Page 432 - Design of Reinforced Masonry Structures

6.82 CHAPTER SIX

3. Acquisition of property rights for installation or construction of noise-abatement barri-
ers or devices
4. Installation or construction of noise barriers or devices (including landscapes for aes-
thetic purposes), whether within or outside the highway right-of-way
5. Acquisition of property to serve as a buffer zone to preempt development that would be
adversely affected by the noise

Of these, installation and/or construction of noise barriers are the abatement measures
most commonly used.
6.10.3.2 Advantages of Concrete Masonry Sound Barrier Walls Widespread use of
concrete masonry as the most commonly used material for noise barrier walls can be cred-
ited to the many desirable properties and features that it possesses. These include design
flexibility, structural capability, durability, and low initial and life cycle costs. A unique fea-
ture of concrete masonry is its unsurpassed aesthetic attribute in that it is available in many
colors, shapes, and surface texture unlike any other material. Sound barriers constitute a
very visible component of highway, and concrete masonry can be advantageously used with
artistic treatment and patterns to enhance highway aesthetics.
But the major advantage of concrete masonry is its greater sound-absorbing capability
than other materials. In other words, concrete masonry sound barriers are more effective in
total noise reduction than those of other materials. Concept of noise reduction by a barrier is
discussed in NCMA TEK-13-3 [6.35], which is summarized here. Effectiveness of a sound
barrier is measured from a quantity referred to as “insertion loss” which is the difference
between the sound level before and after a barrier placed next to a highway. Insertion loss
has five components:
1. Barrier attenuation due to the diffraction of sound waves over and around a barrier
placed in the line-of-sight plane between the source and the receiver
2. Transmission loss of sound through the barrier
3. Reductions in barrier attenuation resulting from multiple reflections caused by double
barriers
4. Shielding attenuation from other barriers between the source and the receiver
5. Loss of excess attenuation already received from soft ground cover
Essentially, a sound barrier acts as an absorber of acoustical energy. Its effectiveness
may be significantly compromised when this energy is permitted to transmit through it to
the receiver. The amount of acoustical energy that can transmit through a barrier depends
on several factors such as the density and stiffness of the barrier material, angle of incidence
of sound, and the frequency spectrum of the sound. The ability of a material to transmit

noise is commonly rated by a quantity called transmission loss (TL), which is related to
the ratio of the incident acoustical energy to the transmitted acoustical energy. For high-
way noise sources and their typical spectral content, the TL of common barrier materials
increases with increasing surface weight of the material. As a general rule, the TL should
be at least 10 dBA above the attenuation resulting from diffraction over the top of the bar-
rier to ensure that barrier noise reduction will not be significantly affected by transmission
through the barrier (less than 0.5 dBA). Typical TL values of common materials are given
in Table 6.10.

6.10.3.3 Design Considerations Highway sound barrier walls may be designed as
either cantilever type as discussed in the previous section or as pier and panel walls. The