Search Results

Minimizing rattle noises is becoming increasingly important for hybrid and electrical vehicles as masking from the internal combustion engine is missing and in view of the functional requirements of the office-like interiors of next generation automated vehicles. Rattle shall therefore be considered in the design phase of component systems. One hurdle is the modelling of the excitation mechanisms and its experimental validation. In this work we focus on excitation by loose parts having functional clearances such as gear systems or ball sensors in safety belt retractors. These parts are excited by relatively large low frequency displacements such as road-induced movements of the car body or low order rigid body engine vibrations generating multiple impacts with broad band frequency content. Direct measurement of the impact forces is in many cases not possible.

Radiation of sound from an open pipe with a hot mean flow presents one of the classic problems of acoustics in inhomogeneous media. The problem has been especially brought into focus in the last several decades, in the context of noise control of vehicle exhaust systems and jet engines. However, the reports on the measurements of the radiated sound field are still rare and scattered over different values of subsonic and supersonic flow speeds, cold and hot jets, as well as different sound frequency ranges. This paper focuses on low Mach number values of the mean flow speed and low frequencies of the incident (plane) sound waves inside an unflanged cylindrical pipe with a straight cut. It presents the results of the far-field radiation pattern measurements and compares them with an existing analytical model from the literature. The mean flow inside the pipe reached Mach number values up to 0.25 and temperature up to 300°C.

For vibration and acoustics vehicle development, one of the main challenges is the identification and the analysis of the noise sources, which is required in order to increase the driving comfort and to meet the stringent legislative requirements for the vehicle noise emission. Transfer Path Analysis (TPA) is a fairly well established technique for estimating and ranking individual low-frequency noise or vibration contributions via the different transmission paths. This technique is commonly applied on test measurements, based on prototypes, at the end of the design process. In order to apply such methodology already within the design process, a contribution analysis method based on dynamic substructuring of a multibody system is proposed with the aim of improving the quality of the design process for vehicle NVH assessment and to shorten development time and cost.

Sensing and acting elements to guarantee the locking functions of seat belt retractors can emit noise when the retractor is subjected to externally applied vibrations. For these elements to function correctly, stiffness, inertia and friction needs to be in tune, leading to a complex motion resistance behavior, which makes it delicate to test for vibration induced noise. Requirements for a noise test are simplicity, robustness, repeatability, and independence of laboratory and test equipment. This paper reports on joint development activities for an alternative test procedure, involving three test laboratories with different equipment. In vehicle observation on parcel shelf mounted retractors, commercially available test equipment, and recent results from multi-axial component tests [1], set the frame for this work. Robustness and reliability of test results is being analyzed by means of sensitivity studies on several test parameters.

Modern vehicles are driven with various speeds over specific rough road tracks to detect the presence of annoying buzz, squeak and rattle sounds. As known in the occupant safety industry the mechanical locking systems of seat belt retractors can be significant noise sources, when excited by road vibrations. A reliable bench test procedure is necessary to quantify the acoustic performance of retractors, verify production quality, and derive realistic acoustic product targets. With this goal, a vibration noise test procedure has been developed condensing the work over three years by the K2 Comet automotive research project X2T1, various OEM retractor noise specifications closed to public and own research. The load case in this specification has been defined as horizontal 60 Hz bandlimited broadband excitation, while the N10 instationary loudness metric has been selected to characterize the retractor acoustic performance.

Seatbelt retractors as important part of modern safety systems are mounted in any automotive vehicle. Their internal locking mechanism is based on mechanically sensing elements. When the vehicle is run over rough road tracks, the retractor oscillates by spatial mode shapes and its interior components are subjected to vibrations in all 6 degrees of freedoms (DOF). Functional backlash of sensing elements cause impacts with neighbouring parts and leads to weak, but persistent rattle sound, being often rated acoustically annoying in the vehicle. Current acoustic retractor bench tests use exclusively uni-directional excitations. Therefore, a silent 2 DOF test bench is developed to investigate the effect of multi-dimensional excitation on retractor acoustics, combining two slip-tables, each driven independently by a shaker. Tests on this prototype test bench show, that cross coupling between the two perpendicular directions is less than 1%, allowing to control both directions independently.

In automotive industry, acoustic trim materials are widely used in order to reach passenger comfort targets. The dynamic behavior of the poro-elastic materials is typically modelled by the Biot theory, which however leads to expensive numerical finite element calculations. One way to deal with it is to use the Patch-Transfer-Function (PTF) sub-structuring method, which couples subdomains at their interfaces through impedance relations. This was done already for systems including locally reacting poro-elastic materials. In this paper, a methodology is presented allowing to numerically assess the PTF impedance matrices of non-locally reacting trim materials using the Biot based poro-elastic model solved by the finite element method (FEM). Simplifications of the trim impedance matrices are introduced resulting in considerable calculation cost reductions. The associated prediction errors are discussed by means of a numerical case study.

Automotive industry is becoming more and more interested in assessing the noise of electric motors, since their integration in many types of road vehicles is rapidly growing in a market oriented to hybridization and electrification. The acoustic characterization of an electric motor is often being performed numerically, having as consequence the fact that the investigation is confined to one specific model belonging to one particular type of motor. This paper proposes an experimental airborne sound characterization methodology, suitable for any type of cylindrical source, based on a set of data acquired following a cylindrical Nearfield Acoustical Holography (NAH) scheme. Such an approach allows the evaluation of sound intensity, as well as pressure level and particle velocity.

In many application fields, such as automotive and aerospace, the full FE Biot model has been widely applied to vibro-acoustics problems involving poro-elastic materials in order to predict their structural and acoustic performance. The main drawback of this approach is however the large computational burden and the uncertainty of the input data (Biot parameters) that may lead to less accurate prediction. In order to overcome these disadvantages industry is asking for more efficient techniques. The vibro-acoustic behaviour of structures coupled with poroelastic trims and fluid cavities can be predicted by means of the Patch Transfer Function (PTF) approach. The PTF is a sub-structuring procedure that allows for coupling different sub-systems via impedance relations determined at their common interfaces. The coupling surfaces are discretised into elementary areas called patches.

Electric high voltage (HV) cables are commonly used in electrified vehicles. These cables are well known for being potential flanking transmission paths for structure-borne sound in a broad frequency range and must therefore be included in the NVH design process. Although many different layouts exist automotive HV cables fundamentally exhibit a layered structure consisting of a conductive core covered by, in sequence, an isolating sheath, a wire mesh shield to minimize electromagnetic interference and an outer jacket. The conductive core consists of hundreds of thin copper or aluminum wires organized in strands which are wound in a helical pattern. An extensive measurement campaign was carried out to dynamically characterize different cable specimen in terms of multi-degree of freedom transfer impedances.

The NVH optimization of new vehicle models can in principle only be carried out in a relatively late stage of the development process, when the geometrical data (CAD) are available and can be used to generate detailed Finite Element (FE) models of the car body. Unfortunately, in this stage of the development process most of the geometrical data are already fixed and countermeasures are limited and expensive. In order to be able to evaluate design concepts in an earlier conceptual stage of the development process existing models of similar predecessor vehicles must be used leading to techniques such as “mesh-morphing” or “concept modelling” (see for instance [1, 2]). Here, a different approach is investigated based on a substructuring technique.