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Published Articles

The Volume 16, No 3, September 2011

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Tuning of the Extended Concentric Tube Resonators

P. Choudary Chaitanaya and M. L. Munjal


It has been shown recently that the acoustic performance of the extended tube expansion chambers can be improved substantially by making the extended inlet and outlet equal to half and quarter chamber lengths, duly incorporating the end corrections due to the evanescent higher order modes that would be generated at the discontinuities. Such chambers, however, suffer from the disadvantages of high back pressure and generation of aerodynamic noise at the area discontinuities. These two disadvantages can be overcome by means of a perforated bridge between the extended inlet and extended outlet. This paper deals with design or tuning of these extended concentric tube resonators. One dimensional control volume approach is used to analyze this muffler configuration. It is validated experimentally making use of the two source-location method. It is thus shown that the inertance of holes plays a role similar to the lumped inertance generated by evanescent 3-D modes at the terminations of the quarter wave resonators in the case of the double-tuned extended tube chambers. The effect of mean flow is also investigated. The resultant transfer matrix is then used to carry out a systematic parametric study in order to arrive at empirical expressions for the correction lengths. Thus, an extended concentric tube resonator can be tuned such that the first three troughs, which characterize the corresponding simple chamber transmission loss (TL) curve, may be eliminated making use of the proposed procedure. In fact, the entire TL curve at low and medium frequencies may be substantially lifted, making the tuned extended concentric tube resonator a viable design option.

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Impact Sound Insulation and Viscoelastic Properties of Resilient Materials made from Recycled Tyre Granules

Francesco Asdrubali, Francesco D'Alessandro


The disposal of used tyres is a significant environmental problem in developed countries. The production of acoustic materials from rubber crumbs can therefore represent a valid alternative to incineration or to the disposal of used tyres into landfill. The goal of the paper is to analyze and optimize the manufacturing process of impact sound insulating materials made of recycled tyre granules mixed with binders. Several prototypes were manufactured in laboratory by means of the developed process and tested in order to evaluate their dynamic stiffness. The influence of grain size, binder concentration and density of the sample was investigated. Thanks to the measured values of the viscoelastic properties, it was possible to model the new materials and to estimate the indexes of impact sound reduction. Finally, the prototypes showing the best properties were produced in bigger size and tested in two overlapping reverberating rooms according to the standard ISO 140-8. Results confirm that the materials manufactured through the developed process have satisfactory acoustic properties, comparable to the ones of commercially available materials.

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Free Vibration Analysis of Punctured Plate by Matching Polynomials and Constraint Technique

Sasank Sekhar Hota, Mrutyunjay Rout, Payodhar Padhi


The hallmark of this paper is the adoption of the constraint technique, in combination with a subparametric, triangular-plate bending element of first-order shear deformation, to maintain uniform mesh size and shape even while dealing with cutouts of arbitrary shapes. The evolution of two cutout models in the present investigation is a distinct improvement over the existing practices of cutout analysis. The use of matching polynomials offers the scope of eliminating the hazards of locking and spurious zero energy modes, while solving problems of very thin plates. Benchmark examples, as well as the author's own problems on free vibration of rectangular plates with different shapes of cutouts, have been solved to exhibit versatility of these models. Mode shapes of plates with different shapes of cutout have also been provided.

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Control of Stick-slip Oscillations in Oil Well Drill Strings Using the Back-stepping Technique

Mohamed Zribi, Mansour Karkoub and C. C. Huang


This paper deals with the problem of suppression of stick-slip oscillations in oil well drill-strings. The back-stepping technique is used to design two control schemes for the system. The first controller is a typical back-stepping controller; the second controller is a sliding back-stepping controller. Moreover, a reduced order observer is proposed to estimate the unmeasurable states of the system. Simulation results of the two proposed control schemes with and without the observer, indicate that both control schemes work well.

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On Some Recent Applications of the Coanda Effect

Caroline Lubert


Over the last quarter century or so, the Coanda principle has become increasingly used in a wide variety of applications, including industrial, medical, maritime technology, and aerodynamics. In addition, its effect has been increasingly observed in the natural world. Devices employing this principle usually offer substantial flow deflection, and enhanced turbulence levels and entrainment compared with conventional jet flows. However, these prospective advantages are generally accompanied by other significant disadvantages such as jet flow detachment, and a considerable increase in associated noise levels. Much of the time, the reasons for this are not well understood. Consequently, in many cases, the full potential offered by the Coanda effect is yet to be completely realized. This paper discusses a variety of recent applications of the principle and describes attempts to understand some of the difficulties associated with it, particularly those related to increased acoustic radiation.

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Letter to the Editor: Critical Distance in Churches: Comments on Carvalho & Lencastre (2000)

P. Jafari Shalkouhi


The goal of this letter is to comment on the article "Catholic Churches, Sound Reinforcement Systems and RASTI" by Carvalho & Lencastre (2000). Carvalho & Lencastre could have calculated or measured critical distance in each church to know the reason of decreasing RASTI in the direct field. It is concluded that in spaces with long reverberation time the number and position of loudspeakers with regard to critical distance must be taken into consideration.

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