Some ceiling-mounted ventilators have a sound-attenuating function due to the resonant effect that takes place within a chamber that consists of a rectangular outer cover and a cylindrical fan unit. In order to control the resonant properties in a reactive manner and maximize the sound-attenuation effect of this resonant system, the chamber is separated by a partial shutting plate, called a "separator," located at the end of an inner cylinder-modeling a fan unit-into two spaces that are expected to approximate a Helmholtz resonator with two degrees of freedom. Variations in resonant properties are shown, experimentally obtained by modulating the dimension of the separator. In this model, the second resonance frequency can be widely changed depending on the clearance between the two spaces, without attenuation of the resonant effect. This is meaningful because the second resonance frequency can be shifted into a frequency range that corresponds to the main component of the radiated sound from an actual ventilator. In addition, it is indicated that the resonance frequencies can be approximated by a simple analytical approximation. Furthermore, detailed considerations of sound fields of relatively delicate models concerned are provided using numerical calculation (BEM) and the effectiveness of the chamber is summarized.

A typical ceiling-mounted ventilator consists of a cylindrical fan unit and a rectangular box as an outer cover. When the box is appropriately closed by an orifice plate in front of the fan, the space enclosed by the box and plate acts as a sound resonator. In the present study, the performance and physical aspect of this resonator are investigated experimentally using a model of a rectangular chamber containing an internal cylinder placed in an infinite circular duct. The sound transmission loss of the chamber is measured while varying the chamber length and the inlet length. The experimental model verified the effect of an actual ventilator. Two resonances appear in the frequency range of interest. These resonance frequencies are determined using a relatively simple estimation. In addition, for modulating the chamber property, a thin circular plate or a thin cylindrical pipe, called a "separator," is attached to the chamber. The performance of the separator, which modulates the resonance frequency, is examined. If the orifice plate and the separator are combined and arranged appropriately, they can promote sound attenuation in actual ceiling-mounted ventilators.

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An axially-short expansion chamber in a duct acts as a resonator muffler. The radial size of the chamber must be altered in order to change the target frequency for noise reduction because the resonance frequency depends on the depth of the chamber. The present study reveals that the resonance frequency can be altered by changing the shape of the expansion to an 'L' shape, even if the radial size of the expansion is fixed. The structures of the acoustic field in the duct are obtained numerically based on a finite difference method for several L-shaped expansions. The computed results show that the resonance frequency decreases with increasing axial length of the expansion, whereas the minimum transmission coefficient is approximately constant. In addition, increasing the inlet length of the expansion enables the resonance frequency to be increased and the minimum transmission coefficient to be reduced. These results are in good agreement with previously reported experimental data.

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