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Laminated steel body panels are used in different applications in vehicles, such as dash panels and wheel wells. A part made out of laminated steel has the potential to provide structure-borne noise reduction and also improve the airborne noise reduction of the part compared to a monolithic part. The use of laminated steel has been more critical when there are deep draws on the part as the deep draws cause localized resonances which degrade the acoustic performance significantly. However, due to lightweighting demands, hybrid laminated panels, commonly known as acoustic patch laminates have become very attractive. This paper discusses the damping and sound transmission loss performances of a dash panel part with monolithic, laminated, and acoustic patch panels.

Standards organizations develop standards depending on the need in the market place. With the change in vehicle design, lightweighting structures, and body panels made out of aluminum and composites, SAE’s Acoustical Materials Committee is developing a new damping standard. This standard is also very suitable in determining the damping performance of materials used in the off-highway applications, where the thickness of the steel body panel is much greater than in the automotive application. The general methodology of this standard is based on the mechanical impedance measurement method and has been developed with the general consensus of automotive engineers, suppliers, and independent test laboratories. This method is essentially based on the fact that a bar is excited at the center by a shaker. The force exerted by the shaker and the corresponding vibration is measured at that point to determine the frequency response function of the mechanical impedance signal.

Traditionally, the damping performance of a visco-elastic material is measured using the Oberst bar damping test, where a steel bar is excited using a non-contacting transducer. However, in an effort to reduce the weight of the vehicles, serious effort is put in to change the body panels from steel to aluminum and composite panels in many cases. These panels cannot be excited using a non-contacting transducer, although, in some cases, a very thin steel panel (shim) is glued to the vibrating bar to introduce ferrous properties to the bar so it can be excited. In the off highway vehicles, although the panels are made of steel, they are very thick and are difficult to excite using the Oberst bar test method. This paper discusses a measurement methodology based on mechanical impedance measurements and has the potential to be a viable/alternate test method to the Oberst bar testing. In the impedance method, the test bar is mounted to a shaker at the center (Center Point method).

Most of NVH related issues start from the vibration of structures where often the vibration near resonance frequencies radiates the energy in terms of sound. This phenomenon is more problematic at lower frequencies by structureborne excitation from powertrain or related components. This paper discusses a laboratory based case study where different visco-elastic materials were evaluated on a bench study and then carried on to a system level evaluation. A body panel with a glazing system was used to study both airborne and structureborne noise radiation. System level studies were carried out using experimental modal analysis to shift and tune the mode shapes of the structure using visco-elastic materials with appropriate damping properties to increase the sound transmission loss. This paper discusses the findings of the study where the mode shapes of the panel were shifted and resulted in an increase in sound transmission loss.