09-05-2014, 04:12 PM
A parametric investigation of the performance of T-profiled highway noise barriers and the identification of a potential predictive approach
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ABSTRACT
Although a considerable amount of research has been undertaken regarding the performance of T-profile
noise barriers, the information available to the practicing highway engineer is confusing. For example,
there is a widespread belief that the performance of a top edge, expressed as an insertion loss relative
to that of the simple barrier on which it is mounted, is constant, irrespective of the relative locations
of the source, barrier and receiver. In order to clarify the situation an investigation has been undertaken,
using computer modelling, of the performance afforded by highway noise barriers with T-profile tops
with different acoustic treatments. The relative insertion loss was found to increase systematically with
increasing top width. Although the relative insertion loss afforded by a reflective T-top is small, signifi-
cant attenuation can be obtained with an absorptive top. Examination of the effect on performance of
the locations of source and receiver relative to that of the noise barrier indicated that, for source and
receiver locations typical of those experienced for highway noise barriers, the relative insertion loss
for a given width of T-top was a function of (a) the path difference between sound travelling to the recei-
ver via the barrier top and direct sound from the source to the receiver and (b) the barrier height. Plots of
relative insertion loss versus the path difference, normalised with respect to barrier heights, for a range of
T-top widths and absorbent treatment, resulted in a collapse of data around well defined trend lines
which offer the potential of being developed into a prediction method.
Introduction
Modifications to the top edges of noise barriers, first investi-
gated by May and Osman [1,2], have been shown to be capable
of increasing noise attenuation over that of a simple barrier of
the same height. Whilst the early work building upon that of
May and Osman was concerned primarily with simple devices such
as the T-profile and multiple edged treatments [3,4], more recent
work has concentrated on more complex devices such as the use
of Quadratic Residue Diffusers [5,6]. Work has also been carried
out on the optimisation of treatment to multi edge barriers and
T-tops [7,8] and also the basic shapes of noise barriers [9].
The interest in the acoustic research community has been lar-
gely focused on demonstrating the potential of a variety of sophis-
ticated top edge devices [10–12]. However, there is continued
interest in the potential use of relatively simple practical top edge
devices based upon the T-profile such as that devised by Garai and
Guidorzi [13]. However, the highway engineer seeking to employ a
T-profile is faced with a limited amount of information, much of it
confusing, on which to base his or her design decisions.
Identification of relevant parameters
The performance of a simple highway noise barrier is deter-
mined by diffraction of sound from source to receiver via the top
edge of the barrier. Early theoretical work by Redfearn related
the attenuation provide by a simple barrier at a point in its shadow
zone to the angle between the direct sound ray from the source to
the barrier top and the diffracted ray from the barrier top to the re-
ceiver location [18]. However, practical predictive techniques tend
to be based upon the work of Maekawa in which the attenuation is
a function of the Fresnel Number given by the ratio of the path dif-
ference between sound reaching a receiver via the barrier top and
the direct path between source and receiver [19]. The simple idea
behind the use of profiled top edges on barriers is to increase the
magnitude of the diffracted angle and thus the path difference be-
tween the diffracted and direct sound paths. The resulting geomet-
rical configurations do not lend themselves to theoretical analysis
and hence research into the performance of profiled barrier tops
has largely been by means of scale model studies or computer
modelling using the Boundary Element Method (BEM).
Identification of relevant parameters
The performance of a simple highway noise barrier is deter-
mined by diffraction of sound from source to receiver via the top
edge of the barrier. Early theoretical work by Redfearn related
the attenuation provide by a simple barrier at a point in its shadow
zone to the angle between the direct sound ray from the source to
the barrier top and the diffracted ray from the barrier top to the re-
ceiver location [18]. However, practical predictive techniques tend
to be based upon the work of Maekawa in which the attenuation is
a function of the Fresnel Number given by the ratio of the path dif-
ference between sound reaching a receiver via the barrier top and
the direct path between source and receiver [19]. The simple idea
behind the use of profiled top edges on barriers is to increase the
magnitude of the diffracted angle and thus the path difference be-
tween the diffracted and direct sound paths. The resulting geomet-
rical configurations do not lend themselves to theoretical analysis
and hence research into the performance of profiled barrier tops
has largely been by means of scale model studies or computer
modelling using the Boundary Element Method (BEM).
The predictive method
It can be seen from Figs. 9–11 that for each barrier height, the
additional insertion loss corresponding to the three different T-
top treatments follow distinct trends which are almost indepen-
dent of the source location. The trends are approximately linear
apart from the highest path differences which correspond to source
and receiver locations very close to the barrier.
Whilst the data shown in Figs. 9–11 could be the basis of a
design method, the possibility of further simplification was ex-
plored. From examination of Figs. 9–11 it can be seen that the
magnitudes of both the basic path differences and relative inser-
tion losses plotted for the higher barriers are greater than those
for the lower barriers. Thus, the effect of plotting the relative
insertion loss as a function of the ratio of the basic path differ-
ence to barrier height was investigated and the results are
shown in Fig. 12.
Conclusions
Although a considerable amount of work has been undertaken
regarding the performance of T -profile noised barriers, the infor-
mation available to the practicing highway engineer is confusing.