This thesis consists of four papers concerning the audibledynamic stiffness of a vibration isolator; includingmeasurements and theoretical modeling. The study focus isrubber isolators, being the mo ...

Resilient devices are commonly used in lightweight structures to decrease sound transmission in a broad frequency band. Applications of such devices may be found in e.g. resilient mounted ceilings in aeroplanes, ships and buildings. A measurement method to characterise the frequency dependency of the transfer stiffness and the input stiffness of the resilient device is presented. The mechanical characteristics of the measurement method are investigated. In addition, some resilient devices used in buildings are analysed with respect to acoustic properties. Parameters such as static load and mountings for the devices are considered and handled by means of statistical analysis.

Structure-borne sound properties of vibration isolators

Measurement of the Acoustic Properties of Resilient, Statically Tensile Loaded Devices in Lightweight Structures

An analytical model for wave propagation in a three-layered sandwich structure with different boundary conditions was developed. The sandwich material that was treated, consisted of a thick lightweight core, and two thin and stiff laminates. Both laminates and core were assumed to be homogeneous and isotropic. The governing equations were derived partly by applying the general field equations (2-D theory) to the core and, partly by utilizing the thin plate/ beam theory (1-D) to the laminates. The dispersion equation for the structure was derived and solved using the winding-integral technique. The boundary conditions across the thickness of the structure were then satisfied at a finite number of the points. The calculated natural frequencies were shown to converge with respect to the number of collocation points and included modes. Comparisons with alternative models, i.e. the Euler-Bernoulli, the Timoshenko, and a complete layered media model, were made. The latter comparison, concerned only the eigenvalues and eigenfunctions for the infinite structure. The first two models are shown to diverge, to a great extent, with the present theory while a good agreement is obtained between modal wave numbers calculated by the present theory and the complete layered media model, respectively. Moreover, measurements, using modal analysis, were carried out on a sandwich beam with two different boundary conditions, i.e., clamped-clamped and free-free. The results of the analytical model, as well as measurements, suggest that the boundary conditions have a considerable effect on the apparent bending stiffness of the sandwich beams. In other words, there is a great discrepancy between the natural frequencies for the sandwich beam with fixed and free ends, respectively. The agreement between the predicted and the measured results is found to be quite satisfactory. Furthermore, a restricted parameter study was carried out. The results of the parameter study indicate that the stiffness relation between laminates and core is of great importance as regards the dynamic behavior of the sandwich structure.

background or methods or result

/pdf/wave-propagation-in-sandwich-structure-5990xwye8l.pdf

Wave propagation in sandwich structure

The tendency to design lighter structures and the increasing demands of customers for comfort, increases requirements of the resilient mounting of machinery (e.g. engines) in vehicles. The conventional passive vibration isolators might not be sufficient to reduce interior noise levels. Additionally, active isolation techniques can be used to obtain a better isolation performance. This research deals with the application of a combination of passive and active isolation (so-called hybrid isolation) for resilient mounting of machinery. Methods are introduced to model this kind of hybrid isolation system and in particular to investigate the different actuator configurations and sensor strategies of the active isolation part.

methods

/pdf/a-modeling-approach-to-hybrid-isolation-of-structure-borne-3gng46956o.pdf

A modeling approach to hybrid isolation of structure-borne sound

An important cause for interior noise in vehicles is structure-borne sound from the engine. The vibrations of the source (engine) are transmitted to the receiver structure (the vehicle) causing interior noise in the vehicle. For this reason the engine is supported by rubber isolators for passive isolation in especially the high-frequency region. To make a good judgment of the characteristics of a vibration isolator in the design process, it is useful to use numerical models. In this paper a cylindrical vibration isolator is modelled numerically with the Finite Element package ABAQUS. The investigation is split in two parts: first a nonlinear analysis is performed for different pre-deformations of the mount. After that, a linear harmonic analysis is superimposed on the pre-deformed isolator. The structure-borne sound is transmitted by the isolator by six degrees of freedom, so the harmonic analysis must be performed for different excitations. With the results the behavior of the isolator can be represented by dynamic stiffness matrices as function of the frequency and predeformation. These matrices can be used to model the passive isolation components as part of numerical models of hybrid isolation systems. These isolation systems describe a combination of active and passive isolation to reduce the structure-borne sound transmission to receiver structures.

background

/pdf/numerical-modelling-of-rubber-vibration-isolators-1e7djm3wmi.pdf

Numerical modelling of rubber vibration isolators

/pdf/procedure-setting-to-determine-the-dynamic-transfer-3e87p4bk6k.pdf

Structure-borne sound properties of vibration isolators

Citations

Measurement of the Acoustic Properties of Resilient, Statically Tensile Loaded Devices in Lightweight Structures

Wave propagation in sandwich structure

A modeling approach to hybrid isolation of structure-borne sound

Numerical modelling of rubber vibration isolators

Procedure setting to determine the Dynamic transfer stiffness of a resilient mounting element in a low frequency range