Resonant Properties of Short Expansion Chambers in a Circular Duct: Including Extremely Short Cases and Asymmetric Mode Wave Incidence Cases

Shirahama²,

Tsubakishita

2008

JEE

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.

Section: Simple Chambers (Types I and Ii)supporting

confidence: 54%

Sound Attenuation by a Resonant Effect of a Rectangular Box Containing a Cylinder as a Model of a Ceiling-Mounted Ventilator

Shirahama²,

Tsubakishita

2008

JEE

“…As some researchers (e.g., Sullivan and Crocker, 1978) have pointed out, a chamber becomes a resonant chamber when l/d < 0.41. In addition, based on the results of our research (see Sadamoto and Murakami, 2002), it can be said that a chamber becomes a slit-type chamber when l/d < 0.1, where slit-type means that the chamber acts as a side-branch type of silencer; this property is realized by the interaction of sounds in the branch axis direction. In other words, the slit acts as a closed-end tube connected to the duct, and the resonant frequency is obtained when a quarter of the incident sound's wavelength becomes equal to the length of the tube with an open-end correction.…”

Section: Figurementioning

confidence: 79%

“…In this section we discuss the plane wave only. We examined several cases, including cases of higher-order modes (Sadamoto and Murakami, 2002), but we omit those data here so as to show the basic character of the slit.…”

Section: Figurementioning

confidence: 99%

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Reduction of Discrete‐Frequency Fan Noise Using Slitlike Expansion Chambers

International Journal of Rotating Machinery

Murakami

2003

As is generally known, discrete-frequency noises are radiated from fans due to rotor-stator interaction. Their fundamental frequency is the blade-passage frequency, which is determined by the number of rotor blades and their rotating speeds. To reduce such noises, several types of silencers have been designed. Among them, the authors noted a slitlike expansion chamber (hereafter referred to asslit, for simplicity) and have studied its performance. A slit is a simple expansion chamber with a very short axial length that is placed in a duct. A slit with a circular cross-section that is concentric with a circular duct may be studied using the same interpretation as is used for a side-branch resonator muffler (closed-end tube connected to a duct); that is, the resonant frequency of a slit depends on its depth (with an open-end correction). It is expected, hence, that a slit might be applicable as a simple and axially compact silencer that is effective on discrete-frequency noises. In this article, the properties of a slit are introduced, and the applicability of a slit to actual rotating machinery is described using experimental data.

“…An expansion chamber in a duct acts as a resonator type muffler when the axial length of the chamber is short (1) . Its effective frequency, or its resonant frequency, depends on the duct and chamber geometry (2), (3) . In order to modulate this frequency in a reactive manner for a fixed geometry chamber, the properties of the gas in the chamber must be changed.…”

Section: Introductionmentioning

confidence: 99%

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

Tsubakishita

Murakami

2008

JSDD

An axially short expansion chamber in a duct acts as a resonator type muffler whose effective frequency is determined by the duct and chamber geometry. To modulate this frequency, the temperature of the air in a very short chamber consisting of two rectangular branches located symmetrically on opposite sides of a rectangular duct was changed. The transmissibility of the sound was measured for a plane wave propagating in the duct through the chamber section. Each effective frequency (resonant frequency), at which the maximum transmission loss was obtained, was summarized and considered using a simple side branch type muffler approximation. The frequency modulates regularly according to the temperature in the chamber. In practice, though, the range of frequency modulation is small versus the change in temperature. However, this method provides a means for the fine adjustment of the effective frequency to a target frequency for noise reduction.