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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.

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 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.

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.

Seat covers made from textiles, leather and vinyl were evaluated for noise absorption. The textiles included woven velours, pile knits and flat wovens. The noise absorption of the covers and the corresponding seat assemblies was tested by the reverberation room method per ASTM C423. The effect of different foams was also tested. For the leather and vinyl covers, the effect of perforation was evaluated. Test results showed distinctive differences between textiles and leather/vinyl with cloth seats having superior noise absorption. Even among the textiles, there are significant differences. Core foam densities affect the characteristics as well. For pile fabrics (woven velours and pile knits), the size of the pile fiber does not affect the acoustic characteristics of the seat. Also, no significant difference was observed between a bonded seat and a conventional (cut and sew) seat.

The area in the neighborhood of the package tray can be a significant path for road noise and exhaust noise. Air extraction routes and loudspeakers add to the difficulty of effective system design. A variety of designs were prototyped and their transmission loss measured in a standard SAE J1400 sound transmission loss suite. The performance of the various designs was compared to an untrimmed piece of sheet metal with embedded air extraction holes. The addition of trim added from 1 dB to 14 dB to the transmission loss. Statistical energy analysis (SEA) models of a variety of package tray systems will also be shown. Both of these techniques can provide design guidance at an early stage of vehicle program development.

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.

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.

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.