Vibroacoustic Performance Research Articles

Dynamic Performance Analysis of a Light Van Body-In-White Structure

Requirement for low emissions and better vehicle performance has led to the demand for lightweight vehicle structures. Lighter gauge panels are being used to construct the body-in-white (BIW) monocoque structure, which is the basic component of the vehicle body. Since lighter gauge panels tend to generate more vibration and interior noise, it is necessary to optimize the dynamic performance of lightweight vehicle structures in order to achieve acceptable levels of vibro-acoustic performance. The design of a light commercial van structure has evolved over the years and through a lightweighting exercise the current BIW is about 10 per cent lighter than the previous BIW even though the volume capacity was increased by 15 per cent and the load-carrying capacity by 18 per cent. In this study, the dynamic performance of the current production light van BIW structure is investigated. Its performance is assessed against the structural dynamic performance standards which have been established for this class of structures. While the input mobility performance was found to exceed the standards easily, the modal mobility performance was found to be unsatisfactory owing to the occurrence of local panel resonant modes in the two side panels. A finite element model of the structure was developed to study the effect of adding stringers to the roof and side panels to eliminate some of the local panel modes and thus to improve the dynamic performance of the structure.

Vibroacoustic characterization of a commercial airliner bulkhead.

The commercial airliner bulkhead design candidate of concern is an internal divider wall consisting of an aluminum forward panel and liner aft panel sandwiching an array of closely spaced interstitial aluminum I-beams. The main purpose of the bulkhead, which separates the cargo bay from the occupied area, is to protect the crew from being compromised by cargo during a hard landing. Occupied area noise requirements also affect the bulkhead design as it is a primary path of noise transmission from the cargo bay. Vibroacoustic performance of the bulkhead analyzed with statistical energy analysis is supported by coupon testing of a representative bulkhead test section. Fibrous acoustic treatments were also evaluated experimentally for noise reduction potential. The experimental methods, results, and component level modeling complement each other by demonstrating a vibroacoustically appropriate bulkhead design and good bulkhead modeling practices.

Sound Transmission Through Simply Supported Finite Double-Panel Partitions With Enclosed Air Cavity

Abstract The vibro-acoustic performance of a rectangular double-panel partition with enclosed air cavity and simply mounted on an infinite acoustic rigid baffle is investigated analytically. The sound velocity potential method rather than the commonly used cavity modal function method is employed, which possesses good expandability and has significant implications for further vibro-acoustic investigations. The simply supported boundary condition is accounted for by using the method of modal function and the double Fourier series solutions are obtained to characterize the vibro-acoustic behaviors of the structure. The results for sound transmission loss, panel vibration level, and sound pressure level are presented to explore the physical mechanisms of sound energy penetration across the finite double-panel partition. Specifically, focus is placed on the influence of several key system parameters on sound transmission including the thickness of air cavity, structural dimensions, and the elevation angle and azimuth angle of the incidence sound. Further extensions of the sound velocity potential method to typical framed double-panel structures are also proposed.

Vibro-Acoustic Damping Simulation of Two Laminated Glass Panels Coupled to Viscothermal Fluid Layer

This paper numerically investigates the vibro-acoustic behavior of two thin film-laminated glass panels coupled to a viscothermal fluid layer. For this purpose, the film-laminate finite element model is coupled to the viscothermal fluid model, introducing an original configuration of a laminated double-glazing system. The two-layer laminated panel finite element is developed using Kirchhoff’s plate theory and by taking into account the shear stress of a confined ultra-thin film at the skin’s interface. Then, the developed equations of the laminated glass panel are based on the theory that introduces a particular behavior rule for an ultra-thin film, which includes effects of stiffness, without taking into account the film thickness. After that, the dynamic model is established in an appropriate form, including effects of viscosity and thermal conductivity of fluid and taking into account the acousto-elastic coupling. Thus, the developed finite element model is defined by a frequency-dependent symmetrical matrix system that is obtained after discretization and minimization of the coupled system energy function. Hence, a modal iterative approach is derived in order to determine the eigenmodes and vibro-acoustic responses of the film-laminated double-glazing system. The validation of the acousto-elastic model is accomplished by comparing the coupled system results against data obtained from the literature. Subsequently, several vibro-acoustic indicators, such as film-laminate strain energies and the transmission loss of the coupled system, for a specified design of a laminated double-glazing system excited by uniform pressure are presented and discussed. The numerical results show the importance of both thin film-lamination and viscothermal fluid effects on double-glazing vibro-acoustic performance.

Vibroacoustic behavior of clamp mounted double-panel partition with enclosure air cavity

A theoretical study on the vibroacoustic performance of a rectangular double-panel partition clamp mounted in an infinite acoustic rigid baffle is presented. With the clamped boundary condition taken into account by the method of modal function, a double Fourier series solution to the dynamic response of the structure is obtained by employing the weighted residual method (i.e., the Galerkin method). The double series solution can be considered as the exact solution of the problem, as the structural and acoustic-structural coupling effects are fully accounted for and the solution converges numerically. The accuracy of the theoretical predictions is checked against existing experimental data, with good agreement achieved. The influence of several key parameters on the sound isolation capability of the double-panel configuration is then systematically studied, including panel dimensions, thickness of air cavity, elevation angle, and azimuth angle of incidence sound. The present method is suitable for double-panel systems of finite or infinite extent and is applicable for both low- and high-frequency ranges. With these merits, the proposed method compares favorably with a number of other approaches, e.g., finite element method, boundary element method, and statistical energy analysis method.

Spectral analysis methods for vehicle interior vibro-acoustics identification

Noise has various effects on comfort, performance and health of human. Sound are analysed by human brain based on the frequencies and amplitudes. In a dynamic system, transmission of sound and vibrations depend on frequency and direction of the input motion and characteristics of the output. It is imperative that automotive manufacturers invest a lot of effort and money to improve and enhance the vibro-acoustics performance of their products. The enhancement effort may be very difficult and time-consuming if one relies only on ‘trial and error’ method without prior knowledge about the sources itself. Complex noise inside a vehicle cabin originated from various sources and travel through many pathways. First stage of sound quality refinement is to find the source. It is vital for automotive engineers to identify the dominant noise sources such as engine noise, exhaust noise and noise due to vibration transmission inside of vehicle. The purpose of this paper is to find the vibro-acoustical sources of noise in a passenger vehicle compartment. The implementation of spectral analysis method is much faster than the ‘trial and error’ methods in which, parts should be separated to measure the transfer functions. Also by using spectral analysis method, signals can be recorded in real operational conditions which conduce to more consistent results. A multi-channel analyser is utilised to measure and record the vibro-acoustical signals. Computational algorithms are also employed to identify contribution of various sources towards the measured interior signal. These achievements can be utilised to detect, control and optimise interior noise performance of road transport vehicles.

Modal acoustic transfer vector approach in a FEM–BEM vibro-acoustic analysis

The aim of the present work is to set up an integrated approach for an automobile vibro-acoustic analysis, useful to assess, visualise and compare vibro-acoustic performance to pre-determined design targets, while identifying and quantifying the forces and sound sources responsible for the current behaviour. Such design approach, based on experimental and numerical procedures, enables the prediction of noise emissions and the correlation with the structural vibration source. Vibro-acoustic prediction in the low- to mid-frequency range is generally performed through finite element method (FEM) or boundary element method (BEM) but in this work a combined usage of the two methodologies is adopted: FEM is used for the structural dynamics and BEM for the acoustic problem resolution. The BE methodology adopted is based on an indirect formulation and on a variational solution scheme. The adopted FEM–BEM approach takes advantage of the Modal Acoustic Transfer Vector algorithm that is particularly useful when big problems are to be analysed. The comparison between numerical and experimental results enables an assessment of the accuracy level.

Theoretical, numerical, and experimental modal analysis of a single-winding gradient coil insert cylinder

The objective of this paper is to find the relatively low-frequency (200-2,000 Hz) mode shapes of a single-winding gradient coil cylinder with intermediate wall thickness. The dynamic behavior of a gradient coil cylinder plays a crucial role in determining and controlling the vibroacoustic performance of magnetic resonance imaging (MRI) scanners. Modal analyses of the gradient coil cylinder were carried out under different boundary conditions to obtain the various mode shapes. Theoretical modes, predicted by using modified Love's governing equations, and numerical modes simulated using a finite element method show close agreement with experimental modal results and reveal the mode shapes for both free-end and fixed-end boundary conditions. These results were further compared to in situ measurements of the mode shapes of the gradient coil cylinder insert during scanning in a 4 Tesla MRI. The general agreement among the analytical, numerical, and experimental mode shapes indicates that a linear combination of basic beam vibration and ring vibration patterns occupy the dynamic vibration modes in the low frequency range. The in situ vibration measurements show that the forcing function developed by the distributed Lorentz forces on the surface of the single-winding gradient coil results in predominantly beam-type bending mode patterns in the low frequency range.

Structural and Acoustic Behavior of Chiral Truss-Core Beams

This paper analyzes the structural and acoustic behavior of sandwich beams with a chiral truss-core. This particular core configuration is considered to exploit some of the unique properties of the chiral geometry and to explore their potential benefits in terms of sound-transmission reduction and vibration isolation. The chiral core is composed of circular elements or nodes, joined by ligaments or ribs. The arrangement of nodes and ribs is such that chiral assemblies exhibit in-plane negative Poisson’s ratio behavior as well as unique deformation patterns. The vibroacoustic performance of the considered beams is evaluated through a numerical model, formulated by employing dynamic shape functions derived directly from the distributed parameter model of beam elements. This formulation allows an accurate evaluation of the dynamic response of the considered structures at high frequencies with a limited number of elements. Furthermore, such a numerical model can be coupled with a Fourier-transform-based analysis of the sound radiated by the structure in a surrounding fluid medium. The structural-acoustic behavior of the proposed beams is investigated in terms of kinetic energy of the constraining layers and sound pressure levels corresponding to an incident pressure wave. A sensitivity study investigates the influence of core configuration and geometry on the beam performance. Moreover, the performance of the chiral core is compared to that of cores with “square” and hexagonal topologies. The results demonstrate the design flexibility offered by the proposed core design, whose configuration is defined by a number of independent parameters that can be modified and optimized to enhance the structural-acoustic performance of the beam.

Vibroacoustic performance of composite honeycomb structures

This paper is concerned with the prediction of vibroacoustic performance of the honeycomb sandwich panels in a close-fitting enclosure. The acoustical transfer function of this panel in the close-fitting enclosure are presented. The noise reduction performance of the honeycomb sandwich panels is investigated through a set of experiments verifying the potential and usefulness of the honeycomb panel in practice. The results have confirmed that the absorption at the mass-spring-mass resonance in the case of a honeycomb panel (comparatively stiff) with a thin plate is more efficient with minimum weight. Adding absorption minimizes the effect of the 'mass-air-mass' resonance that usually impairs the low-frequency transmission loss, and offers good performance at higher frequencies. The potential of composite honeycomb sandwich panels for reducing low frequency noise (<500 Hz) is demonstrated.

Vibroacoustic Optimization of Stiffening Ribs and Damping Material Distribution on Sheet Metal Parts

Vehicle noise and vibration levels are basic parameters in passenger comfort. Both static and dynamic stiffness of sheet metal parts is commonly increased by means of stiffening ribs. Vibrations are also reduced by adding a layer of damping material on the floor, the roof, the firewall and other parts of the vehicle. In common practice the panels to be treated are ribbed according to criteria based on the designer’s experience, rather than on well defined design procedures and are uniformly covered by a layer of damping material. However, these are not efficient design solutions, especially with regard to the effectiveness of vibration reduction and to weight containment. In this paper a novel approach to achieve an optimal distribution of stiffening ribs and damping material will be presented. The proposed method is based on a Genetic Algorithm (G.A.) procedure which takes into account both the vibroacoustic performance and the weight and cost reduction. A simple case study will be illustrated to demonstrate the capabilities of the developed procedure.

Une stratégie de modélisation conceptuelle pour la prise en compte de performances vibro-acoustiques en préconception d'un berceau automobile

The assessment of mechanical performances, in the automotive engineering domain, is mainly at present time the result of late finite element analysis processes (FEA) which remain computationally expensive, limiting their use to the analysis of a limited number of design alternatives. But, in the conceptual design stage, the quality depends on the comprehension and on the exploration of the design space. This paper describes a strategy for a construction of conceptual models. It consists in building a first simplified model and then in resulting in a metamodel, i.e., a mathematical function that rapidly assesses a design performance. This strategy has been applied to assess acoustical performances of an automotive subframe. Now, this is possible to take vibro-acoustic performances into account since the determining stage of envelop volumes allocation for sub-systems, a stage that is necessary in concurrent engineering for the automotive architecture deployment. In this example, we show that designers have obtained useful information from the exploitation of this conceptual model.

Wave beaming effects in two-dimensional cellular structures

Cellular structures like honeycombs or reticulated micro-frames are widely used insandwich construction because of their superior structural static and dynamicproperties. The aim of this study is to evaluate the dynamic behavior oftwo-dimensional cellular structures, with the focus on the effect of the geometry ofunit cells on the dynamics of the propagation of elastic waves withinthe structure. The characteristics of wave propagation for the consideredclass of cellular solids are analyzed through the finite element model ofthe unit cell and the application of the theory of periodic structures.This combined analysis yields the phase constant surfaces, which definethe directions of waves propagating in the plane of the structure for theassigned frequency values. The analysis of iso-frequency contour linesin the phase constant surfaces allows the prediction of the location andextension of angular ranges, and therefore regions within the structures wherewaves do not propagate. The performance of honeycomb grids of regularhexagonal topology is compared with that of grids of various geometries,with the emphasis on configurations featuring a negative Poisson’s ratiobehavior. The harmonic response of the considered structures at specifiedfrequencies confirms the predictions from the analysis of the phase constantsurfaces and demonstrates the strongly spatially-dependent characteristicsof periodic cellular structures. The numerical results presented indicatethe potentials of the phase constant surfaces as tools for the evaluationof the wave propagation characteristics of this class of two-dimensionalperiodic structures. Optimal design configurations can be identified inorder to achieve the desired transmissibility levels in specified directionsand to obtain efficient vibration isolation capabilities. The findings fromthe presented investigations and the described analysis methodology willprovide invaluable guidelines for the prototyping of future concepts ofhoneycombs or cellular structures with enhanced vibro-acoustics performance.

The acoustic emission of a distributed mode loudspeaker near a porous layer.

Experimental and theoretical modeling of the vibro-acoustic performance of a distributed mode loudspeaker (DML) suggest that their acoustic emission can be significantly affected by the presence of a porous layer. The amplitude of the surface velocity of the panel and the acoustic pressure on the porous surface are reduced largely in the vicinity of structural resonances due to the additional radiation damping and visco-thermal absorption phenomenon in the porous layer. The experimental results suggest that a porous layer between a rigid base and a DML panel can considerably alter its acoustic emission in the near field and in the far field. This is illustrated by a reduction in the level of fluctuations in the emitted acoustic pressure spectra. These fluctuations are normally associated with the interference between the sound emitted by the front surface of the speaker and that emitted from the back. Another contribution comes from the pronounced structural resonances in the surface velocity spectrum. The results of this work suggest that the acoustic boundary conditions near a DML can be modified by the porous layer so that a desired acoustic output can be attained.

Prediction of the vibro-acoustic transmission loss of planar hose-pipe systems

Vibro-acoustic energy travels through hose walls as longitudinal waves and flexural waves, apart from the sound waves through the fluid medium inside. Longitudinal waves in the hose wall are coupled to the sound waves inside by means of the hose-wall Poisson’s ratio. Both in turn get coupled to bending or flexural waves because of the energy transfer or interaction at the bends. For any of these three types of waves incident on one end of a hose, waves of all the three types may be transmitted on the other end because of their dynamical coupling with one another. Therefore, in the present paper, expressions have been derived for the 3×3 transmission loss matrix for a two-dimensional or planar piping system in terms of elements of the overall 8×8 transfer matrix of the system. These expressions have then been used in a comprehensive computer program to evaluate the vibro-acoustic performance of hoses, with particular application to the automotive climate control systems with gaseous as well as liquid media. Finally, parametric studies have been made that have led to some general design guidelines.