On the basis of the boundary surface control (BSC) principle, it is possible to control the sound field within a volume by controlling the sound pressure and particle velocity on a surface. It is, however, impossible to realize an active noise control (ANC) system, which controls both the sound pressure and particle velocity at all points on the surface. Therefore, we have to control these at discrete points on the surface. In conventional numerical studies, it is known that the discretization of the surface decreases the effectiveness of the ANC system. To determine the amount of discretization, which is equal to the number of error sensors, we have to know the relationship between the discretization of the surface and the effectiveness of the ANC system quantitatively. In this study, we aim to establish a method of quantifying the relationship between the discretization of the surface and the control effectiveness in the ANC system on the basis of the BSC principle. Furthermore, we derive the arrangement of the error sensors required to obtain a desired effectiveness within a circular area using the derived equation.
Combating noise signals is still a very important and challenging research topic. Previously, we presented a subtractive-beamformer-based noise reduction algorithm using paired microphones, which was shown to be effective in reducing directional noise. However, its basic assumption, namely a perfectly coherent noise field, is generally not satisfied in real-world environments. In this paper, we develop a general expression for the original algorithm we suggested earlier, on the basis of a generalized subtractive beamformer and by relaxing the strict assumption to that of an arbitrary noise field. Following ideas similar to those of the original algorithm, the generalized algorithm with a generalized sidelobe canceller (GSC)-like structure is derived. A theoretical analysis is then presented to show the linkage between this generalized algorithm and the original algorithm, and to show its noise reduction performance in theoretically defined noise fields. Finally, the superiority of the proposed noise reduction algorithm to other comparable algorithms was verified by experiments using multichannel recordings.
Electret condenser microphones for mobile system terminals should be more robust than those for ordinary consumer equipment. Generally, the diaphragm electret can achieve flat temperature characteristics with respect to sensitivity because the temperature dependence of the diaphragm stiffness and gain characteristics of the FET offset each other. However, the fixed electrode electret using a PET diaphragm is not sufficiently robust with respect to temperature because the PET membrane has the same temperature coefficient as FETs, so there is no offsetting and it is difficult to compensate the temperature characteristics. In order to improve robustness, this paper proposes a temperature dependence evaluation method for acoustic characteristics of the electret condenser microphone. It then reports the temperature dependence of the dominant parameter that affects the sensitivity of the microphone and describes a method to design a microphone with stable temperature characteristics. In a new trial, silicon, an inorganic material, is applied to the diaphragm. It is subsequently demonstrated that an impedance converter composed of a source-follower circuit is effective for a high temperature environment of up to 80°C. Finally, the possibility of an electret condenser microphone with flat temperature characteristics is proposed.
In recent years, several studies on acoustic communication systems utilizing gas pipe lines have been carried out. However, in conventional acoustic communication technology, the transmission rate cannot be improved because of the reverberant signals arising from reflections at the bends and branches in pipes. To reduce the effect of the reverberant signals and to improve the transmission data rate, we studied a multi-carrier modulation system. In this study, to avoid intersymbol interference, we employed special modulation symbols composed of multicarrier frequencies which change cyclically. To evaluate the proposed system, we constructed an experimental setup simulating a pipe system equivalent to that of a six-story building as a communication path. We achieved a transmission rate of 3,840 bps using the proposed method.