herein propose a vibration simulation method for a beam-plate structure using a dimension-reduced modeling method. The target structure is modeled as a composite structure consisting of two-dimensional plate elements and one-dimensional beam elements, which are coupled based on the implicit finite-difference approximation scheme. By applying such a low-dimensional element, a faster simulation that requires less memory, as compared with a three-dimensional discretization scheme, is made available. To validate the method, the vibration characteristics obtained by the proposed scheme are compared to the measured results for mode...

Modeling and formulation of microperforated panels in the finite-difference time-domain method

A microperforated panel (MPP) was first developed by Maa and is recently regarded as one of the most promising absorption materials for next generation. Considering the approximation formula suggested by Maa, impedance of the perforations cannot be directly implemented in time-domain calculations because the impedance depends on frequency. Some new ideas are therefore necessary to take into account the effects of an MPP in time-domain analyses. Herein, a method to consider effects of an MPP in the finite-difference time-domain (FDTD) analyses is proposed. An MPP is considered as a boundary condition and the frequency dependence in the calculation is removed by replacing the frequency-dependent terms with constant values which are derived based on the multiple regression analysis. Additionally, the stability condition of the MPP boundary for one-dimensional sound field is developed. Absorption characteristics of MPP absorbers with back air layers of various depths are calculated numerically and compared wi...

Three-dimensional space sound absorbers using microperforated panels

A microperforated panel (MPP) is one of the most promising alternatives of the next-generation sound absorbing materials. It has a large potential for various use. However, its conventional use is to place it with a rigid-back wall and an air-cavity in-between. Therefore, its arrangements in a room are limited on to room boundaries, and in some cases, this may cause a problem in room design and its strength: for example, in Japan, it is not allowed by regulation to use it for a wall. Therefore, the authors have been looking at the possibility for using MPPs for space sound absorbers without any backing structure. This can allow MPPs for wider use: a three-dimensional shape MPP space absorbers can be easily produced if a suitable material is used. They can be placed easily in various places: they can be put on the floor or hang from the ceiling, etc. Besides, MPPs can be made of designable, lightweight materials, which can add designability to the absorbers. In this paper, the main results of our studies o...