Abstract

Finding methods to provide a quiet passenger compartment in a car are highly sought after by automobile manufacturers. The ability to reduce noise inside the vehicle enhances the perceived value of the vehicle to the consumer and offers a competitive advantage to the manufacturer.

Several methods to reduce noise and its sources are employed, one of which reduces noise in the passenger compartment using sound absorbing materials attached to various components such as doors, quarter panels, trunk sides and floors, headliners and others.

This study attempts to quantify the characteristics of several cellulosic-based nonwovens to act as efficient absorbers, reducing the overall sound level in the passenger compartment as measured by ASTM C-384 "Impedance and Absorption of Acoustic Materials by the Impedance Tube Method."

The results of testing demonstrate that each of the cellulosic-based nonwoven composites contribute to the absorptive properties of the components and are effective for overall noise reduction in the vehicle. The individual acoustic characteristics of the various vehicles determine the type and amount of material required to provide the best results.

Introduction

Various blends of cellulosic-based nonwovens manufactured by the carding and needlepunch process are tested for their ability to absorb sound energy. They are compared to targets of acoustic absorption established by automotive manufacturers for use with vehicle interior trim components such as door absorbers, headliners, trunk liners and others with the goal of reducing the noise level in the vehicle.

Discussion

The ability of a nonwoven material to absorb sound or unwanted noise in the passenger compartment of the vehicle is based on dissipation of the energy of the sound wave upon passing through the material and being redirected by the fibers, and also upon conversion of some of the energy into heat. The amount of original energy less the remaining unabsorbed energy results in the measurement referred to as the absorption coefficient. This absorption coefficient is often used to rank the order of different materials to reduce the noise level in the vehicle when composites of these materials are attached to various components in the car, such as doors, pillars, headliners and trunk compartments.

These components are typically placed between the sound sources (such as vibrating steel panels, windows passing air and tires) and the receivers (occupants of the vehicle). While it is commonly accepted that the most effective way to reduce sound is at its source, several issues, such as cost, smooth ride and vehicle weight, make it necessary to use sound absorbing materials as "bandages" for noisy interiors.

For this study, the low cost, renewability, biodegradability and recyclability of the natural cellulosic fibers as the matrix materials, make them attractive as a potential sound-absorbing nonwoven. The carding and needlepunch structuring process was chosen to produce the samples because it has long been used to make padding that can be molded into shapes for attaching to the components in the vehicle (Table 1). Polypropylene fiber in the cellulosic composites make the nonwovens moldable. "Seconds" quality of polypropylene was used in the present set of experiments to produce products at lower cost.