06-12-2012, 11:38 AM
Sound Walls: Absorptive versus reflective design and effectiveness
Absorptive versus.pdf (Size: 286.13 KB / Downloads: 208)
ABSTRACT
The overlap of commercial development and urban residential sprawl has created an intense awareness of noise in America, and a demand for better noise abatement practices. The primary noise sources which elicit the most fervent public resistance are road & traffic noise, and commercial developments including the explosive trend in Big Box stores. Sound barrier walls have been one of the most common and effective abatement treatments for such applications. Due to the availability and relatively low cost, reflective materials like concrete, brick or block have been the traditional manufacturing components of sound walls. As the public’s knowledge of noise and noise treatments has evolved, however, so has its demand for more efficient sound wall performance. As a result, sound walls comprised of absorptive materials have grown in popularity amongst architects, developers, contractors and the general public. Thus there is an ongoing, vigorous discussion on the differences between absorptive and reflective sound walls, and which type is best suited for specific applications.
INTRODUCTION
Sound barriers are an effective means to reduce the noise impact from sound sources affecting sound-sensitive receivers. Common sound sources include roads & highways, retail and big-box developments, mechanical & hvac equipment, construction sites, etc. Receivers may include homes or apartments, schools, hospitals, office buildings or even public parks. When noise becomes an issue between such sources and receivers, the use of sound barriers may be an ideal solution.
SOUND BARRIERS – REALISTIC EXPECTATIONS
Although often overlooked, sound barriers can be an effective sound attenuation and noise reduction option. Sound barriers are most effective at mid- and high-frequencies, while low frequency sounds may require the use of longer and taller sound walls for mitigation.
While the sound insertion loss of a sound barrier can be limited, it can be often optimized to provide sufficient reduction of the offensive sounds. The height and length relationship of sound barriers is well documented. At a minimum, the sound barrier should at least block the line-of-sight between the sound source and the receiver. Additionally, the sound waves that travel around the ends and over the top of the sound barrier can be significant, as well as the sound waves reflecting off of other nearby buildings and structures as shown in Figure 1.
BENEFITS OF SOUND ABSORPTION ON SOUND BARRIERS
A key factor that is often overlooked on sound barrier selection is the effect of the surface design on overall performance. Most common building materials such as wood, metal and masonry have hard surfaces and thus reflect sound. i.e. they are considered “Reflective” barriers. Thus when sound strikes the surface of a reflective barrier, some energy is transmitted through the wall but the bulk is reflected back in the general direction of the noise source. Depending upon the roughness and shape of the surface, (and the wavelength of the sound), the sound may be fractured in different directions.
As with interior building materials, the use of sound absorptive materials in a sound wall can be beneficial in eliminating unwanted noise. Additionally, the physical geometry and location of the barriers can impact sound mitigation. For example, having two reflective sound walls – one on each side of a roadway – forms a “sound canyon” resonating with reflective sound from and between each wall, see Figure 2. The same configuration using absorptive sound walls eliminates such reflected noise. This is an obvious example of an application where the use of an absorptive sound wall should be considered.
Service Drive and Roadway Sound Barriers
Consider the placement of a sound barrier between some houses and the back of a shopping center, see Figure 3. The drives behind the stores are often used for deliveries by medium “bob tail” trucks and heavy delivery or even over the road “semi” trucks with tall side trailers. The truck engine and running gear are perhaps at a nominal 6’ tall but the trailers are 10 to 12’ tall. Thus as truck moves through one of the drives the sound reflects between the side of the truck and trailer and the sound barrier wall. Sound travels at over 1,000 fps so there will be multiple reflections of sound between the two that produces a reverberant sound buildup. Thus the sound levels are increased and also the height of the sound source is effectively increased.
Mechanical Equipment Noise
Consider the placement of mechanical and hvac equipment such as air cooled outdoor chillers, cooling towers, and emergency engine/generators. Often the pieces of equipment are placed behind or beside an industrial, hospital, educational or commercial building. This equipment is usually close to a property line. When residential homes and apartments are adjacent to such commercial property, specific (low) sound levels are mandated due to zoning regulations. Sound level limits in the 45 to 50 dBA range at night are not unusual. Many times simply meeting zoning requirement is not enough to eliminate nuisance complaints from neighbors, so sound levels approaching the general background sounds are desired.
In many cases, screen walls are typically used to hide the equipment, see Figure 5. Since there is a significant amount of sound reduction needed, these walls also need to serve double-duty as a sound barrier. Screen walls comprised of reflective materials like concrete, metal, wood or brick will often create sound buildup in the receiver’s area due to sound reflecting off of the screen walls and the sides of the building, which are typically reflective and much taller than the screen wall.