When vehicles are in motion, friction between the vehicle's body and the air touching the vehicle will take place. Such friction renders an aerodynamics effect that noise will be generated because of the gradient in the air pressure field induced by the friction. This pressure field will propagate to generate noise that can be heard at significant distances. Additionally, the contact of grooved tires on pavement surfaces occurring at high speeds creates a substantial sound pressure field as well as engine operations and exhaust systems. This type of noise is called traffic noise since it is originated by moving vehicles. Its acoustic spectrum is of multiple frequencies. The majority of the spectrum falls within the frequency range of 250 Hertz and 4000 Hertz[4]. The noise within this frequency range can be easily heard by the human ear, and can cause great discomfort. To control the propagation of this traffic noise, common practice is to build noise barriers along highways so that noise will be contained and absorbed within barriers, and will not propagate to any significant distance.

However, most highway noise barriers are built with pre-cast concrete or concrete blocks/slabs. The study shows that these barriers are of very high acoustic reflectivity (95% and above[5]) and of low sound absorption for the frequency band of highway noise between 250 Hertz and 4000 Hertz. So the effectiveness of concrete noise barriers in controlling vehicle noise is far from being satisfactory.

With the drastic increase in highway traffic in the last two decades, the effort to develop new and better noise-reduction barriers for highways as well as airport and other applications has been intensified. It is predictable that such an intensification will continue because noise poses an increasingly environmental threat. In recent years, some notable progress has been made in this respect. It has been reported a section of polycarbonate noise wall was built in 1996 near Culver City park in Los Angeles, California. The polycarbonate noise reduction panels are developed by Quitite International, a company based in Los Angeles, California, and the panels are made by Lexan® polycarbonate plastic produced by General Electric. In addition, a jet engine testing shelter was installed also by using Lexan® polycarbonate plastic at Albany airport, Albany, New York in 1997 [6]. Another development is the noise barrier system developed by Carsonite International in Early Branch, South Carolina, and the noise barriers are lightweight hollow panels made of tongue-and-groove planks of reinforced composite material filled with crumbed tire rubber. A few sections of Carsonite noise barriers have been built in Long Beach, California. Traditional noise barrier walls have a flat surface. Now new designs are experimented with non-flat surface textures (Figure-1).

Figure-1 Grooved noise barrier walls near Highway 101 in Tempe, Arizona

These newly developed noise barriers exhibit a much better performance than concrete with respect to the capability of sound absorption and transmission loss, but the noise reduction is not the only criterion. In fact, there are other crucial criteria in constructing noise barriers. These criteria include: (1) cost effectiveness, (2) technology maturity, (3) durability, (4) low cost and convenience in installation, (5) low cost and convenience in maintenance and repair, and (6) aesthetics. The conventional concrete noise barriers meet those criteria very favorably. For example, the average cost to build one foot of concrete noise barrier (typical 6 to 8 feet tall) is about $20 ($20/ft). Polycarbonate plastic or composite noise barriers are very costly, and much less competitive in those criteria in comparison to concrete ones. This is why so far the progress made in replacing concrete noise barriers with aforementioned new noise reduction materials is very limited.