The improvement on the sound insulation of small openings for natural ventilation by the internal cavity of the sleeve

摘要

We compared the theoretical and measured sound reduction indices when a silencer wasinserted into a sleeve with circular aperture and examined its sound reduction index withthe aim of suppressing the deterioration of sound insulation due to axial first-order-moderesonant transmission through small openings for natural ventilation. The silencer wasproduced by opening a slit in the middle of a circular pipe with closed ends and wrapping anonwoven fabric around the slit to increase flow resistance. The sound transmissioncoefficient with respect to the zeroth-order mode in the cross-sectional direction of a smallopenings for random incidence was theoretically calculated using a two-port equivalentcircuit for frequencies lower than the value at which the radius of the small openings is0.293-times of the wavelength. At these frequencies, high-order-mode transmission isnegligible. The decrease in the sound reduction index that is caused by resonanttransmission was suppressed by inserting the silencer with the nonwoven fabric. Thetheoretical and measured sound reduction indices were in good agreement, although therewas a difference in the resonance frequency of the silencer at which the maximum soundreduction index was obtained.