Noise Abatement Information
Technical Note 45
Brick Masonry Noise
Barrier Walls
Feb. 1991
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.
INTRODUCTION
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.
ACOUSTICAL CONSIDERATIONS
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
FIG. 1
Noise Path With Barrier Wall
FIG. 2
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
FIG. 3
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.
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