Iljae Lee

Helmholtz resonator with extended neck

Acoustic performance of a concentric circular Helmholtz resonator with an extended neck is investigated theoretically, numerically, and experimentally. The effect of length and shape of, and the perforations on the neck extension is examined on the resonance frequency and the transmission loss. A two-dimensional analytical method is developed for an extended neck with constant cross-sectional area, while a three-dimensional boundary element method is applied for the variable area and perforated extension. Lumped and one-dimensional approaches are also included to illustrate the effect of the higher order modes. For a piston-driven model, predicted resonance frequencies using lumped, one-dimensional, and two-dimensional analytical methods are compared with those from multidimensional boundary element method. Analytical and computational transmission loss predictions for pipe-mounted model are compared to the experimental data obtained from an impedance tube setup. It is shown that the resonance frequency may be controlled by the length, shape, and perforation porosity of the extended neck without changing the cavity volume.

Analytical approach for sound attenuation in perforated dissipative silencers

The acoustic attenuation performance of perforated dissipative circular expansion chambers with inlet/outlet extensions is investigated. The eigenvalues and eigenfunctions of the sound field are analytically determined in the extended inlet/outlet circular ducts, upstream/downstream end annular dissipative chambers, and the central perforated dissipative expansion chamber. Utilizing the continuity conditions of velocity/pressure at the interfaces the transmission loss is predicted by a two-dimensional analytical approach. For a specific configuration, such predictions are compared with both experiments and a three-dimensional computational solution based on the substructure boundary element technique, showing a reasonable agreement. The analytical results for the effect of the absorbent resistivity, duct porosity, and geometryon the acoustic attenuation performance are discussed in detail.

Helmholtz resonator lined with absorbing material

A closed-form, two-dimensional analytical solution is developed to investigate the acoustic performance of a concentric circular Helmholtz resonator lined with fibrous material. The effect of density and the thickness of the fibrous material in the cavity is examined on the resonance frequency and the transmission loss. With the expressions for the eigenvalue and eigenfunction in the cavity, the transmission loss is obtained for a piston-driven model by applying a pressure/velocity matching technique. The results from the analytical methods are compared to the numerical predictions from a three-dimensional boundary element method and the experimental data obtained from an impedance tube setup. It is shown that the acoustic performance of a Helmholtz resonator may be modified considerably by the density and thickness of the fibrous material without changing the cavity dimensions.

Acoustic impedance of perforations in contact with fibrous material

Acoustic impedance of perforations in contact with fibrous material, as well as air alone, is experimentally determined in the absence of mean flow. Different porosities (2.1, 8.4, 13.6, and 25.2%) and hole diameters (0.249 and 0.498cm) are applied for perforations, along with two fiber filling densities (100 and 200kg∕m3) to illustrate the effect of variations in such parameters on the perforation impedance. A modified impedance tube setup is developed for the measurement of the acoustic impedance of perforations in contact with fibrous material, and the air alone. The complex wave-number and characteristic impedance of the fibrous material are also experimentally obtained in this study to utilize such properties for the calculation of acoustic impedance of perforations in contact with the material. The experimental results show that both resistance and reactance of the perforations in contact with air decrease as the porosity increases. It is also shown that the fibrous material significantly increases ...

Analytical approach for sound attenuation in perforated dissipative silencers with inlet/outlet extensions

The acoustic attenuation performance of perforated dissipative circular expansion chambers with inlet/outlet extensions is investigated. The eigenvalues and eigenfunctions of the sound field are analytically determined in the extended inlet/outlet circular ducts, upstream/downstream end annular dissipative chambers, and the central perforated dissipative expansion chamber. Utilizing the continuity conditions of velocity/pressure at the interfaces, the transmission loss is predicted by a two-dimensional analytical approach. For a specific configuration, such predictions are compared with both experiments and a three-dimensional computational solution based on the substructure boundary element technique, showing a reasonable agreement. The analytical results for the effect of the absorbent resistivity, duct porosity, and geometry on the acoustic attenuation performance are discussed in detail.

Impact of perforation impedance on the transmission loss of reactive and dissipative silencers

The effect of perforation impedance on the acoustic behavior of reactive and dissipative silencers is investigated using experimental and computational approaches. The boundary element method (BEM) is applied for the prediction of transmission loss of silencers with different perforation geometries. The variations are considered in the porosity (8.4 and 25.7%) and hole diameter (0.249 and 0.498 cm) of perforations for both reactive and dissipative silencers, as well as the fiber filling density (100 and 200 kg/m3) for the latter. The acoustic impedance for a number of perforations in contact with air alone and fibrous material has been incorporated into the predictions, which are then compared with the measured transmission loss using an impedance tube setup. The results demonstrate the significance of the accuracy of the perforation impedance in the predictions for both reactive and dissipative silencers.

Effect of leakage in Helmholtz resonators

The effect of leakage in Helmholtz resonators has been experimentally and numerically investigated. Transmission loss of a Helmholtz resonator having a gap between the cavity and main duct is measured using an impedance tube setup. The effect of leakage on the transmission loss is examined using different amounts of gap openings. Experimental results are then compared with the predictions from the boundary‐element method. The study shows that the leakage increases the resonance frequency substantially and widens the transmission loss. Hence, the leakage needs to be taken into account for accurate predictions of Helmholtz resonators.

Acoustic impedance of perforations in contact with absorbing material

Acoustic impedance of perforations in contact with fibrous material is experimentally determined for various hole geometries and filling densities in the absence of mean flow. Two setups are developed to measure the perforation impedance and the acoustic properties of the absorbent, while yielding empirical expressions for both. The experimental results show that both real and imaginary components of the perforation impedance substantially increase as a result of the contact with the fibrous material, whereas they generally decrease with increasing porosity for a given filling density. The empirical expressions developed in this study are implemented in a boundary element method to predict the transmission loss of dissipative silencers, which is then compared with the experimental results.

Dissipative silencers with an extended inlet/outlet and baffles

The acoustic characteristics of a single‐pass perforated dissipative silencer were investigated experimentally and numerically by Selamet et al. [J. Acoust. Soc. Am. 109, 2364 (2001)]. The current study extends this work by considering variations in the internal structure of the dissipative silencer. In addition to the boundary element method (BEM) introduced earlier, a multi‐dimensional analytical approach is now developed to investigate the wave modes and transmission loss. Both methods are then employed to study the effect of an extended inlet and outlet on the acoustic behavior of the silencer. BEM is further used to explore the effect of baffles and air space inside the dissipative chamber. The location and number of baffles inside the dissipative chamber are shown to have a significant influence on the transmission loss.

Acoustic characteristics of a three‐chamber hybrid silencer

The acoustic characteristics of a hybrid silencer consisting of two identical single‐pass, concentric, perforated dissipative chambers combined with a reactive Helmholtz resonator in between are investigated computationally and experimentally. Transmission loss predictions from a three‐dimensional boundary element method are compared with experimental results obtained from an impedance tube setup in the absence of mean flow. In addition to the overall design, the effects of filling material density in dissipative chambers and the neck geometry of the reactive resonator are examined. The dissipative chambers are found to be effective at high frequencies, while the reactive resonator is shown, in general, to improve the acoustic attenuation at low frequencies typical of airborne noise in internal combustion engines running at low‐ to mid‐speed range. The acoustic behavior near the resonance frequency is determined, however, to be sensitive to the duct length connecting dissipative and reactive chambers. Pot...