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Published Articles


The Volume 13, No 3, September 2008




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On the acoustic matching of straight, curved and twisted tubes

Luis M.B.C. Campos and Pedro G.T.A. Serrao


https://doi.org/10.20855/ijav.2008.13.3227


The acoustic matching of straight, curved and twisted rigid-walled tubes with the same constant ectangular crosssection is considered. The starting point is the acoustic pressure in an helicoidal tube of constant rectangular crosssection, which is specified exactly by Bessel functions of non-integer order. By expanding the Bessel functions in Fourier Series, the acoustic fields can be readily matched at the junction of the straight, curved or twisted tubes. The matching conditions are the continuity of acoustic pressure and particle displacement. In the case of matching of two straight tubes there is a reflection and a transmission coefficient, which depends only on the ratio of free wave impedances in the incidence and transmission media; in the case of matching involving curved (bent or twisted) tubes, e.g. straight to twisted, twisted to straight or twisted to twisted, there is one reflection coefficient and a series of partial transmission coefficients into each mode. This is illustrated by plotting the partial ransmission coefficients for all modes with no less than 1% of the total energy, for the acoustic matching of a straight and a twisted tube, with various cross-sections, and radius of curvature of flexion and torsion.


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A comparison and review of theories of the acoustics of porous materials

Michael R. F. Kidner and Colin H. Hansen


https://doi.org/10.20855/ijav.2008.13.3228


This article reviews the research on acoustic waves in porous media. Particular emphasis is placed on the relation- ship between the full Biot?Allard model and the simpler approximations presented by Zwikker and Kosten, Morse and Ingard, and others. A comparison of several models used to predict the absorption characteristics of porous materials is presented.


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Application of microperforated panel absorbers to room interior surfaces

Kimihiro Sakagami and Masayuki Morimoto


https://doi.org/10.20855/ijav.2008.13.3229


Microperforated panel (MPP) absorbers are promising as a basis for the next generation of sound absorbing mate-rials. A microperforated panel absorber is backed by an air cavity with a rigid back wall, and its typical use is for a sound-absorbing ceiling. However, MPPs have some limitations and disadvantages. MPPs are typically made of a thin metal or plastic panel. Such a thin panel is often not suitable for an interior finish because thin limp panels do not have enough strength. In particular, an interior finish of room walls requires appropriate strength. This problem prevents practical applications of MPPs as an interior finish of room walls. In order to solve this problem, the following treatments were considered and their acoustical effects are discussed: (1) the thickening of an MPP to make it firmer, (2) the use of an elastic support to stiffen an MPP, and (3) attaching a honeycomb structure to
MPPs to stiffen the construction.


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Ultrasonic Studies on High Pressure Transmitting Fluids used for Hydrostatic Pressure Measurements up to 1.0 GPa

Sanjay Yadav, Manju Singh, Om Prakash, A.K. Bandyopadhyay and V. R. Singh


https://doi.org/10.20855/ijav.2008.13.3229.497


In the present investigation, an attempt was made to study the ultrasonic propagation parameters viz. ultrasonic velocity (V ), attenuation (A), acoustic impedance (Z), bulk modulus (k0), adiabatic compressibility, (b) and relaxation time (l ) of some of the high pressure transmitting fluids (PTFs) used for hydrostatic pressure measurement up to 1.0 GPa, using an ultrasonic pulse echo transmission technique at 2.5 MHz. All the ultrasonic measurements are performed at room temperature of (21 +- 1:5)C within the measurement uncertainty of 0:5%. Efforts are made to correlate the results with the suitability of these fluids (obtained from the characterization of a controlled clearance piston gauge (CCPG)) to generate pressure up to 300 MPa using J-13, 680 MPa using di(2-ethylhexyl) sebacate (commercially known as BIS), and 1000 MPa using a mixture of 5% white gasoline (G), 10% J-13 and 85% BIS (GJBIS). Although, we have not studied the ultrasonic parameters as a function of applied pressure, yet interestingly, the adiabatic compressibility (b) is found to decrease with an increase in suitability of pressure range except for (G), which has minimum viscosity as 0.6897 mm2/s in comparison to other fluids, which have viscosity in the order of 10.0 mm2/s or more. Ultrasonic measurements were also performed to investigate the effect of temperature on ultrasonic velocity in BIS, the most common pressure transmitting fluid (PTF) used all over the world and also in the newly developed mixture of GJBIS. The studies would open new lines to measure the ultrasonic velocity as a function of applied pressure and temperature in the future.


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