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Jean-Louis Guyader, INSA de Lyon (France)
E. Parizet, Renault D.R. (France)
Some experiments on populations of industrially identical structures have shown a large variation of their vibroacoustic behaviour. They indicates that small structural defects can lead to strong differences on vibration and acoustic radiation properties. This problem is presently important for automotive industry, because it leads to a non negligible percentage of cars that have a non perfect acoustic. The phenomenon was also theoretically shown on populations of coupled plates having small defects in their junction angle ; and was called hypersensitivity. The explanation of such behaviour is a very strong change in the coupling of flexural and in plane motions, when the angle of connection vary. However, the phenomenon only exist of quasi flat junctions. An experiment made on real body parts of a car will be presented to demonstrate that small geometric imperfections due to the process of assembling substructures, produce the hypersensitivity phenomenon. The experiment was based on mobility measurements. We started with simple parts then coupled together in order to build a polpulation of complex structures. We will show in the presentation, at what level of the process of conecting substructures the hypersensitivity appears. A last part of the presentation, will be devoted to a theoretical approach of non perfectly described structures. We will present how the uncertainty on the equation of motion, characterized through a residual energy, is related to the uncertainty on localization of resonance frequencies.
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TOPBarry M. Gibbs, University of Liverpool
The structure-borne sound power delivered by a vibrating machine into a supporting or connected structure is determined by both the machines activity and the dynamic characteristics at the contacts. It is a relatively simple matter to measure activity in the form of velocity at the contacts of the free source under normal operating conditions. The structural dynamics are not so easy to predict or measure and the full mobility matrix method of prediction is data and time consuming. However, visual inspection of the measured nobilities of machine bases can give engineering insights on how best to treat machines as multi-point and multi-component sources. Mobility data from a measurement survey of a range of machinery bases is described and results compared in order to confirm the generality of the point and transfer mobilities. The role of moments also is discussed and although it is demonstrated that it is much less likely that generalisations can result, the special but common case of building services machinery on concrete plantroom floors is shown to allow some simplifying assumptions.
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Yoshihiro Kikushima, Mechanical Engineering Laboratory (Japan)
Nobuo Tanaka, Mechanical Engineering Laboratory (Japan)
This paper considers the design of distributed parameter modal sensors called smart sensors, with a particular emphasis on filtering the combination of appropriately weighted vibration modes providing a specific performance index in control strategy. First, by considering a practicability of the distributed parameter smart sensors using PVDF film sensors, one-dimensional smart sensor is presented. It is found that the approach done by the one-dimensional sensors holds only the necessary condition for sensing the transformed mode. This problem is overcome by introducing multiple one-dimensional smart sensors. Moreover, the design procedure for the multiple one-dimensional smart sensors for measuring the transformed mode is established. Then, an experiment is conducted, demonstrating the validity of the smart sensors. Finally, using the smart sensors, the minimization of the total acoustic power radiated from a vibrating plate is carried out.
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Yan K. Tso, Defence Science and Technology Organisation (Australia)
Ross G. Juniper, Defence Science and Technology Organisation (Australia)
This paper describes an investigation of the noise problem in an aluminium motor vessel. The problem was due to the high noise level in the work deck area which exceeded operational health and safety (OH&S) requirements by 9 dB at cruising speed. It was identified in this paper that the deck noise was due to contributions from three sources. These sources were: (i) the radiated noise due to vibration of the deck structure, (ii) the noise transmitted from the engine room to the deck and (iii) the noise transmitted from the exhaust vent to the deck. Various control measures for reducing the deck noise are discussed and preliminary results on deck noise measurements of the modified vessel are presented
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Ben Cazzolato, University of Adelaide (Australia)
Colin H. Hansen, The University of Adelaide (Australia)
The problem of actively minimising the transmission of harmonic sound through a curved panel into a contiguous cavity using structural sensors is investigated both numerically and experimentally. It is well known that a control system that simply aims to minimise the structural vibration does not necessarily lead to a reduction in sound transmission. However, by considering the dynamics of the coupled system, it is possible to derive an orthonormal set of structural "radiation modes" which are orthogonal with respect to their contribution to the acoustic potential energy of the internal coupled acoustic space. Minimisation of the amplitudes of these "radiation modes" is guaranteed to result in a reduction of the interior potential energy, in contrast to minimising the normal structural modes. Sensing of the orthogonal sets of "radiation modes" is accomplished by using "smart sensors" made from either shaped PVDF film or a linear combination of accelerometers, adjusted to sense a particular radiation mode.
sv970349.pdf (Scanned) TOPRoger J. Pinnington, ISVR, University of Southampton
The equations of motion of a fluid-filled tube moving with axisymmetric motion are described. This leads to a dispersion plot describing the frequency dependence of four wavenumbers. One wave is predominantly controlled by the fluid and wall stretching, the others by wall motion. Homogeneous rubber tubes were found to soften with increasing pressure while braided rubber tubes stiffened significantly with increasing pressure.
sv970190.pdf (Scanned) TOPJie Pan, The University of Western Australia (Australia)
The prediction of resonance frequencies and mode shapes of distributed structures requires high convergence rate and accuracy. When approximated methods, such as the Rayleigh-Ritz method, are used, these requirements are largely dependent upon the base functions used to express the mode shape functions. This paper shows that while base functions selected to satisfy all the boundary conditions of the structure, their linear independency may be reduced. This will results in ill conditioned mass matrix in the generalised eigenequations (large matrix condition number) and prediction error. On the other hand, the base functions with good linear independent property may not satisfy part of the boundary conditions. This paper propose the use of adjustable base functions to allow the selection of the base functions capable of satisfying the all boundary conditions and keeping the condition number of the mass matrices sufficiently small. As a result, the two requirements can be simultaneously satisfied.
sv970184.pdf (Scanned) TOPR.A. Novak, Ingemansson Technology AB (Sweden)
A residential building with high sound insulation was recently built in Stockholm, This building was designed to have a sound insulation between the apartments, both horizontally and vertically, that was substantially higher then normal buildings in Sweden. Since the building costs are high this new building should at the same time not be more expensive to build. In order to achieve this a special type of supporting structure was used. In connection to this project some theoretical and experimental studies were made, One of these studies concerned the sound insulation of light-weight double walls, which were used between the apartments, and another concerned the radiation factorof finite plates. In this paper,an evaluation of the influence of the plate size on the radiation factor and the sound reduction index is presented. The radiation factor is obtained from the Fourier transform of the plate velocity over the excited area of the plate and it is presented for both rectangular and square plates of different sizes.
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Jean-Louis Guyader, INSA de Lyon (France)
Thierry Loyau, INRS (France)
The prediction of noise radiated from machinery is a very difficult problem because of the complexity of the vibration fields producing noise. The classical Green's function formulation solve the problem using pure tone signals wich necessitate a very good knowledge of vibrations, including modulus and phase, this is quite impossible in machinery due to structural complexity. The necessity of finding appropriate methods to predict radiation from machinery is then obvious. Our idea is to predict the acoustical behaviour using a quantity not so sensitive as the pure tone pressure radiated. As in S.E.A. we will introduce frequency band average quatratic quantity, in order to get a method that reduces the information necessary for calculations and also the computing time. The theory for a simply supported baffled plate was previously presented in [1], [2]. The main assumption was dicussed and prediction compared with classical calculations and experimental data. The present paper extend the method to the case of the hood of a machinery internally excited. Comparaison of theoretical and experimental data on a circular saw will be shown in order to see the limitation of the method when applied to non plane structures.
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V.U. Kirpichnikov, Krylov Shipbuilding Research Institute (Russia)
A.A. Juravleva, Krylov Shipbuilding Research Institute (Russia)
Lyudmila Ph. Drozdova, Baltic State Technical University (Russia)
The high-frequency forced vibrations of a homogeneous plate that contacts with acoustic media are investigated. The roots values of the dispersion equation of the plate bending vibration with account for the shear and rotary inertia of it cross section and for internal losses were analyzed. It is showed the frequency range where this equation has not a single but three pairs of real roots appears just when losses are equal to zero. The waves caused by the plate - media cooperation dont disappear but transmit along the plate with decreasing that is proportional to the loss value when the internal losses are counted. A physical meaning of wave transforming at some frequency ranges when the fast decreasing waves transform to the nonuniform waves is explained. The process of the acoustic pressure forming near the plate by elastic waves is investigated at the case when the pressure is a sum of the steepest path integral and the line integral around the dispersion equation poles that are at the physical Riman sheet. The four areas of the main influence of a continuos spectra and a discrete wave spectra are established.
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V.U. Kirpichnikov, Krylov Shipbuilding Research Institute (Russia)
V.V. Savenko, Krylov Shipbuilding Research Institute (Russia)
Lyudmila Ph. Drozdova, Baltic State Technical University (Russia)
An analytical solution of sound radiation of an infinite homogeneous plate is derived. The plate is in contact with acoustic media by a single surface and is excited by a point force directed at arbitrary angle to it plane. An physical analysis of roots of a dispersion 14-ordered equation of jointed bending and longitudinal-shear plate vibrations is executed. It is estimated that correlation of this type vibrations is caused by an asymmetric reaction of media. A physical meaning of the six components of the farfield sound pressure is presented. The dependencies of this components levels from angles of a force inclination and of a direction to a view point are analyzed for a case of a metal plate vibrations that is contacted with water surface. The differences of the components levels of sound pressure at frequencies that are lower and above the coincidence frequency are showed.
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A T Moorhouse, University of Liverpool
J.M. Mondot, Centre d'Entreprise et d'Innovation (France)
Barry M. Gibbs, University of Liverpool
A source descriptor for structure-borne sound sources is presented which is similar to airborne sound power. It has units of power and is valid for multiple point contact and for moment as well as force excitation. The power delivered when the source is installed is a fraction of this source descriptor and is determined by the degree of mobility matching between source and receiver. Results are presented for a domestic central heating pump and centrifugal fans on various receiver structures.
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N.S. Feng, University of New South Wales (Australia)
E.J. Hahn, The University of New South Wales (Australia)
In analysing the vibration behaviour of turbomachinery, a problem exists in modelling foundation-pedestal support systems whose natural frequencies are within or close to the operating speed range. Often, a finite element model of the foundation-pedestal support is unavailable and it is impractical to determine its modal properties experimentally. As a first approximation, one could regard the foundation as rigid and replace the pedestals by equivalent supports having mass and stiffness properties which correctly reflect the system unbalance response over the operating range. This paper outlines a method for identifying such pedestal properties for general rotating machinery using measurements of the motion of the pedestals and of the rotor. The proposed technique assumes a knowledge of the stiffness and damping properties of the support bearings (which may be hydrodynamic) but does not require a knowledge of the rotor, nor of the unbalance excitation; but merely that it be sufficient to provide measurable motion data. Numerical experiments show that excellent identification of pedestals is feasible even with the two digit measurement accuracies attainable with field instrumentation, suggesting applicability to practical turbomachinery where there is significant vibration at the pedestals though relatively insignificant vibration in the foundation itself.
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Katrien Wyckaert, LMS International (Belgium)
G. Toniato, CRF (Italy)
K.Q. Xu, KU Leuven (Belgium)
Thispaper describes the process of obtaining vibratory and acoustical response predictions within the car body, due to engine excitation. The approach that is followed is to model the high modal density vibratory and acoustical behaviour of the car body by a set of measured frequency response functions, while the engine carrying crossmember is represented by a finite element model. The coupling interaction of the crossmember with the car body is calculated based on FRF based substructuring techniques. The modeling process further includes experimentally derived engine excitation forces. Coupling predictions of crossmember and car body are carried out and design changes evaluated.
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Yuejun Li, Australian Defence Force Academy (Australia)
Hugh M. Williamson, Australian Defence Force Academy (Australia)
Jue Dai, Australian Defence Force Academy (Australia)
Christopher J. Norwood, Defence Science and Technology Organisation (Australia)
Finite element techniques were investigated for determination of surface mobility of an infinite plate excited over a square contact area by a uniform force distribution using the effective point mobility concept. Ordinary point and transfer nobilities were obtained using harmonic response analysis in FEM. Then surface nobilities were calculated for different sizes of square contact area, based upon these ordinary mobilities. The comparison between the numerical FEM results and theoretical prediction shows that the surface mobility obtained by both methods generaIly decreases with increasing area of the square and that both results have dips at intervals in Helmholtz number which are approximate multiples of pi.
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Jue Dai, Australian Defence Force Academy (Australia)
Hugh M. Williamson, Australian Defence Force Academy (Australia)
Yuejun Li, Australian Defence Force Academy (Australia)
In the study of structure-borne sound and vibration isolation, mobility is used to reflect the characteristics of power transmission of a supporting structure. When the contact area between an exciting machine and its supporting structure is larger than the governing wave length, surface mobility is applied. Surface mobility is influenced by many factors such as the pattern and frequency of excitation and the dimensions and shape of the contact area. By means of analyses of these factors, the effects of power transmision through the contact area can be obtained under different conditions. In this paper, the effective point mobility and surface mobility over rectangular contact areas, for the assumption of a uniform conphase velocity distribution, are studied. An infinite homogeneous thin plate is chosen as a supporting structure. The resulting force distribution and effective point mobility within the contact area are calculated. Using effective point mobility, surface mobility is also calculated. By this method, not only is the total power transmission calculated from an excitor to the thin platelike support, but also the detailed pattern of power transmission within the contact areas can be predicted.
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C. Karra, UPRESA CNRS (France)
Mabrouk Ben Tahar, Universite de Technologie de Compiegne (France)
A total variational formulation of vibro-acoustic interaction between vibrating membrane and a visco-thermal fluid layer was investigated. This formulation combines a new variational formulation by integral equations of the fluid, which taking into account of acoustic and entropic waves coupling, with a classical variational formulation of the membrane. This formulation has been implemented numerically for the problems with revolution geometry. The obtained numerical results are compared to analytical ones developed for three models : perfect fluid model, visco- thermal fluid model which coupling acoustic and entropic waves (developed in this work) and visco-thermal fluid model which coupling acoustic, entropic and shear waves. These comparisons showed the validity of our formulation proposed in this work and the importance of the effects of entropic and shear waves on the damping of modes of coupling fluid-structure system and the decrease of their natural frequencies in the case of the micro-cavities.
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