Resonant Properties of a Short Expansion Chamber Symmetrically Located on Opposite Sides of a Rectangular Duct

Abstract

A short expansion chamber in a duct acts as a resonator type muffler. The properties of this type of chamber have already been studied in detail by the authors for circular duct-chambers. In this paper, short chambers are applied to rectangular ducts. These chambers consist of two rectangular branches located symmetrically on opposite sides of a rectangular duct. The transmission properties of plane waves through the chambers were examined experimentally. For a very short axial chamber length, the resonant frequency was determined in the same manner as for a side-branch muffler with open end correction. Though considered proportional to depth in circular chambers, it becomes constant in rectangular chambers with fixed axial length, regardless of chamber depth. When the chamber length is close to half of the chamber height (corresponding to the upper limit of resonator-type mufflers), the resonant frequency is close to, but in excess of, the cut-off frequency of the chamber's second resonant mode. The reason for this discrepancy becomes clearer subsequent to examination of the sound field contours calculated using the finite difference method.

Sound-Attenuation Effect of a Partially Shut Rectangular Chamber at the End of an Inner Cylinder, Modeling a Ceiling-Mounted Ventilator

“…As experienced in our earlier research that studied the properties of some simple narrow expansions in a duct (7) , the resonance effect of a cavity tends to become weak as the inlet of the cavity becomes narrow. From a comparison between the measured and calculated results, the accuracy of the numerical calculation can be ascertained.…”

“…At 2.53 kHz, the resonance condition is obtained when the node reaches not at, but just in front of point N shown in Fig.9, which is the sound source-side corner of the chamber inlet. This condition was experienced in our earlier research, which had a simple chamber of L 1 = L 2 and having a chamber shape similar to the duct shape (7) .…”

Modulation of Sound Attenuation Properties of a Short Expansion Chamber by Changing the Temperature in It

“…If the branch is sufficiently deep (b >> h), the resonant frequencies of the chamber having the configuration in Fig.1 depend on the total height of the chamber (2b + h) (7) . On the other hand, in the case of a relatively shallow chamber (b < h in this research) with very short axial length (l/h < about 0.3), the chamber properties can be described by an analogy to a single side-branch muffler (a quarter-wavelength tube attached to a duct) (5) . Its resonant frequency is calculated by…”

“…The duct aspect ratio w/h does not affect the sound property because only a plane wave is considered in this research. However, from an experimental point of view, it is preferable that w is not too wide in order to avoid the generation of higher-order mode waves that have a sound distribution in the z direction (5) . If the branch is sufficiently deep (b >> h), the resonant frequencies of the chamber having the configuration in Fig.1 depend on the total height of the chamber (2b + h) (7) .…”

“…In this configuration, the sound attenuation performance of the chamber is relatively easy to understand by a concept of a side branch. The property of it has been investigated in detail in our previous study (5) . While a side branch-type silencer should have a certain depth for attenuating low frequency sounds, its depth in this research is relatively shallow.…”