Reduction of Sound Power Radiated into a Free Field by a Continuous Silencer

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Passive reduction of sound power radiated into a free field has been investigated. A passive source whose mouth surrounds a primary source continuously, which is named a continuous passive source, is proposed because it is thought to be effective to gain the area of the mouth. A model analyzed here consists of a primary source radiating a pure tone and a secondary continuous passive source having a circular ring-shaped mouth. The relation between acoustic impedance of a passive source and sound power radiated was analyzed. The optimal acoustic impedance of a passive source and the minimum and maximum sound power were deduced as functions of a product of a wavenumber and a radius of a continuous passive source. The radiation characteristics of a continuous passive source were also analyzed. The expression of the radiation component of acoustic impedance and their approximation in the practically important range of frequency were found. It was shown that this passive approach can theoretically achieve the same maximum reduction in sound power as an active control approach. It was also found that the result from this approach is consistent with the result from another passive approach using multiple passive point sources, which are arrayed in a circle surrounding a primary point source. A cylindrical silencer and an annular silencer, which have a circular ring-shaped mouth on an infinite baffle and a cavity with the same thickness as the mouth, were studied as examples of a continuous passive source. Its optimal design for the minimization of sound power radiated and the effects of its characteristics on sound power reduction were discussed. The experiments were carried out and it has been verified that the continuous passive source proposed here is effective.