岩本 宏之

<p>Noise countermeasures are an important engineering problem, and acoustic analysis methods for noise prediction and reduction have been widely studied. In general acoustic analysis, the temperature of the air is often assumed to be constant. However, the temperature gradient of the space may have a large effect on the sound pressure. For example, in a thermoacoustic engine, the sound is generated when the air vibrates due to the temperature gradient inside the pipe. Applying this effect may reduce noise by using the temperature distribution. Therefore, the purpose of this study is to formulate an acoustic space with a temperature gradient using a concentrated mass model. This model is consisted of the masses, springs and dampers. In addition, the relationship between temperature gradient and sound pressure is analyzed. In this paper, as a first step, a model using a linear spring is constructed, and the validity of the proposed model is confirmed by numerical calculation for a cylindrical pipe.</p>

<p>In active noise control<tt>，</tt>if total acoustic power is minimized<tt>，</tt>absorption and blowout of acoustic energy around the control source are balanced<tt>，</tt>and a zero power phenomenon occurs in which the time average of control power becomes zero. In this study, we aim at generalization of this phenomenon, the sound source is regarded as a flat plate structure embedded in an infinite baffle, and the generation condition of the zero power phenomenon is considered. First, we will clarify the reciprocity when the zero power phenomenon occurs in the case of three point sources. Next, the reciprocity of the zero power condition is clarified based on the control law derived from the sound sources emitted from the three flat plate structures.</p>

<p>This paper is concerned with active control of acoustic radiation power of a multi-walled structure. In the past literature, optimal control of acoustic radiation power for a double-walled structure in which both walls of the acoustic space are surrounded by flexible walls has been studied. It has been clarified that the reduction level of an actively controlled doublewalled structure is greater than that of an actively controlled one-walled structure in the sense of sound transmission control. Thus, the number of flexible walls have an impact on the control effect. However, research on modelling and control of a triple or more walled structure have not sufficiently done. Therefore, this paper presents how to model and control a triple walled structure.<tt> </tt>First, in order to facilitate further numerical analysis of multiple-walled structures, we model triple-walled structures using block inverse matrices, which is different from the conventional method. Next, the validity of the theory using the block inverse matrix is verified by comparing with the result of the numerical analysis for the triple-walled structure in the previous research. Finally, we verify the optimal control of acoustic radiation power by two control elements in a triple-walled structure from the numerical perspective, comparing its characteristics with the case of control by one control element.</p>

<p>In active noise control，if total acoustic power is minimized，absorption and blowout of acoustic energy around the control source are balanced，and a zero power phenomenon occurs in which the time average of control power becomes zero. In this research，we aim at generalization of the said phenomenon，and consider the generation condition of the zero power phenomenon in the case of multiple control sound sources. Next，based on the derived control rules，we will clarify the change in acoustic power due to the increase in the number of sound sources and the position from the viewpoint of numerical analysis. Finally，the zero power control law in the case of expanding the control sound source to n control sources is derived.Numerical analysis using the derived control law to prove importance.</p>

<p>This paper is concerned with active control of acoustic radiation power of a multi-walled structure. In the past literature, optimal control of acoustic radiation power for a double-walled structure in which both walls of the acoustic space are surrounded by flexible walls has been studied. It has been clarified that the reduction level of an actively controlled double-walled structure is greater than that of an actively controlled one-walled structure in the sense of sound transmission control. Thus, the number of flexible walls have an impact on the control effect. However, research on modelling and control of a triple or more walled structure have not sufficiently done. Therefore, this paper presents how to model and control a triple walled structure. First, the target structure is analytically modelled by using modal coupling method that utilizes modal amplitudes of the uncoupled subsystems. Next, the optimal control law for minimizing the acoustic radiation power of the triple wall structure is derived by using the elementary radiator method which calculates the acoustic radiation power based on the minute and divided elements of the panel. Finally, numerical simulations of the control system are conducted, confirming the effectiveness of the proposed method.</p>

<p>This paper is concerned with active vibration control aiming at the minimization of vibrational energy of a flexible beam. The purpose of the paper is to present the methodology of designing the smart sensor in which the reduction of its output results in the minimization of the vibration energy. Firstly, this paper begins with the derivation of an ideal feedforward control law for minimizing the vibration energy. This is followed by the derivation of the control law for minimizing the smart sensor output. Next, shaping function of smart sensor such that the control law for minimizing the kinetic energy is coincident with that for minimizing the smart sensor output is numerically designed. Numerical simulation on adaptive feedforward control in conjunction with the desined smart sensor is finally conducted. It is found that if the disturbance is pure tone, the kinetic energy is minimized by the proposed method. On the other hand, if the disturbance is a white noise, the control effect deteriorates compared to the case of the sinusoidal disturbance. This is caused by instability of the ideal control law for minimizing the kinetic energy.</p>

<p>Suppression of noise and vibration in machine products is an important problem, and many methods have been studied. In particular, structural-acoustic coupled effects due to the weight reduction of machines cannot be ignored. Therefore, research on efficient analysis methods for coupled systems is required. In structural-acoustic coupled analysis, the finite element method (FEM) that the acoustic space is described by sound pressure and the structure is described by displacement is often used. In this method, eigenvalue analysis requires a long computational time because the mass and stiffness matrices are asymmetric. In our previous study, we proposed a concentrated mass model (CMM) of quadrilateral elements for two-dimensional acoustic space. This model consists of mass points and connecting springs, and has variable displacement in its acoustic space. Since the CMM has symmetrical mass and stiffness matrices, structural-acoustic coupled analysis can be performed efficiently. The effectiveness of the CMM was shown in the past for an acoustic space-membrane vibration coupled system. In this paper, we propose the CMM of arbitrary element shape in structural-acoustic coupled analysis. Furthermore, we propose a coordinate transformation method to improve calculation speed by reducing the degree of freedom while preserving the symmetry of coefficient matrices. The effectiveness of the proposed method is confirmed by numerical calculation.</p>

This paper presents a methodology of generating a sound field in which the sound energy around the speakers is high while that in the other region is relatively low. Although directional loudspeaker that propagates sound energy only for the specific direction has been discussed in the past literature, such a method leaks sound to outside of the specific area because of reflection of the sound wave. To overcome this problem, a novel methodology of generating localized sound which is based on the phenomena that radiated sound from vibration of a rectangular plate under the specific conditions decays drastically more than normal distance decay effect. First, spatial distribution of a rectangular plate vibration at a frequency where acoustic power become extremely low is verified, clarifying localized sound is generated by coupling certain modes each other. Next, a matrix-type speakers array is used to imitate the spatial distribution that generates localized sound in order to achieve generating a sound field not only at a single frequency but in band-limited spectra. As a result, efficacy of speakers array to generate of localized sound is revealed.

The objective of this study is to generate a quiet space in a coupled rectangular cavity by suppressing a travelling wave caused by vibration of a flexible panel. Firstly, this study begins with description of an enclosed sound field from a wave point of view by presenting a one-dimensional transfer matrix method for three-dimensional space. Next, a concept of modal coupling method is utilized to model a coupled rectangular cavity. Furthermore, introducing multiple control sound sources in an arbitrary plane in the target space, a control law is derived that eliminates a travelling wave passing through the control plane. Finally, numerical simulations are carried out to demonstrate the validity of the proposed method. As a result, it was clarified that the proposed method enables to generate a quiet space by cancelling transmitted waves.

This paper is concerned with an eigenvalue problem of a vibro-acoustic coupling system. In the conventional method, the dynamics of a vibro-acoustic coupling system is calculated with modal coupling methodology which uses eigenfunctions of a rigid-walled cavity. Therefore, particle velocity on the surface of a panel is always calculated as zero, while it should be coincident with the velocity of the panel. To overcome the problem, this paper presents a new methodology for deriving accurate eigen-pairs of a vibro-acoustic coupling system. First, a transfer matrix is introduced which can describe the characteristics of the sound field. This is followed by the derivation of the sound pressure on the coupled rectangular panel. Then, the vibrational velocity of the panel is derived with the sound pressure being used as the input. Furthermore, the eigenvalue problem is formulated based on the equations of vibration and sound field. Finally, the numerical simulation is carried out, demonstrating the validity of the proposed method.

The objective of this paper is to experimentally implement the feedback wave control method. Firstly, based on the collocation of sensors and actuators, cluster control is introduced. Next, linking the cluster control with the active wave control, it is clarified that the active wave control system can be realized to a limited extent. Finally, experiments on the reflected wave absorbing control using clustered direct velocity and displacement feedback are carried out. The experimental results show perfect wave absorption is achieved at the target frequency.