Noise Abatement Information
Technical Note 45
Brick Masonry Noise Barrier Walls
Abstract: Because our national highway system has grown significantly over the last few decades, public awareness of traffic noise on neighborhood communities has increased. Neighborhood associations and governmental bodies look for ways to reduce traffic noise without adversely affecting the surrounding environment. A solution to this problem lies in brick masonry noise barrier walls. Brick masonry noise barrier walls can easily blend into the environment and give residential communities protection from unwanted highway noise.
Key Words: acoustics, brick, noise barrier walls.
Continued growth of our national highway system combined with an increase in public awareness of environmental issues has focused on a need to evaluate the impact of traffic noise associated with highway systems on neighboring communities. When noise levels exceed acceptable limits, community action generally alerts governmental bodies to the problem or potential problems. Governmental bodies then investigate measures to prevent or alleviate noise problems.
The severity of the noise and the stage at which the problem is identified determine the measures available to reduce the impact of highway noise. Measures to alleviate highway noise include traffic controls and regulations, modification of the highway configuration, land-use planning and zoning, and brick noise barrier walls.
When new highway systems are in the planning and design stages, a comprehensive analysis of and consideration to noise abatement measures can be given. However, when existing highway systems are renovated or if restrictions are placed on the routing of new highway systems or use of adjacent land, the most practical solution to noise control may be the use of noise barrier walls to isolate the highway noise sources from the surrounding communities.
Three major types of noise barriers are currently being used in the United States: earth berms, walls and berm-wall combinations. Of these three, the noise barrier wall is typically the most common means of achieving noise abatement and is the primary topic of this Technical Notes.
This Technical Notes, the first in a series, addresses acoustical, visual, structural, construction, detailing and maintenance considerations of brick masonry noise barrier walls. The other Technical Notes in this series addresses the structural design of brick masonry noise barrier walls.
To understand the function of a noise barrier wall or how the wall reduces the noise level perceived by a receiver, it is necessary to discuss some of the fundamental principles involved in sound propagation and noise reduction.
When there are no obstacles or barriers between highway noise sources and receivers, sound travels in a direct path from the source to the receiver (Figure 1). When a noise barrier wall is placed between the noise source and the receiver, the barrier disperses the sound along three paths: a diffracted or bent path over the top of the wall, a reflected path away from the receiver and a transmitted path through the wall (Fig. 2).
Direct Noise Path
Noise Path With Barrier Wall
Diffraction of sound over the top of the wall produces a shadow zone behind the barrier. The boundary of this shadow zone is outlined by a straight line drawn from the noise source over the top of the barrier wall (Fig. 3). All receivers located within the shadow zone will experience some degree of sound attenuation. The amount of reduction or attenuation is directly related to the diffraction angle Ø-. As this angle increases, the barrier attenuation increases. Thus, barrier attenuation is a function of the wall height and the distances between the source, barrier and receiver. Two other factors also affect the amount of attenuation: the sound transmission characteristics of the material from which the barrier is constructed and the length of the barrier.
Noise Barrier Shadow Zone
The sound transmission characteristics of a material are related to its weight, stiffness and loss factors. The sound transmission characteristics of materials can be assessed and compared by means of transmission loss values. The sound transmission loss is related to the ratio of the incident noise energy to the noise energy transmitted through the material. Typically, transmission loss values can be expected to increase with increasing square foot surface weights of barrier materials. Table 1 lists the transmission loss values at a frequency of 550 hertz (Hz) for materials commonly used in noise barrier wall construction. 550 Hz is the accepted frequency used to determine the transmission loss of highway noise barrier wall materials. As a general rule for design, the transmission loss value should be a minimum of 10 decibels (dB) above the attenuation resulting from the diffraction over the top of the barrier. The transmission loss values for brick masonry are at the higher end of the range and sound transmission through a brick barrier will not significantly affect the attenuation. However, when less massive materials are used, the transmission loss values may not be adequate and the noise reduction provided by the barrier can be severely affected.
The actual acoustical design of a barrier system to determine the length and height requirements are beyond the scope of this Technical Notes. A detailed discussion of noise barrier acoustical design procedures and considerations can be found in Reference 1.