This paper reviews the progress in the technology of active noise control (ANC) with some examples and discusses what are the key points in this technology. In the first place, the principle of the ANC is briefly explained and it is shown that the cancellation of noise by the sound wave radiated by the additional sound source can be considered to be equivalent to the control of the internal impedance of additional sound source. And an example of the noise control by the adaptive directivity control is explained. Then, some method to decrease the closed loop gain are shown as the instability is usually fatal in the ANC system. Next, the FIR filter to synthesize the signal to drive the additional source is introduced as an indispensable equipment. It is finally pointed out that the adaptive control of the coefficients of FIR filter is the most important technology in the active noise control because the optimum values of the coefficients vary with the changes in the noise transmission path.

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Active control of sound fields is an emerging technology which has a great potential to become superior to many passive low frequency noise control methods. It is based on fundamental physical principles which include acoustical coupling of sources, destructive wave interference, and control of enclosure modes. The implementation of active noise control requires placing secondary sources and microphones into the controlled field. Most systems use adaptive filters to control the secondary sources to achieve maximum noise reduction. The engineering applications will require developing optimization techniques for secondary sources and controlling microphone locations. Also new types of robust actuators and controlling sensors will be needed. A brief summary of existing and future applications is provided.

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This paper reports the recent tendency in the active vibration control of flexible structures which are fundamental components in machinery. In the first half of the paper, it covers basic problems such as making a reduced-order model of the structure required for designing a vibration controller and its state equation representation, proper arrangements of sensors and actuators, and a method of prevention of the spillover instability occurred by ignored residual modes. In the last half, it deals with classification of vibration control methods and a vibration controller design using LQ control theory. In particular, the design method of the active dynamic absorber which is widely known in mechanical engineering and civil engineering is explained in detail.

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In today's advanced society, a much higher performance is being demanded of buildings and other structures against the threat of natural disasters such as earthquakes and strong winds, than the ordinary efficiency relevant to safety and amenity that are provided by the present design philosophy and standards. The seismic response controlled structure functions actively against earthquakes and can indeed respond to this demand. Especially in recent years with major advanced technology in electronics and computers, research and development of active seismic response controlled structures has quickly gained wide attention in both Japan and the U. S. Practical application to one of actual buildings has been already realized. From a practical view point, this paper presents mainly the state of the art of active vibration control for architectural structures in both Japan and the U. S.

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This paper outlines the use of ASFC system in auditoriums and describes their principles, functions and features. My personal experiences with some of the systems are also reported, and problems related to this type of system are discussed from the viewpoint of an acoustical consultant. The design concept of the system installed in the Tokyo Metropolitan Art Space is presented as an example of introducing an ASFC system.

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The active noise control system using motional feedback (MFB) loudspeaker is proposed which is effective in the low frequency range. Although FIR digital filter is used for usual control system, it is difficult to realize ideal MFB type active control system because the group delay in FIR filter affects the phase characteristics. In this paper, we realize the IIR filter using DSP with high speed A/D and D/A convertors to reduce the group delay. The positive insertion loss is obtained from 21 to 94Hz on the model experiment. However, the obtained insertion loss is lower than one obtained in numerical simulation because of an oscillation about 300Hz.

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As a trial to improve the performance of passive noise barriers by applying the active control technique, basic theoretical and experimental investigation has been made. At first, the basic conception of the “active noise barrier” is presented, in which an active control system composed of a noise detector, an error sensor, a secondary sound source and an adaptive signal processing unit is attached to a passive barrier. As a preliminary study, a simple numerical calculation was performed according to the Fresnel-Kirchhoff diffraction theory, and the adaptability of the active control has been basically confirmed. Next, 1/2 scale model experiments were performed in a hemi-anechoic room to inve stigatethe effectiveness of the active noise barrier. In this experimental study, the method combining the “filtered x LMS” algorithm and the “block processing technique” was applied as the adaptive signal processing technique. As a result, it has been found that the active noise barrier is effective for sound attenuation in low frequencies.

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