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The SEA model of wind noise requires the quantification of both the acoustic as well as the turbulent flow contributions to the exterior pressure. The acoustic pressure is difficult to measure because it is usually much lower in amplitude than the turbulent pressure. However, the coupling of the acoustic pressure to the surface vibration is usually much stronger than the turbulent pressure, especially in the acoustic coincidence frequency range. The coupling is determined by the spatial matching between the pressure and the vibration which can be described by the wavenumber spectra. This paper uses measured vibration modes of a vehicle window to determine the coupling to both acoustic and turbulent pressure fields and compares these to the results from an SEA model. The interior acoustic intensity radiating from the window during road tests is also used to validate the results.

In an effort to reduce mass, future automotive bodies will feature lower gage steel or lighter weight materials such as aluminum. An unfortunate side effect of lighter weight bodies is a reduction in sound transmission loss (TL). For barrier based systems, as the total system mass (including the sheet metal, decoupler, and barrier) goes down the transmission loss is reduced. If the reduced surface density from the sheet metal is added to the barrier, however, performance can be restored (though, of course, this eliminates the mass savings). In fact, if all of the saved mass from the sheet metal is added to the barrier, the TL performance may be improved over the original system. This is because the optimum performance for a barrier based system is achieved when the sheet metal and the barrier have equal surface densities. That is not the case for standard steel constructions where the surface density of the sheet metal is higher than the barrier.

A 1/4 scale model vehicle profile has been tested in a wind tunnel with speeds up to 360 km/h. In order to simulate the free field flow over the vehicle, the top surface of the wind tunnel is contoured. A CFD simulation of the free field flow at various speeds is used to identify the desired top streamline. Then the boundary layer growth on the top surface is calculated and the top contour is adjusted accordingly. Since this contour changes very little with flow speeds of interest, an average contour is used for a fixed top surface of the wind tunnel. Pressure drop measurements are used to verify the flow similarity to the CFD model. Wind noise measurements using surface mounted pressure transducer arrays are used to determine the acoustic loads on the vehicle surfaces.

Road tests on a pickup truck have been conducted to determine the acoustic loads on the back panel surfaces of the vehicle. Surface mounted pressure transducers arrays are used to measure both the turbulent flow pressures and the acoustic pressures. These measurements are used to determine the spatial excitation parameters used in an SEA model of the transmission loss through the back panel surfaces. Comparisons are made between tests on different road surfaces and at different speeds to identify the relative contributions of acoustic and wind noise.

Speech communication from the front seat to the rear seat in a passenger vehicle can be difficult. This is particularly true in a vehicle with an acoustically absorptive interior. Speech Transmission Index (STI) measurements can quantify the speech intelligibility, but they require specialized signal processing. The STI calculations can be simplified if it is assumed that reverberation and echoes play an insignificant role in an automobile. A simplification of a STI measurement is described that uses a stationary reference speech signal from a talker mannequin in the driver's seat to create a signal at the rear passenger positions. On-road noise measurements are used for the noise level and the calculated signal to noise ratio is used to calculate a simplified STI value that tracks closely to a full implementation of the STI method for sedans. In fact, this method is very similar to the techniques described in the Articulation Index (AI) and Speech Interference Index (SII) standards.

The automotive industry is often interested in controlling noise radiated from trim pieces in the passenger cabin. In general, there is a small air gap that separates these trim pieces from the sheet metal that is the actual source of the noise. It is common practice to place an acoustically absorbent material in this space to reduce radiated noise. In this paper the in situ noise control performance of a variety of materials is examined by placing them in a test fixture that simulates the sound field in the vicinity of vehicle pillar trim. In this fixture a noise source is positioned behind a piece of sheet metal. A flat plastic sheet that is similar in composition to pillar trim is placed a small distance away from the sheet metal. The sides and rear of the fixture are sealed so that the plastic sheet is the only significant radiator of the sound radiated from the sheet metal.

Acoustical materials are widely used in automotive vehicles and other industrial applications. Two important parameters namely Sound Transmission Loss (STL) and absorption coefficient are commonly used to evaluate the acoustical performance of these materials. Other parameters, such as insertion loss, noise reduction, and loss factors are also used to judge their performance depending on the application of these materials. A systematic comparative study of STL and absorption coefficient was conducted on various porous acoustical materials. Several dozen materials including needled cotton fiber (shoddy) and foam materials with or without barrier/scrim were investigated. The results of STL and absorption coefficient are presented and compared. As expected, it was found that most of materials are either good in STL or good in absorption. However, some combinations can achieve a balance of performance in both categories.

The sound transmission loss of a limp barrier can be predicted with the field-incidence mass law formula. The formula in general use is, however, only valid when predicting transmission losses for materials with relatively high surface densities and/or at high frequencies. In fact, the formula will predict transmission losses that are less then 0dB for very lightweight materials (or at very low frequencies). This is clearly not possible, but is caused by unsatisfied assumptions. In addition, the field-incidence mass law makes some assumptions about the effective angle of incidence of sound as it impinges the barrier. In fact, various authors use slightly different assumptions and this leads to differences in the mass law equations in the literature. This paper will examine the general equation for the mass law and will look at the effect of the common assumptions when calculating the performance of very light materials and/or at low frequencies.

Composite multi-layered systems are of particular interest in the automotive industry since the design of the various components in an efficient sound package requires a good predictive model. The state of the art in this matter shows that the medium and high frequency ranges are well mastered in terms of predictive tools based on infinite models. But this is not the case for the lower frequency range. The paper will start with a discussion of the medium and high frequency range where, for example, the Transfer Matrix Method (TMM) is an efficient framework to predict the acoustical properties of multi-layer materials. Emphasis will be put on correlation data obtained with a variety of multi-layer systems. In the low frequency range the use of infinite models leads to significant discrepancies. In the present paper the authors propose a finite “hybrid type” formulation which combines the advantages of both single layer and multi-layer approaches of stratified composite structures.

The optimization of acoustical properties of multi-layered materials used in the automotive industry requires a good understanding and characterization of the various component layers. This is a particular concern in the case of headliners where performance must be balanced with packing space demands. These composite structures when used with flexible urethane foams provide good stiffness and light weight, but their acoustic performance can be sub-optimal. Measurements undertaken with poro-elastic high airflow resistivity foams highlighted frame resonances which, if exploited, might significantly improve the acoustical performance of this system. A new modeling technique based on a pseudo-macroscopic description of the poro-elastic material in the framework of a four-pole network will be used to explain these frame resonances. This formulation exploits the electro-acoustical analogy in transmission line theory.

High frequency responses of structural-acoustic systems may be predicted by statistical energy analysis (SEA) or energy finite element method (EFEM). To combine the good features of these two techniques, a simplified energy finite element method, referred to as EFEM0, has been developed recently. The EFEM0 technique, which is compatible with SEA, integrates the joint coupling procedures for discontinuous systems and the finite volume formulation for continuous system. The EFEM0 models have been verified either analytically or experimentally for one- and multi-dimensional systems. In this study, the EFEM0 technique is applied to a passenger van for a noise control investigation. The general scheme is to incorporate the EFEM0 coupling factors into a SEA model in order to release some SEA assumptions and improve the SEA model, especially for relatively high damping, strong coupling and direct field cases.