The transmission loss of a partition depends on the sound fields in the adjoining rooms, which are often taken to be diffuse. A diffuse sound field is a random field, representing a conceptual ensemble of rooms with the same volume and reverberation time, but otherwise any possible arrangement of boundaries and small objects that have a wave scattering effect. Uncertainty due to random wave scattering is therefore inherently present in diffuse transmission loss values. In this work, closed-form expressions are presented for quantifying this uncertainty, such that it becomes possible to estimate by how much the transmission loss of a particular room-wall-room system may deviate from the nominal, ensemble average value which corresponds to omnidirectional incidence and halfspace radiation. The first expression is exact yet its evaluation requires knowledge of the dynamic stiffness of the partitioning structure and of the radiation impedances of the surrounding fluids. The second expression is approximate and depends on energetic quantities only, so it is also applicable in an experimental setting. The expressions are validated in a Monte-Carlo simulation study involving a single wall.
In the framework of developing a non-destructive damage identification technique, vibration monitoring is a useful evaluation tool that relies on the fact that the occurrence of damage in a structural system leads to changes in its dynamic properties. The damage identification techniques are based on the observed shifts in eigenfrequencies and modeshapes and relate the dynamic characteristics to a damage pattern of the structure. The presented technique makes use of the calculation of modal bending moments and curvatures to derive the bending stiffness at each location. Damage identification results are compared with results from a classical sensitivity based updating technique. The basic assumption in both techniques is that damage can be directly related to a decrease of stiffness in the structure. Damage assessment techniques are validated on the progressively damaged prestressed concrete bridge Z24 in Switzerland, tested in the framework of the BRITE-EURAM project SIMCES. A series of full modal surveys are carried out on the bridge before and after applying a number of damage scenarios.
When using the analysis of vibration measurements as a tool for health monitoring of bridges, the problem arises of separating abnormal changes from normal changes in the dynamic behaviour. Normal changes are caused by varying environmental conditions such as humidity, wind and most important, temperature. The temperature may have an impact on the boundary conditions and the material properties. Abnormal changes on the other hand are caused by a loss of stiffness somewhere along the bridge. It is clear that the normal changes should not raise an alarm in the monitoring system (i.e. a false positive), whereas the abnormal changes may be critical for the structure's safety. In the frame of the European SIMCES-project, the Z24-Bridge in Switzerland was monitored during almost one year before it was artificially damaged. Black-box models are determined from the healthy-bridge data. These models describe the variations of eigenfrequencies as a function of temperature. New data are compared with the models. If an eigenfrequency exceeds certain confidence intervals of the model, there is probably another cause than the temperature that drives the eigenfrequency variations, for instance damage. Copyright © 2001 John Wiley & Sons, Ltd.
No abstract is provided for this article.
Finite element (FE) model updating technique belongs to the class of inverse problems in classical mechanics. According to the continuum damage mechanics, damage is represented by a reduction factor of the element bending stiffness. In this study, a global optimization method called ‘Coupled Local Minimizers’ (CLM) is used for updating the finite element model of a complex structure. In CLM, the local optimization processes are coupled so that better solutions than multistart local optimization consisting of independent runs are obtained. This is achieved by minimizing the average cost function of the local minimizers subjected to pairwise synchronization constraints. An augmented Lagrangian which contains the synchronization constraints both as soft and hard constraints is used and a network is derived in which the local minimizers communicate and exchange information through the synchronization constraints. In this study, the finite element model updating method is applied on a complex structure with a complex damage pattern and 24 design variables using CLM. The damage scenario on the structure is based on the hinge pattern obtained from nonlinear dynamic time history analysis. The results show that damage is detected, localized and quantified very accurately by the FE model updating algorithm used. In the second phase of the paper, two levels of noise, namely; moderate and high noise are applied on the modal parameters. In the presence of noise, damage is located and detected very accurately. The extent of the damage is also quantified precisely and the MAC values as well as the relative eigenfrequency differences are improved substantially. In the third phase of the study, the CLM method is compared with other local optimization methods such as the Levenberg–Marquardt algorithm, Sequential Quadratic Programming and Gauss–Newton methods and the results show that the CLM algorithm gives better results in FE model updating problems compared to the above-mentioned local optimization methods.
The cross-sectional shape of the metal studs in a double-leaf wall has a significant influence on its sound insulation. In a recent study, a numerical optimization of the stud shape resulted in designs that offer a significant increase in broadband airborne sound insulation, both when compared to conventional C-shaped studs and commercially available acoustic studs, but at the expense of a much higher material use. In the present work, a multi-objective optimization of the stud shape is aimed at, so as to find an optimized trade-off between the stud material cost and the sound insulation of the overall system across the entire building acoustics frequency range. This is achieved by combining a computationally efficient, iterative multi-objective optimization scheme with a computationally efficient numerical sound insulation prediction model of sufficient accuracy. Pareto fronts, which represent the set of optimal combinations of sound insulation and stud material use, are computed for a range of plasterboard wall systems that differ in cavity depth, number of sheets of plasterboard and stud type (symmetric or point-symmetric). For nearly all walls with symmetric studs, the Pareto fronts show that the introduction of a single deep indentation in the stud results in a high increase in sound insulation at a limited increase in material cost compared to simple C-shaped studs while two or three deep indentations result in a higher material cost with a performance close to that of a wall with decoupled leafs. For nearly all walls with point-symmetric studs, the Pareto fronts show that the introduction of two deep indentations results in a high increase in sound insulation at a limited increase in material cost compared to Z-shaped studs while four deep indentations result in a higher material cost with a performance close to that of a wall with decoupled leafs. It is shown that often, the acoustic performance of commercially available acoustic stud shapes can be further improved with less material used to produce the stud and sometimes even a shallower cavity.
No abstract is provided for this article.
No abstract is provided for this article.
No abstract is provided for this article.
This contribution presents a numerical approach to quantify the response of an absorber in a diffuse reverberation room. Conventionally, this is done by considering an infinite absorber coupled to an acoustic halfspace. It is, however, well known that the diffuse absorption coefficient for a finite absorber can be quite different due to what is referred to in literature as the edge effect. A finite size correction has been developed previously, but it is only applicable to homogeneous absorbers and is based on a computationally costly quintuple integration. This contribution presents an alternative approach in which a deterministic model, e.g. using the finite element or modal transfer matrix method, is coupled with a statistical model of the room using a hybrid deterministic-statistical energy analysis framework. With this framework, also the theoretical uncertainty on this diffuse sound absorption that is inherent in the diffuse field assumption can be quantified, i.e. the variance of sound absorption results that can be theoretically expected across an ensemble of reverberation rooms of the same volume. The methodology is numerically and experimentally validated for several absorber types.
No abstract is provided for this article.
No abstract is provided for this article.
As part of the Brite-EuRam project BE96-3157 SIMCES (System Identification to Monitor Civil Engineering Structures) the three span box bridge Z24 in Switzerland was monitored during almost one year before it was artificially damaged. In the preceding monitoring period the influence of environmental conditions, such as humidity, wind and especially temperature, on the bridge eigenfrequencies was studied. The goal of the subsequent damage tests, corresponding to realistic and relevant cases, was to prove that damage could be detected, localised and quantified by considering changes in eigenfrequencies and modeshapes. Some of the main conclusions are that ambient vibrations treated by proper system identifications algorithms can provide accurate results for eigenfrequencies and modeshapes, that it is mandatory to filter beforehand the influence of environmental conditions and that small, stiffness degradation producing damage can be detected if the corresponding eigenfrequency diminutions surpass 1%.
