Structure-borne Transmission Research Articles

Evaluation of the performance of technologies for reducing ships' machinery noise using a small-scale ship-like structure in a Water Basin

Ocean ambient levels have increased in the last decades, especially in the low-frequency domain (under 500 Hz). This increase is partly due to underwater radiated noise (URN) from commercial ships. Excessive URN harms marine life and is, therefore, considered a pollution that needs to be reduced. At low speeds, machinery is the primary noise source on ships. Mitigation technologies exist to limit machinery’s contribution to URN. While implementing these technologies is costly, a lack of quantitative data regarding their exact performances usually results in limited concrete ship applications since the cost-to-benefit ratio is imprecise. This study aims to quantify better the performance of standard noise mitigation technologies using a small-scale ship-like structure in a water basin. The basin’s acoustic field is first characterized with and without the structure. The structure is then equipped with different mitigation technologies. A loudspeaker and a vibration shaker are fed with pink noise or measured signals on actual machinery. Hydrophones, microphones, accelerometers, and force sensors measure the response in the basin and on the structure. The performance of each tested technology is evaluated and ranked in terms of URN reduction. The relative contributions of airborne and structure-borne transmission paths on URN are also examined.

An in-plane flexible ring model for the analysis of the free and forced response of a rolling tyre

The increased demand for vibroacoustic comfort as well as regulations on noise and vibration levels have made the NVH (Noise, Vibration, Harshness) performances of a vehicle to become one of the fundamental design criteria. Therefore, predictive models for the analysis of noise and vibration transmission mechanisms represent interesting tools to support the R&D department of the automotive companies. Focusing the attention on passenger's comfort, the vibrations induced by the tyre/road interaction propagate from the contact area to the hub and finally inside the cockpit through structure-borne transmission paths. This can be regarded as one of the major contributors to the car cabin interior noise at low frequencies (between 20 and 500 Hz). Simplified models able to interpret the waves propagating inside the tyre structure and influenced by the angular speed may support the studies in this research field. To this end, an analytical model based on the theory of the flexible ring on elastic foundation has been developed. It allows analysing the tyre dynamics in both static and rolling conditions. Model parameters have been calibrated based on an Experimental Modal Analysis of the static tyre. The free response of the tyre shows the bifurcation effect at different rolling speeds, while a cleat test simulation has been carried out to investigate the forced response of the tyre.

Sound isolation of recording studios

Sound isolation to and from quality recording studios is critical to their success. Proper design based on informed understanding of both airborne and structureborne transmission is essential, because retrofitting isolation can be challenging or even prohibitive, and because one-size-fits-all recommendations may be unsatisfactory. This presentation will review some issues and evaluation methods that help to guide successful, cost-effective designs. Also discussed will be examples including a world-class studio with remarkable history that was encroached by an expanded loud facility, floated constructions that were not actually floated, and an approach that has helped to convince clients of the level of required isolation.

A Theoretical Model for Investigating the Structural Dynamics of a Rolling Tyre

Abstract The noise generated by the rolling tyre contributes significantly to the car's interior noise. This is caused by the tyre-road contact, and at low frequencies (0–500 Hz) is mostly transmitted inside the cockpit through the structure-borne transmission path. In support of the studies in this research field, an interpretative model of the tyre-wheel system accounting for the effects induced by the angular speed represents a useful tool. To this aim, we implemented an analytical model based on a flexible ring on an elastic foundation to analyze the dynamics of the tyre-wheel system, in both static and rotating configurations. We fine-tuned the parameters of the tyre based on data coming from experimental modal analysis of the static tyre. Particular attention has been paid to the system's free and forced responses, commonly analyzed with the so-called cleat test. The results are discussed interpreting the behavior in different reference systems.

Optimization of Noise Transfer Path Based on the Composite Panel Acoustic and Modal Contribution Analysis

The noise of a cab directly affects the comfort and labor efficiency of the operators. The optimization of the structure-borne transmission path can obviously reduce the cab noise. The method of panel acoustic contribution analysis (PACA) is used to reduce structure noise. However, most studies only consider the panel acoustic contribution of a single frequency, without considering the contribution of major frequencies synthesis to confirm the optimized panels. In this paper, a novel method is proposed based on composite panel acoustic and modal contribution analysis and noise transfer path optimization in a vibro-acoustic model. First, the finite element model (FEM) and the acoustic model are established. Based on the acoustic transfer vector (ATV) method, a composite panel acoustic contribution analysis method is proposed to identify the panels affecting the noise of the field point. Combined with the modal acoustic contribution of the modal acoustic transfer vector (MATV) method, the noise field point is confirmed in the area which has the most significant influence. Second, the optimization algorithm NLOPT which is a nonlinear optimization is applied to design the areas. The noise transfer path optimization with vibroacoustic coupling response can quickly determine the optimal thickness of the panels and reduce low-frequency noise. The effectiveness of the proposed method is applied and verified in an excavator cab. The sound pressure level (SPL) the driver’s right ear (DRE) decreased obviously. The acoustic analysis of the composite panel acoustic contribution and modal acoustic contribution can more accurately recognize an optimized area than the traditional PACA. This method can be applied in the optimization of the structure-borne transmission path for construction machinery cab and vehicle body.

Measurements and modelling of dynamic stiffness of a railway vehicle primary suspension element and its use in a structure-borne noise transmission model

The noise inside railway vehicles is transmitted by both structure-borne and airborne paths and, although there are many sources, the rolling noise is often the most important. This paper focuses on the structure-borne transmission of rolling noise in a metro vehicle. Measurements are presented first of the vertical and lateral dynamic stiffness of a primary suspension element consisting of conical rubber/metal elements. Results are presented for various constant preloads over the frequency range 60–600 Hz. An analytical model of the suspension element is also developed, based on a mass-spring system and including wave motion within the rubber elements. The dynamic stiffness results are used in a finite element model of the running gear, consisting of the bogie frame, wheelsets and suspension elements. The excitation is provided by the combined wheel/rail roughness at the contact point. This model is used to calculate the blocked forces at the connection points between the secondary suspension elements and the car body. The blocked forces are combined with measured vibro-acoustic transfer functions from these mounting points to the vehicle interior to determine the structure-borne noise inside the vehicle. The proposed methodology is validated against measurements during operation in terms of acceleration levels, blocked forces and structure-borne noise levels inside the vehicle, showing reasonably good agreement. Including the dynamic stiffness for the primary suspension leads to improved agreement between 100 and 500 Hz compared with using a constant stiffness.

The sound insulation and directivity of the sound radiation from double glazed windows.

The sound insulation and directivity of the radiated sound from double glazed windows have been measured by different researchers. Previously, airborne sound insulation models have been used to predict the associated measurement results with limited success. In this paper, the importance of accounting for the structure borne sound transmission between two glazing elements via the window frame on the prediction results is demonstrated. The decreased stiffness of the wall cavity as the depth is increased is the reason why sound transmission via the window frame needs to be considered. The reciprocity argument provided by Davy for the prediction of the directivity of sound radiating into a room is validated and it is shown that once the structure borne transmission is considered, an additional weighting term is not needed to compensate for the extra wall collisions which the sound experiences when radiated at grazing incidence.

Investigation on vibration source and transmission characteristics in power transformers

The tank vibration of power transformers comes from alternating forces acting on the core and windings. It is meaningful to study the vibration sources and the transmission characteristics in transformers for interpreting the constitution of the tank vibration specifically. This paper experimentally investigates the coupling between the winding vibration and the core vibration, as well as the vibration transmission characteristics on a single phase power transformer the winding vibration coupled with the core vibration has a broader frequency band than the responses caused by the electromagnetic force. For transmission characteristics, the oil-borne transmission is the main path for the winding vibration, while the structure-borne transmission highly depends on the modal characteristics of the tank. This work improves the understanding of the relationship between the internal vibrations and the tank responses.

Optimized Mounting Positions for Vibratory Machines in Buildings Based on Structure-Borne Sound Power Transmission and Machine Stability

Optimized mounting positions for isolated vibratory machines in buildings require a minimum transmission of structure-borne sound power from the machine to the floor structure and relative stability of the isolated machine. Previous work by Mak and co-investigators has indicated the importance of using structure-borne sound power to assess vibration isolation and to select the best mounting positions by considering the structure-borne sound power transmission. This paper is a first attempt to utilize both the structure-borne sound power transmission and the rotational velocity (or the stability) of the machine to select the optimized mounting positions for an isolated vibratory machine. The results reveal that a vibratory machine should be symmetrically installed on diagonal lines of the receiving floor structure.

Methods for calculating flanking noise transmission

Airborne sound isolation between spaces is determined not only by the direct sound transmission through the separating assembly, but also flanking transmission, which is the structureborne transmission of vibration due to acoustical excitation in the source room. In many cases, the flanking paths dominate, such as where the separating assembly has a high level of sound transmission loss, and the ability to calculate these paths is therefore crucial if the isolation is to be accurately predicted. Methods for calculating impact noise have been developed in Europe [standard EN ISO 12354] and are currently in use in Europe and Canada; however, the methods are not commonly known or used in the United States. The authors report on use of these methods in calculating flanking noise transmission in recent projects.

Sound transmission loss of ETICS cladding systems considering the structure-borne transmission via the mechanical fixings: Numerical prediction model and experimental evaluation

Nowadays it is common to increase the acoustic and thermal performance of building façades using the External Thermal Insulation Composite System (ETICS). In this paper a prediction model to evaluate the sound transmission loss provided by such systems, also considering the mechanical bridges that connect the insulating slabs to the basic wall, is presented. The model is developed within the transfer matrix method framework and it is based on a well established decoupled approach. The different formulations presented by several authors to take into account the structure-borne sound transmission through the mechanical connections in multilayer and double-plate elements are not suitable to analyse these kind of building partitions, since they are addressed only to thin plate elements and lightweight porous or fibrous materials. Unfortunately, the different layers that the ETICS consists of need a more sophisticated definition within the transfer matrix method. Therefore, a new model, developed for these specific elements, is proposed. The computation of all the terms and the necessary assumptions are accurately described and their validity is verified with experimental data. Finally, the reliability of the model is investigated by comparing the numerical results with measured transmission loss.

Sound transmission through a double panel structure periodically coupled with vibration insulators

In this paper, sound transmission through an aircraft sidewall representative double panel structure is investigated theoretically and parametric and validation studies are conducted. The studied configuration is composed of a trim panel (receiver side panel) attached to a ribbed skin panel (source side panel) with periodically spaced resilient mounts. The structure is considered infinite in order to use space harmonic expansion. The partition is also assumed planar for simplicity. The model allows for a 3D incident field and the panels can be metallic and/or composite. A four-pole formulation is employed for modeling of the mounts and the absorption provided by the fiberglass that fills the cavity between the leaves is addressed with an equivalent fluid model. The investigation of mount stiffness, damping and spacing show that properly designed mounts can increase the TL significantly (up to 20 dB of difference between rigid and resilient mounts). However, they can create undesirable resonances resulting from their interaction with the panels. The influence of cavity absorption is also studied and results illustrate the fact that it is not worth investing in a highly absorbent fiber if the structure-borne transmission path is not adequately insulated, and likewise that it is not worth investing in highly resilient mounts without sufficient cavity absorption. Moreover, the investigation of panel damping confirms that when structure-borne transmission is present, raising skin damping can increase the TL even below coincidence, but that on average, greater improvements are achieved by raising trim damping. Finally, comparison between the periodic model and finite element simulations for structure-borne transmission shows that the average level of transmitted energy is well reproduced with the periodic approach. However, the modes are only captured approximately due to the assumption of an infinite structure.

FEM/SEA Hybrid Method for Predicting Mid and High Frequency Structure-Borne Transmission

Launch Vehicles are subject, at lift-off and during flight ascent, to acoustic and aeroacoustic loads, which are random in nature. Because electronic units are very sensitive to mid and high frequency loads, it is important to numerically predict and specify the vibration levels to be applied to units for qualification test. The general objective of the activity presented in this paper is to develop a methodology to predict mid and high frequency structure-borne transmissions in launch vehicles. As the loads of interest are random, it has been chosen to investigate energy-based modeling approaches, combined with the Finite Element Method. For energy-based modeling, the structure is divided into subsystems. For high frequency predictions, the purely numerical Power Injection Method, derived from Statistical Energy Analysis, is used to estimate the Coupling Loss Factor between structural subsystems. For the mid frequency predictions, an approach close to Statistical Energy Analysis, called Statistical Energy Analysis-Like (SEA-like), is investigated. In this approach, a relation between total energies of subsystems and input powers is established, by identifying a matrix composed of Energy Influence Coefficients. The objective of the study is to establish the methodology to compute with accuracy, using the Finite Element Method, Coupling Loss Factors and Energy Influence Coefficient. It is shown that the excitation of subsystems by 'Rain on the Roof' loads defined by the 'Influence Circle' and the Optimal Latin Hypercube methods provide accurate coupling data. A validation of the methodology on academic and industrial cases is presented.

Reduction of noise in an excavator cabin using order tracking and ultrasonic leak detection

During operation, heavy construction equipment can generate high levels of noise, which can adversely affect the health and job performance of operators. This study examined the noise transmitted into the operator's cabin of a crawler excavator during operation. The sources of the noise generated by the excavator and the air-borne and structure-borne transmission paths into the cabin of the excavator were identified and characterized. Order tracking, which utilizes tachometer measurements to relate the occurrence of an event to a multiple of engine rotational speed, was used to identify noise and vibration components related to excitation from the excavator's engine and hydraulic systems. The results of the analysis showed 1st order engine rotational imbalance, 3rd order engine combustion, and 9th and 18th order hydraulic pressure fluctuations were the primary components of noise and vibration generated by the excavator. An ultrasonic leak detector was used to identify small sound leaks into the cabin. Finally, passive noise control treatments were designed to attenuate and dissipate the acoustic energy transmitted into the excavator cabin. These cabin noise control treatments included covering the floor with an acoustic barrier, adding damping to the windows, lining the ceiling and walls with absorptive foam, and sealing leaks found along window, door, and panel edges. Experimental results showed that the noise control treatments reduced the overall A-weighted sound pressure level in the cabin by up to 3 dB.

Low-frequency dynamic analysis of automotive door weather-strip seals

Automotive car door weather-strip seals play an important role in the vibro-acoustic isolation of passenger compartments. The research on weather-strip seals is mainly focused on the sound transmission characteristics of the joint, while the structure-borne vibration transmission characteristics of the component have received little attention. Previous research has shown the value of the non-linear finite element (FE) method for the static analysis of the mechanical characteristics of weather-strip seals, and defined relevant seal performance factors that are considered in door weather-strip seals design. This study concerns the low-frequency dynamic analysis of the mechanical characteristics of the weather-strip seals, and focuses on the detailed modeling of the structure-borne transmission characteristic of the component. The structure-borne transmission characteristic of the weather-strip seals is an important performance factor to be considered in door weather-strip seal design for NVH applications. In operating conditions, when the car door is locked on the car body, the weather-strip seal can be considered subjected to a small-amplitude vibration superimposed on the seal compression level determined by the door–body gap distance. A detailed non-linear FE model of the weather-strip seal is used to simulate the seal structure-borne transmission characteristic as a function of compression and frequency. A setup is built to measure the material properties used in the numerical simulation, and to experimentally validate the structure-borne transmission characteristics of the weather-strip seal.

MRI system vibration and noise considerations in hospital design

Magnetic Resonance Imaging (MRI) systems are increasingly being located on above-grade floors of hospital buildings. The floor structures that support these systems need to provide vibration environments that satisfy the system’s criteria, and airborne and structure-borne transmission of noise to neighboring areas needs to be taken into account in order to provide acceptable acoustical environments in these spaces. The fundamental operating principles of MRI systems are reviewed in relation to the systems’ susceptibility to vibration and to their sound producing mechanisms. Vibration criteria for some widely used systems are discussed and spectra of airborne and structure-borne noise are illustrated. Means for vibration control and for the attenuation of structurally transmitted sound are delineated, together with architectural considerations for limiting noise transmission to spaces adjacent to MRI suites.

The quantification of structure-borne transmission paths by inverse methods. Part 1: Improved singular value rejection methods

Structure-borne sound from installed machinery is often transmitted into a receiver structure via many connection points and several co-ordinate directions at each of them. In order to quantify the contributions from the various connection points, the operational forces at the interfaces, or an equivalent set of forces at some other locations, should be determined. These forces may be combined with measured transfer functions to determine their contributions to the sound at the receiver locations. Inverse methods are becoming widely used, in which a matrix of measured accelerances is inverted at each frequency and used with operational acceleration data to find the forces. Due to poor conditioning of this matrix, however, the results can often be unreliable. In this paper, using both simulations and measurements, an assessment is made of the success and failure of various strategies for dealing with the problems of ill conditioning, in particular over-determination and singular value rejection. In each case the test structure is a rectangular plate, and a wide frequency range is covered to include regions of both low and high modal overlap. Critical for the rejection of singular values is a suitable threshold. It is established that previously used thresholds, based on estimates of error in either accelerances or operational responses, cannot be used universally. An alternative approach is developed in which the accelerance matrix is perturbed by a different amount for each sample of the operational responses. Based on this approach a more robust strategy is proposed which takes account simultaneously of the effect of errors in both the accelerances and operational responses.

The quantification of structure-borne transmission paths by inverse methods. Part 2: Use of regularization techniques

The inversion of an ill-conditioned matrix of measured data lies at the heart of procedures for the quantification of structure-borne sources and transmission paths. In an earlier paper the use of over-determination, singular value decomposition and the rejection of small singular values was discussed. In the present paper alternative techniques for regularizing the matrix inversion are considered. Such techniques have been used in the field of digital image processing and more recently in relation to nearfield acoustic holography. The application to structure-borne sound transmission involves matrices, which vary much more with frequency and from one element to another. In this study Tikhonov regularization is used with the ordinary cross-validation method for selecting the regularization parameter. An iterative inversion technique is also studied. Here a form of cross-validation is developed allowing an optimum value of the iteration parameter to be selected. Simulations are carried out using a rectangular plate structure to assess the relative merits of these techniques. Experiments are also performed to validate the results. Both techniques are found to give considerably improved results compared to singular value rejection.

Structure-Borne Vibration Transmission in a Tire and Wheel Assembly

Abstract Vibration transmission through tires due to forces generated by the road-tire interaction during rolling over a rough road surface is an important source of noise in the vehicle interior over a wide frequency range. Vibration transmission along the tire tread and across the sidewalls to the rim was measured by striking the tread with an impact hammer in generating transfer functions for comparison with results from an SEA model of the wheel. The effects of tire and rim resonances are noted. Transfer functions to acoustic levels in the tire cavity were also measured.

Parametric evaluation of transmission characteristics in aircraft-like structures

The paper develops a model for the coupling between a beam and a ring which represents the joint between a wind and a fuselage ring frame. The model is used to study the transmission of structureborne disturbances from the beam to the ring. Such disturbances could arise on an actual aircraft due to a wing-mounted propeller and engine. The effects of joint placement and synchrophasing (synchronizing the phase of the propellers and/or engines) are studied as methods of noise control. The results show that the optimum phase shift between propeller inputs depends on the location of the joint between the wing and the ring frame. High and mid-wing designs generally experience lower levels of disturbance transmission than low-wing aircraft, but the combination of wing placement and phase synchronization determines the amplitude of structureborne transmission.