As metropolitan areas expand, sound-barrier walls continue to be the most popular solution to traffic noise. Masonry dominates the sound-wall market in certain geographic areas, such as California and Michigan, while facing stiff competition elsewhere, particularly from precast concrete.

Because of its density, masonry is highly effective at reducing noise transmission, but its advantage over other materials lies mainly in its potential for beauty, its flexibility, and its economy, especially when life-cycle costs are considered.

Sound travels in a direct path. A noise-barrier wall between the source and the receiver diffracts the sound over the top of the wall, reflects it away from the receiver, and transmits it through the wall.

Noise reduction is a function of wall height and the distances between the source, barrier, and receiver. The weight and stiffness of the barrier material also determine what the receiver hears. "Very heavy walls stop noise best," according to Michael Schuller, president of Atkinson-Noland & Associates, Boulder, Colo., and a technical consultant with the Rocky Mountain Masonry Institute (RMMI). But caution must be taken when using the reflective properties of hard materials. "If you build a wall on just one side of the highway, you create more noise on the other side," observes Michael Chusid, an architectural consultant with Chusid Associates, Studio City, Calif. Consequently, sound walls should be erected on both sides of a road.

However, each situation must be evaluated acoustically before a sound-barrier solution is proposed. For example, even when noise barriers are built on both sides of a highway, "hard, reflective surfaces can make the sound worse for people who live on the opposite side if their houses are built on elevated terrain," says David Woodham, a principal of Atkinson-Noland & Associates.

The surface texture of a barrier also affects its performance in reducing noise.

The two most common structural systems for masonry noise barriers are the cantilever wall and the pier-and-panel wall. Both systems are economical, each having advantages and disadvantages.

Wood and, in recent years, precast concrete have been masonry's main competitors in the sound-wall market. Masonry consistently has better long-term value, requiring much less maintenance over time while retaining its ability to abate noise effectively. Precast concrete, like masonry, offers excellent acoustical performance and durability; but masonry normally has an aesthetic edge, offering the widest variety of colors and textures. Precast concrete often seems to cost less, but that is largely because masonry walls are more likely to over-designed.

When masonry is specified for noise barriers, concrete masonry is generally selected because it is less expensive than clay brick. But the difference in price diminishes with the addition of architectural features, such as special textures and integral color. In addition, some municipalities and homeowners' groups favor the look of brick, which often blends in with area homes.

The Michigan Department of Transportation (MDOT) prefers to specify concrete masonry that looks like clay brick. Having a strong relationship with the state department of transportation has certainly paid off for Michigan's masonry industry.

In Colorado, the active promotion of masonry sound walls is more recent but no less focused and determined, thanks to the Denver-based Rocky Mountain Masonry Institute. Masonry has been used extensively for sound-barrier walls in Colorado only during the past 2 or 3 years. Many times masonry initially costs more simply because it is over-engineered.

To help CDOT better understand masonry, RMMI and Atkinson-Noland & Associates developed a slide presentation called "Effective & Economical Design of Masonry Sound Walls." The presentation describes the benefits and costs of four different masonry sound-wall types, showing efficient, cost-effective sample designs for 9 foot 4 inch walls, which could be easily compared. It covers two conventional wall systems: the cantilever wall and the pier-and-panel wall and introduces two alternative systems: a post-tensioned CMU cantilever wall and a prefabricated post-tensioned brick wall.

As a result of the presentation, CDOT has agreed to let a local masonry contractor build a post-tensioned CMU sample section of a conventional cantilever wall.

California has hundreds of miles of sound-barrier walls, the most in the nation, and roughly 85% are masonry. The typical sound wall in California is 8-inch reinforced concrete masonry, integrally colored, laid 6 to 16 feet in height, with seismic loads controlling the design. "Masonry is still the leader, but there is a movement afoot to have something other than a masonry wall," civil engineer Dan Kirkland says. "Some of the proprietary systems have a lighter weight material. Some include polyurethane." Several boast environmental benefits-for example, a very thin metal enclosure in which shredded rubber is placed.

Of the 27 systems he has reviewed, only three have been approved, but one looks especially promising: an innovative system of lightweight but high-strength polycarbonate block modules that resemble glass block made by QUILITE Noise Barriers. But this product and the other proprietary systems are still experimental.

Masonry, in contrast, is a proven top performer in sound-barrier walls: beautiful, durable, and effective. The industry needs to be aggressive like RMMI to make sure masonry's share of the sound-wall market continues to grow.