Volume 126, Issue 6, December 2009
Index of content:
- ARCHITECTURAL ACOUSTICS 
Determination of elastic constants of generally anisotropic inclined lamellar structure using line-focus acoustic microscopy126(2009); http://dx.doi.org/10.1121/1.3245032View Description Hide Description
A methodology for measuringelastic constants of different phases in materials with lamellar microstructure by line-focus acoustic microscopy is developed. The materialmicrostructure investigated is modeled by generally anisotropic multilayers arbitrarily inclined to the sample surface on which acoustic microscopy measurements are performed. To calculate surface acoustic wave(SAW) propagation in such structures quasi-static effective elastic constants are determined and compared with calculated frequency-dependent constants. As a model material, practically important, Ti–6Al–2Sn–4Zr–2Mo alloy is selected. Time-resolved line-focus acoustic microscopy experiments are performed on a Ti-6242 single colony (Ti–6Al–2Sn–4Zr–2Mo alloy) and on a Ti–6Al -phase single crystal for which elastic constants of different phases are determined using inversion of measuredSAWvelocities. To validate the experimental methodology, SAWvelocities in an -cut quartz crystal are measured as a function of sample orientation angle and compared with predictions based on the known elastic moduli of quartz.
126(2009); http://dx.doi.org/10.1121/1.3257182View Description Hide Description
Low-frequency noise is difficult to deal with by traditional porous material due to its inherent high acoustic impedance. This study seeks to extend the effective range of sound absorption to lower frequencies by filling a low density gas, such as helium, in the porous material. Compared with conventional air-filled absorptionmaterial, the helium-filled porous material has a much reduced characteristic impedance; hence, a good impedance matching with pure air becomes more feasible at low frequencies. The acoustic properties of a series of helium-filled porous materials are investigated with a specially designed test rig. The characteristic of the sound propagation in a helium-filled porous material is established and validated experimentally. Based on the measuredacoustic properties, the sound absorption performance of a helium-filled absorber (HA) of finite thickness is studied numerically as well as experimentally. For a random incidence field, the HA is found to perform much better than the air-filled absorber at low frequencies. The main advantage of HA lies in the middle range of oblique incidence angles where wave refraction in the absorber enhances sound absorption. The advantage of HA as duct lining is demonstrated both numerically and experimentally.
Comparison of three measurement techniques for the normal absorption coefficient of sound absorbing materials in the free field126(2009); http://dx.doi.org/10.1121/1.3242355View Description Hide Description
Three different techniques for evaluating the absorption coefficient of sound absorbing materials in free field conditions are discussed. One technique measures the acoustic impedance at one point nearby a specimen, the other two techniques evaluate the impedance from the transfer function of two sound pressures and two particle velocities at two points. These are called “PU-method,” “PP-method,” and “UU-method,” respectively. An iterative algorithm to estimate the acoustic impedance of the locally reactive specimen in the spherical wave field is also applied. First, the effect of receiver positions, specimen areas, and source heights to the measured normal absorption coefficient is investigated by the boundary element method. According to these investigations, the PU-method is most stable against the effect of specimen area, and the UU-method is easily affected by that effect. Closer source to the specimen distance is advantageous for the signal to noise ratio of these measurement techniques, but correction for the effect of the spherical wave field has to be applied. As a finding, the iterative algorithm works for all of three techniques. Finally, the PU-method is applied experimentally with a pressure-velocity sensor and a loudspeaker in a hemi-anechoic room. As a result, the calculated results have been verified.
126(2009); http://dx.doi.org/10.1121/1.3257209View Description Hide Description
Numerical and experimental studies were undertaken to characterize the noise transmission and scattering properties in higher-order modes across the tee-junction of a rectangular duct used in ventilation and air-conditioning systems. To measure these properties, a formulation of a transmission matrix based on the transfer function and a two-microphone method was devised. The measurement of modal sound transmission and scattering coefficients is demonstrated for a duct element in a rectangular duct. The results of numerical simulations were verified by experiments. The results show that sound transmissions of fundamental mode and higher-order modes across the main duct are high at the eigen-frequencies of the main duct and sidebranch. Weak modal coupling of the branch-modes and the traveling wave in the main duct is observed at or very close to the eigen-frequencies of the sidebranch, which shifts excitation of the higher-order branch-modes at higher frequencies. A decrease in sound transmission and increase in sound scattering into higher-order modes occur with excitation of the axial branch-mode. Excitation of the longitudinal branch-mode due to branch-end reflection also results in lower sound transmission of higher-order modes across the junction along the main duct.
126(2009); http://dx.doi.org/10.1121/1.3245033View Description Hide Description
Sound transmission through hollow structures found its interest in several industrial domains such as building acoustics, automotive industry, and aeronautics. However, in practice, hollow structures are often filled with porous materials to improve acoustic properties without adding an excessive mass. Recently a lot of interest arises for granular materials of low density that can be an alternative to standard absorbing materials. This paper aims to predict vibro-acoustic behavior of double panels filled with porogranular materials by using the patch-mobility method recently published. Biot’s theory is a basic tool for the description of porous material but is quite difficult to use in practice, mostly because of the solid phase characterization. The original simplified Biot’s model (fluid-fluid model) for porogranular material permitting a considerable reduction in data necessary for calculation has been recently published. The aim of the present paper is to propose a model to predict sound transmission through a double panel filled with a porogranular material. The method is an extension of a previous paper to take into account the porogranular material through fluid-fluid Biot’s model. After a global overview of the method, the case of a double panel filled with expanded polystyrene beads is studied and a comparison with measurements is realized.