Field of the invention

This invention relates to a noise barrier, particularly of the type for use at road sides to reduce traffic noise.

Background to the Invention

Traffic noise barriers are known for use along the edges of roads so as to shield noise sensitive areas from traffic noise from the road.

Many different forms of noise barriers have been constructed in the past, using a wide variety of materials. The simplest takes the form of a vertical, flat topped wall, or a specially designed wooden fence. Such a barrier faces towards a source of noise, and acts to block the passage of sound waves from the source to a noise sensitive area beyond the barrier.

However, sound waves will diffract around the top edge of such a barrier. This means that a certain amount of noise is still able to pass around the barrier to the noise sensitive area beyond.

Also known are noise barriers of more complex cross-sectional shapes than the simple vertical screen. Some, for instance, take the form of a vertical screen with a horizontal screen across its top, giving an overall T-shaped cross-section to the barrier. Others which have been investigated in the past have an upturned V-shaped cross-section, comprising two angled screens connected at the apex of the V; or have instead an approximately semi-circular or a part-trapezoidal cross- section.

Different barrier geometries give different levels of sound attenuation. It is desirable to improve attenuation, and to seek a barrier which will perform more efficiently than

currently available models. Thus, it is an aim of the present invention to provide a noise barrier, particularly for use as a traffic noise barrier at road sides, which can reduce noise to a greater extent than is possible using conventional noise barriers.

Statement of the Invention

According to the present invention there is provided a noise barrier comprising a main screen, which in use faces substantially towards a source of noise so as to attenuate the noise level on the opposite side of the barrier from the source, and an additional screening element connected to the main screen, which additional element also faces, in use, substantially towards the noise source and is displaced from the main screen in a direction either towards or away from the noise source, thus serving further to attenuate the noise level on the opposite side of the barrier from the source, the main screen and the additional screening element both comprising substantially sound-opaque regions which extend downwardly from their upper edges over at least the height of the shorter of the main screen and the additional screening element, and the lower edge of the shorter of the two being higher, in use, than the lower edge of the taller of the two.

Where the relative lengths of the main screen and the additional screening element are referred to in the above definition, these lengths are the (in use) vertical lengths of the main screen and additional element.

Thus, in the barrier of the invention, an additional screening element or diffracting surface, is provided, as well as the single surface conventionally used in noise barriers. The additional screening element is connected to the main screen rather than having its own separate foundations, which yields practical advantages, in terms of production and use, over the use of a series of single-screen barriers one behind the other to improve attenuation. Both the main screen and the additional screening element face substantially towards the

noise source, in use, and hence are usually substantially parallel to one another. Such a construction has been found greatly to improve the efficiency of the barrier in terms of noise attenuation.

The use of one shorter and one taller screening element, with the lower edge of the shorter being higher, in use, than the lower edge of the taller, has been found to yield, in various constructions of noise barrier, an improved level of noise attenuation. The shorter of the two elements has a much greater impact on noise from the noise source when its lower edge is higher than that of the taller.

Preferably, the additional screening element is shorter than the main screen.

Either or both of the main screen and the additional screening element may have an acoustically absorbing surface facing towards the noise source in use. Suitable acoustically absorbing materials are known, and can be used to coat the relevant surfaces of either or both of the screens so as further to reduce the level of noise capable of passing beyond the barrier. With the application of a suitable sound absorbing material and the introduction of an additional noise screening element, as provided by the present invention, the efficiency of a noise barrier (in terms of the noise attenuation which it achieves) can be improved by an order of three dB(A) .

A noise barrier in accordance with the invention may comprise more than one additional screening element, a preferred number of additional screening elements being two. Whilst there may be more than two, this might lead to practical problems in construction, and to an unacceptable width in the barrier as a whole.

Where the barrier comprises two additional screening elements, both are preferably shorter than the main screen. The lower

edges of both additional screening elements are preferably higher, in use, than the lower edge of the main screen.

In one type of noise barrier according to the invention, the upper edge of the shorter of the main screen and the additional screening element is no higher than the upper edge of the taller of the two. A more preferred barrier comprises two additional screening elements, both of which are shorter than the main screen, the upper edges of the main screen and the two additional screening elements being, in use, at substantially the same height.

It should be noted that it is the position of the highest edge of the noise barrier as a whole which has the greatest effect on noise attenuation from a source. Where any screening elements shorter than the overall height of the barrier are positioned with their upper edges too far below the highest edge of the barrier, the effect of such shorter screening elements on the overall attenuation is greatly reduced. However, where the upper edge(s) of the additional screening elemen (s) coincide(s) with that of the main, taller, screen, a significant improvement in attenuation, compared to that found for a single noise screen, is achieved.

The additional screening element or elements are preferably displaced from the main screen, in use, in a direction either towards or away from the noise source. Thus, the additional screening element or elements are placed either in front of the main screen (i.e. closer to the noise source) or behind it (i.e. further away from the noise source) . The additional screening element or elements are preferably substantially parallel to the main screen. In one preferred embodiment, two additional screening elements are arranged one in front of and one behind the main screen, forming a "trident"-like overall construction. Alternatively, all of the additional screening elements may be in front of, or all behind, the main screen, preferably behind (ie, away from the noise source) for safety reasons concerned with vehicle impact on the road. In this

latter situation, the main screen may need additional reinforcement, due to reduced stability when more than one additional screening element is located to the same side of the main screen.

Although not all of the additional screening elements need to have acoustically absorbing surfaces, it is preferred that at least one of them does.

One or more of the additional screening elements (preferably the one nearest to the noise source in use) may be angled slightly so that the face which it presents towards the noise source in use is tilted upwardly. This would reduce the amount of noise reflected back from the noise barrier towards the source, which would be desirable if, for instance, a noise sensitive area existed beyond the noise source (e.g. on the opposite side of a road to that on which the noise barrier were to be positioned in use) .

The main screen and additional screening element(s) may be connected together by means of one or more brackets (e.g. metal brackets) bolted to both of the screens. Preferably, a series of such brackets are bolted to both the main screen and the additional screening elements at appropriate points (for instance, at 2m intervals) along their lengths.

Alternatively or additionally, cross-members may be used to connect the main screen and the additional screening elements. This provides at least one three-sided enclosure, of substantially U-shaped cross-section, as a noise diffracting member. The cross-members will typically be in the form of substantially horizontal screens, perpendicular to and positioned between the upstanding main and additional screens. They may also have acoustically absorbing surfaces. The absorbing surfaces are preferably on the internal surfaces of the enclosure of U-shaped cross-section.

The main screen will typically take the form of a vertical screen made from a suitable material, such as one of those

used for conventional noise barriers. For instance, the screen may take the form of a series of slotted posts of steel or concrete, between which are placed sound-absorbing panels made from wood, concrete or metal. Examples of commercially available absorbing panels include hollow metal box panels having a perforated surface and containing an acoustically absorbing material; and moulded panels made from a composition of wood shavings and cement.

The additional screening element or elements may be made from the same or similar materials to those from which the main screen may be made.

The noise barrier will typically be of total height approximately 2m, and the thickness of each screen between about 10 and 100 mm, depending on the material from which the screen is constructed. The height of the additional screening element or elements might typically be around 0.5m. The separation between the main screen and its nearest additional screening element would conveniently be about 0.5m.

A noise barrier according to the present invention may be constructed by adding an additional screening element or elements to an existing, single-screen noise barrier.

The present invention will now be described by way of example only and with reference to the accompanying illustrative drawings, of which:

- Figure 1 shows schematically how noise may be attenuated using a series of separate screens;

- Figure 2 is a section through a noise barrier in accordance with the invention;

Figure 3 is a part plan view of the noise barrier shown in Figure 2;

Figures 4-7 show, in section, alternative noise barriers in accordance with the invention;

Figures 8-10 show the levels of noise attenuation which can be achieved using barriers in accordance with the invention;

Figures 11 and 12 show, in section, further designs of noise barrier in accordance with the invention;

Figures 13 and 14 are graphs showing the levels of noise attenuation achieved using two of the barriers shown in Figure 12;

- Figure 15 shows, in section, another noise barrier in accordance with the invention;

Figure 16 is a graph showing levels of noise attenuation achieved using the barrier shown in Figure 15;

- Figure 17 is a section through yet another noise barrier in accordance with the invention;

Figures 18 and 19 are graphs showing the variation of noise attenuation with noise frequency for a noise barrier in accordance with the invention;

Figure 20 shows an experimental set-up used to test noise barriers in accordance with the invention;

Figure 21 shows dimensions and relative insertion losses for barriers tested using the set-up shown in Figure 20;

Figure 22 shows cross-sections through barrier panels used in the tests;

Figure 23 shows the normalised A-weighted one-third octave spectrum for typical highway traffic noise, used in the analysis of results taken during the tests;

- Figures 24 and 25 show graphically results obtained from tests on some of the barriers shown in Figure 21; and

- Figure 26 shows spectra for measured and calculated insertion loss for a barrier in accordance with the invention.

Detailed Description of the Drawings

Figure 1 shows schematically how noise from a source region 30 is attenuated by, in this case, a series of conventional single-screen noise barriers 31. Each barrier 31 serves to block the passage of sound waves from the source to the region beyond the barrier, as shown. It will also absorb the sound waves to a greater or lesser extent, depending on the material of which the barrier is made and whether or not it is coated with an acoustically absorptive material.

However, the top edge of each screen also diffracts the sound waves to a certain extent, causing them effectively to bend around the edge. Thus, a certain amount of noise does inevitably pass beyond each screen. It is therefore useful to provide a series of screens, so that any noise passing beyond the first is further attenuated by the second, etc.

The provision of several separate noise barriers, however, as shown in Figure 1, is cumbersome, costly and generally inconvenient. Each barrier 31 needs its own separate foundation. Whilst increasing the number of barriers in the series improves attenuation, it also increases the amount of space taken up by the barriers, not always practical in real- life situations such as at roadsides.

Figure 2 shows, in section, a typical noise barrier in accordance with the invention. The barrier comprises a main screen 35 and a parallel facing additional screen 36. The additional screen is connected to the main screen by means of a metal bracket 37, bolted to both of the screens.

The screens are made from conventional materials. For instance, the main screen 35 may comprise a series of slotted steel or concrete posts between which are placed absorbing panels. The panels may be made of wood, concrete, metal or other suitable materials.

Both the main screen 35 and the additional screen 36 are of a length (along the direction perpendicular to the plane of Figure 2) which is appropriate for the particular situation in which they are to be used. A series of brackets, such as 37, would typically be used to connect them together along their lengths, as seen in the part plan view of the barrier, shown in Figure 3. These brackets might typically be bolted to the two screens at 2m intervals along their lengths.

In the barrier shown in Figures 2 and 3, the provision of an additional screen serves to improve noise attenuation of a conventional single screen barrier, in the manner illustrated in Figure 1. However, the additional screen is in this case connected to the main screen, and does not have separate foundations.

Figure 4 shows another noise barrier in accordance with the invention. This comprises a main vertical screen 1, which in use faces substantially towards the noise source 2 whose position is indicated by the arrow. Screen 1 blocks the passage of noise from source 2, thus reducing the level of noise able to pass through the barrier to the noise sensitive area beyond.

The barrier includes an additional screen 3, which serves as an additional block against noise from source 2. This further reduces the level of noise reaching beyond the barrier. Any sound able to pass beyond screen 3, for instance due to diffraction around the top edge of the screen, is further blocked by main screen 1.

The height of main screen 1 is in this case 2m, that of additional screen 3 being l . The two screens are placed lm apart.

The main and additional screens 1 and 3 have acoustically absorbing surfaces, indicated by the dashed line 4. Substantially horizontal cross-member 5, also having an acoustically absorbing surface, connects additional screen 3 to main screen 1. There is thus defined a three-sided noise diffracting member, of approximately U-shaped cross-section, made up of elements 3, 5 and 1.

Cross-member 5 serves to prevent sound from entering the space between screens 1 and 3 from below. It also provides an extra sound absorbing surface, thus improving attenuation by the barrier. The absorbing surfaces 4 are inside the U-shaped diffracting member, so as to absorb any reflected sound within this inner region.

Figure 5 shows an alternative noise barrier in accordance with the invention, which comprises a main vertical screen 6 and an additional screen 7. These are connected together by means of metal brackets (not shown) such as 37 in Figure 2. The brackets are bolted to both the main screen 6 and to additional screen 7 at appropriate points along their lengths.

The use of brackets to connect together screens 6 and 7 means that there is no U-shaped diffracting member such as that on the barrier of Figure 4. This may reduce attenuation to a small extent. However, it does avoid the accumulation of debris and moisture, which would tend to occur in the U-shaped member of the barrier shown in Figure 4.

As in the noise barrier of Figure 4, the additional screen 7 is parallel to main screen 6 and displaced therefrom in a direction towards the noise source 8. Both screens face towards this source, so as to reduce the level of noise from the source passing to the noise sensitive area beyond the barrier.

In the noise barrier shown in Figure 5, only the main screen 6 has an acoustically absorbing surface, indicated by dashed line 9.

Another embodiment of the present invention is shown in Figure 6. Here, the noise barrier comprises a main vertical noise screen 10 and two additional screens 11. The screens are all substantially parallel, the additional screens being placed one in front of and one behind the main screen 10 relative to noise source 12.

Cross-members 13 link the two additional screens 11 to main screen 10. Two three-sided noise diffracting members, substantially U-shaped in section, are thus defined. The screens have acoustically absorbing surfaces 14 at the inner surfaces of these two U-shaped elements.

The alternative noise barrier shown in Figure 7 comprises a main screen 15 and additional screens 16 and 17, positioned in similar fashion to additional screens 11 in the barrier shown in Figure 6. In this particular case, main screen 15 has an acoustically absorbing surface 18 and additional screen 17 has an acoustically absorbing surface 19. Both surfaces 18 and 19 face towards the noise source 20.

Screens 15, 16 and 17 are connected together by means of appropriately positioned brackets (not shown) .

Figure 8 is a table showing the results of investigations carried out on various designs of noise barrier, including some in accordance with the invention. Figures 9 and 10 are continuations of the table in Figure 8.

The cross-sectional shapes of designs (a) - (s) of noise barrier are shown in the left-hand column of the table, together with their dimensions in metres. Designs (f) , (g) , ( ), (i), (j), (k), (1), (m), (p), (q) , (r) and (s) fall within the scope of the present invention. Some, as indicated in the table, are coated with acoustically absorbing surfaces.

The table includes figures for insertion loss (a measure of noise attenuation) , in decibels, for each barrier at six different positions, 1-6, of a sound receiver relative to the barrier. These positions are defined as follows:

The position of the barrier relative to the noise source remained constant throughout the investigations.

The table also includes figures for the mean insertion loss (mean IL) for each barrier, over the six different receiver positions, and for the difference in mean insertion loss (AIL) between the value for that barrier and the value for the conventional, single-screen barrier (a) . The positive values of AIL for those barriers in accordance with the invention show that improved efficiency, in terms of noise attenuation, can be achieved using such barriers as alternatives to those previously available.

Figure 11 shows further designs of noise barrier, 1-4 and 7- 10B, in accordance with the invention. The cross-sectional shape and dimensions of each barrier are shown, together with values for the average insertion losses (Av IL) in decibels measured for each. All measurements were taken with the noise source and the barrier positioned on hard ground, such as a typical road surface.

Figure 12 shows yet further designs of noise barrier, (1) - (5) , of which (2) - (5) are constructed in accordance with the invention. Again, the cross-sectional shape and dimensions are shown for each. Acoustically absorbing surfaces are

labelled 40. Each barrier is shown positioned 15m from a noise source 41 (horizontal distance - not shown to scale) .

Design (1) of barrier is a conventional design comprising two separate noise screens placed one behind the other relative to source 41. Distance D between the screens may be either 0.5 or lm. The screens may have acoustically absorbing coatings on all of their surfaces, or on none of their surfaces (rigid barriers) , or just on the two facing surfaces, as shown. The barrier was tested on both hard ground and grassland.

Designs (2) - (5) each comprise a main noise blocking screen and at least one additional screen connected thereto (usually by means of brackets, not shown in Figure 12) . These were all tested on rigid ground only. The heights H of the additional screens vary between 0.25 and 1.5m, as indicated to the right of the drawings.

It should be noted that design (2) incorporates a U-shaped enclosure, made up of the main screen, the additional screen and a horizontal cross-member (shown hatched) connecting the two.

For the barrier design (1) in Figure 12, the noise level predictions are as follows:

D (m) 0.5 1.0

RIGID 1.38 (1.03) 1.60 (1.47) BARRIERS

ABSORBING BARRIERS (all 2.12 (1.67) 2.92 (2.32) surfaces)

INTERIOR SURFACES ONLY 1.44 2.28 ABSORBING

The figures given are of mean insertion loss, in decibels, relative to the mean insertion loss for a single rigid 3m high barrier placed a horizontal distance of 15m from the same noise source. Results are given where appropriate for both hard ground and soft ground (grassland) . The figures for soft ground are parenthesised.

Figures 13 and 14 are graphs of mean insertion loss (dB) vs. height H of the additional screen, for barrier designs (2) and (3) in Figure 12. The insertion loss is again measured relative to that for a single rigid barrier under the same noise conditions. Figure 13 shows how insertion loss varies with H when the distance D between the main screen and the additional screen is lm; Figure 14 when D=0.5m. In each case, results are shown for completely rigid barriers (i.e. no absorbing surfaces) and for those with absorbing coatings on their interior surfaces.

On the whole, insertion loss increases with height H of the additional screen. H is of course also equal to the depth of the U-shaped enclosure in design (2) .

The variation of insertion loss with H for designs (3) , (4) and (5) of Figure 12 is illustrated by the following table:

The values given are of mean insertion loss (dB) relative to the mean insertion loss for a single rigid barrier, height 3m, placed a horizontal distance of 15m from the same noise source. Again, the values are greater for greater H. Design (5) is clearly _. superior to design (4), which in turn is superior to design (3) . All three are superior to the conventional single-screen barrier.

For design (3) , "side A" is the inside face of the additional screen, "side B" the inside face of the main screen.

Figure 15 shows in section a noise barrier in accordance with the invention, the noise blocking characteristics of which were investigated for different dimensions of the constituent screens. The barrier comprises a main screen 50 and two additional screens 51 and 52, all connected together by means of brackets (not shown) . Different barriers of this same general shape were considered, having different values for the dimensions D, H and T shown in the figure. In some cases, the surfaces labelled IJ and KL were coated with an acoustically absorbing material; in others, the barrier surfaces were rigid.

The figures for mean insertion loss for the different barrier con igurations are tabulated below. Also included are figures for the mean insertion loss relative to that for. a single rigid barrier under the same noise conditions.

Using the same design of barrier, the effect on insertion loss of decreasing the thickness T of the additional screens 51 and 52, whilst keeping other variables fixed, was also investigated. The results are shown below.

D = 0.5m; H = 0.5m; surfaces IJ and KL were absorbing. Tests .were conducted on both hard ground (e.g. a road surface) and grassland.

As can be seen from these results, reducing T marginally reduces the efficiency of the noise barrier as a whole.

Figure 16 is a graph of the variation of mean insertion loss with the height H of the additional screens 51 and 52, for the barrier shown in Figure 15. For these tests, D was kept at 0.5m, T at 0.2m. The tests were conducted on both hard ground and grassland, using either a rigid or an absorbing central barrier (main screen 50) . Plotted values for mean insertion loss are calculated relative to those for a single rigid barrier under the same noise conditions.

Figure 17 shows in cross-section another noise barrier in accordance with the invention, which comprises a main screen 60 and two additional screens, 61 and 62, connected to it by means of cross-member 63. Both additional screens are

positioned in front of main screen 60 relative to the noise source 64. Dimensions shown are in metres.

The values for mean insertion loss for this barrier were found to be as follows: a) barrier as shown in Figure 17 = 16.1 dB b) as in a) , but with all faces coated with an acoustically absorbing material (σ* = 20,000) = 17.2 dB c) as in b) , but with the absorbing material having σ* = 250,000; test conducted on soft ground = 10.3 dB d) as in a) ; test conducted on soft ground = 9.6 dB

* σ = flow resistivity (Nsm ^"4 )

Figures 18 and 19 show the variation of insertion loss with noise frequency for a noise barrier in accordance with the invention. The cross-sectional shape of the barrier is illustrated in the inset drawings. It comprises a main screen 70 and two additional screens 71 and 72, connected by means of cross member 73 which creates an enclosure 74 of U-shaped cross-section above the main screen 70.

Both experimental and theoretical values for insertion loss are plotted. Figure 18 shows the results for a noise source 75 placed a horizontal distance of 15m from the base of the noise barrier (see inset drawing) . Figure 19 shows the results when the same source 75 is positioned 15m away from the barrier and 0.88m above the ground.

All surfaces of the barrier are rigid and reflective. The tests were conducted on a hard ground surface.

Further Experimental Tests

Full-scale tests of acoustical performance were carried out on a range of promising traffic noise barrier shapes, including many in accordance with the present invention, which had previously been identified by mathematical and scale modelling work.

A test facility was established in order to examine effectiveness under full-scale conditions. This consisted of a 20m length of noise barrier with interchangeable barrier panels, a large flat asphalt surface and a transportable speaker system capable of sufficient output to represent typical traffic noise. Screening performance was measured up to 80m behind the barriers over a flat grassland area and at heights above the ground of 1.5 and 4.5m. It was concluded that the average increase in acoustic screening of 2m high multiple-edge barriers compared with a simple plane reflecting barrier of identical overall height ranged from 2.4 to 2.7 dB(A) depending on detailed design.

The tests measured the resulting noise levels for each barrier option. These measurements were used to determine the performance of each barrier system and allowed comparisons to be made with the performance of simple reflecting barriers of various heights.

Test Method A test facility was constructed on a flat section of grassland free of reflecting surfaces which might interfere with recorded noise levels. A level section 101 (see Figure 20)of asphalt surface (remainder flat grassed area) was laid, approximately 40m long and 11m wide. Adjacent to one edge a 20m long noise barrier 102 was constructed (see Figure 20) . This consisted of nine I-section posts into which could be slotted 0.5 deep panels. Enough panels of different types were available for a 2m high barrier to be constructed which was either fully reflective, absorptive on one side or

absorptive on both sides. The lowest 0.5m of the barrier consisted of a permanent concrete panel with plane reflective surfaces. Post extensions enabled the maximum height to be raised to 3m. In addition, aluminium brackets and clamps were provided so that multiple edge barriers up to 2m wide could be fitted to the barrier.

In Figure 20, 103 indicates speaker positions, 104 the microphones and 105 an anemometer.

Figure 21 shows scale drawings of the designs that were tested, the multiple edge designs being in accordance with the present invention. As in Figure 20, all dimensions shown are in metres.

The noise source consisted of a dual cone speaker system rated at 800W which was capable of delivering noise levels above HOdB at lm from the speaker in the frequency range 100Hz to 3.2kHz. High levels were needed to ensure that background noise did not add significantly to the noise produced by the speaker especially at the largest distance behind the barrier. The aim was to achieve levels at these distances which were 15dB above background for each one-third octave band in the range of interest. By producing random noise at approximately HOdB in each band at lm from the speaker face, this requirement was generally met. It was not possible to produce levels of HOdB simultaneously in all octave bands with the speaker system; instead, it was achieved by broadcasting separately in four broad frequency bands (100 to 200Hz, 250 to 500 Hz, 630Hz to 1.25kHz and 1.6 to 3.2kHz). A reference microphone was positioned lm from the speaker and on the axis to enable adjustments to be made for any variations in speaker output. A specially constructed gauge enabled the exact location to be achieved. Measurements were adjusted to the reference output of HOdB. Random noise in these frequency bands was recorded on a digital tape recorder, and the tape was edited so that 20-second periods of noise in each broad frequency band could be replayed in sequence. The speaker was mounted on a specially constructed trolley, which could be

adjusted for height and inclination, so that it could be readily transported to various positions in front of the barrier.

The noise broadcast from the speaker was sampled for a 16- second period at three microphone positions behind the barrier. The microphones were located at distances of 20, 40 and 80m from the barrier. Separate runs were carried out at microphone heights of 1.5 and 4.5m. The speaker was placed at distances of 5.5 and 7.8m in front of the barrier, and the axis of the speaker was 0.5m above the asphalt surface. These distances correspond to the effective positions of the traffic source with respect to noise barriers sited at the edge of the carriageway on all dual carriageway roads and motorways respectively which are assumed in the standard traffic noise prediction model used by the Department of Transport in the United Kingdom (Department of Transport and Welsh Office, 1988: "Calculation of Road Traffic Noise"; HMSO, London). The axis was aligned accurately so that it was horizontal to the test surface and in line with the three microphones located behind the barrier.

A computer-based sound level meter used for noise measurement allowed the simultaneous monitoring of four microphone channels. The levels in each one-third octave band were saved in a spreadsheet for later analysis.

Because measurements were taken over a period of several months, meteorological variables were also recorded so that, where necessary, the data could be adjusted before making comparisons.

Barrier Options

The barrier options which were tested in full scale were identified from the promising, results of mathematical modelling work carried out. on a wide range of possible designs. Figure 21 shows the cross-section of the designs tested. The main barrier panels inserted between I-section posts were produced commercially and were all 2.5m long, 0.5m

deep and 120mm thick. They were constructed from aluminium sheet in the form of a hollow box with a mineral fibre panel mounted behind the traffic face of the panel. In the case of the sound-absorptive panels, the traffic face was perforated to allow sound to enter the cavity and energy to be largely dissipated in the fibrous material. Figures 22 (a) and (b) show the constructional details of panels which are respectively absorptive on one side only and capable of absorbing sound on both faces. 107 are the perforations in the sound-absorptive panels; 108 is the fibre panel portion.

The side panels were constructed from plywood 12mm thick and several coats of varnish were applied to ensure the surface was acoustically reflective over a wide range of frequencies. It was expected from the mass of the material used that any sound transmitted through the plywood panels would be reduced by at least 15dB, while laboratory test results from the panels manufactured from aluminium have indicated the reduction would be approximately 25dB(A) .

Analysis

Analysis was carried out using a suitable spreadsheet program which allowed the one-third octave levels measured during the four broadcast periods to be adjusted for source strength and then weighted and combined to produce results expected for a typical traffic noise source. The sound pressure levels in each one-third octave band were first normalised for a source level of HOdB at the reference microphone. The levels were then weighted using the normalised A-weighted highway traffic noise spectrum based on measurements taken in various European countries and reviewed within the noise barrier standards committee CEN TC 226/WG6/TG1. This spectrum is shown in Figure 23. The one-third octave levels were then combined to produce an overall SPL (sound pressure level) .

For each of these overall adjusted levels, the averaged wind- speed component was computed in the direction of source to receiver. The components were calculated for each one-second

sample of wind-speed and direction, and averaged over the four 16-second broadcast periods.

Generally, a set of measurements at normal incidence consisting of twelve levels (resulting from measurements at six microphone and two speaker positions) was taken on at least six occasions under different wind conditions, and it was then possible to regress the levels at each microphone position against the averaged wind-speed component to obtain the predicted levels at zero wind component with reasonable accuracy. To avoid excessive wind noise affecting recorded levels, measurements were not taken when wind speeds exceeded 4m/s.

The twelve wind-adjusted measurements obtained from all source/receiver positions were then averaged to yield a single value representing the mean noise level behind the barrier for a standardised noise source. The relative insertion losses of the barrier options were then calculated by comparing these mean levels with the mean levels for the 2m high simple reflecting barrier. The absolute value of insertion loss for a particular barrier was calculated from the difference between the average noise level behind the barrier and the average noise level at the control site without the barrier.

For measurements along the line at 30° from the normal to the barrier the average noise level was calculated from eight noise levels obtained from four microphone and two speaker positions (measurements were not taken at the 80m position) . The predicted value at zero wind component was computed using a similar procedure to that employed for measurements at normal incidence.

Theoretical Model The numerical model used is based on the boundary element method (BEM) and has been described in detail elsewhere (D.C. Hothersall, 1992: Proceedings of the Eurosymposium in Nantes: The Mitigation of Traffic Noise in Urban Areas, 251-262: "Modelling the Performance of Noise Barriers".) It is a two-

dimensional model which in three dimensions is equivalent to an infinite coherent line source, and an infinitely long barrier parallel to the source. Along its length, the barrier is of uniform cross-section and surface covering. The model calculates the wave field behind the barrier at a particular frequency by solving a reformulation of the Helmholtz wave equation in terms of an integral equation. For this purpose, the barrier and ground surfaces are divided into boundary elements of length no greater than L/2 (where L is the wavelength) . The effects of ground cover and absorptive surface treatment of the barrier can also be predicted. Homogeneous, still air conditions are assumed in all computations.

The calculations carried out using the BEM were for the same source, receiver and barrier configurations as the experimental investigations. The same methodology was used to process the results. Further discussions of assumptions and approximations are omitted here, since not directly relevant to the practical performance of the barriers under test.

Results

The performances of the multiple-edge barrier systems at normal incidence, based on the average noise levels at the six measurement positions and two speaker positions, are shown in Table I. Listed are the A-weighted sound pressure levels for the traffic noise source at zero wind component. The measured relative insertion losses compared with a simple reflective barrier are also given in Table I and Figure 21 (figures in circles) .

Generally, the measured results and theoretical predictions for relative insertion losses were found to be in good agreement.

For the multiple-edge barriers, insertion loss improvements at normal incidence ranged from 2.4 to 2.7dB(A). The addition of absorptive material, larger side panels and

increased separation all had relatively small effects on the performance of multiple-edge barriers. These additions had a maximum effect of only 0.3dB(A) at normal incidence.

The performance of selected barrier systems at the six measurement positions with the speaker at 7.8m from the barriers are shown in Figures 24 and 25. At 4.5m above ground, average noise levels are generally higher than those at 1.5m and the decrease in noise level with distance is close to 6dB(A) per doubling of distance. The decrease between 50 and 80m from the barrier at a height of 1.5m follows the trend of the 4.5m measurements. Between 20 and 40m, the decrease at 1.5m is generally less pronounced. These effects are probably due to a combination of changes in path length difference and geometric spreading of the wavefronts. Where the path length difference does not change appreciably with increasing distance, as is the case at the 4.5m height; the decrease is mainly due to geometric spreading. At the lower microphone height, the path difference changes considerably between 20 and 40m. This results in the 20m position being in a significantly deeper acoustical shadow than the 40m position. The spreading of the wavefront produces a decrease but this is offset by the decrease in barrier screening. Generally, for the multiple-edge barriers, the gains in insertion loss are evident at all receiver positions and do not vary appreciably.

To obtain a further understanding of the way in which the various profiles improved screening performance, the spectra of insertion loss for the barrier profiles were examined. Figure 26 shows spectra of insertion loss for the lm wide multiple edge profile. The source was at the 5.5m position, and the receiver was 1.5m above the ground, 40m from the barrier. The experimental results have been derived by calculating the difference between themeasured sound pressure level with the barrier present and the control sound pressure, level for a similar geometry without the barrier. In each case, a single spectrum was used which had been measured in conditions where the wind speed was close to zero. The

spectrum has been adjusted for source strength variations using the reference microphone. Theoretical spectra for the same conditions are also shown.

Discussion

The experimental results indicate the advantage of multiple- edge barrier shapes for consistently increasing insertion loss over a considerable area. This is true even using relatively thin (here, 12mm plywood) side panels.

In addition to the effects on screening of the extra path length for waves passing over these barrier options compared with a simple barrier, other factors are likely to be important. For multiple-edge barriers, diffraction will occur at each of the edges with a consequent reduction in the intensity of the sound energy finally diffracted into the area behind the barrier system. It is interesting to note that the multiple-edge barriers are generally not effective below a frequency of approximately 400Hz. This corresponds to a wavelength of nearly lm which is approximately the width of the components added to the simple barrier. This agrees with simple theory since it would not be expected that sound of larger wavelengths would interact with this structure to produce significant effects.

In practice, the multiple-edge barriers are likely to be relatively easy to fabricate since the side panels can be fixed to the barrier support posts. In addition, the average insertion loss of 2.5dB can be achieved without the use of absorption materials which are relatively costly to fabricate. Existing reflective barriers could be upgraded by attaching panels either side of the traffic face. Since it is not necessary for these additional panels to protrude above the top edge of the barrier, this should not significantly increase wind loading and since the weight of the side panels need not be large, existing posts should be adequate. In situations where additional noise control is required, but where the loss of view caused by higher barriers would cause concern to residents living alongside the road, the use of

multiple-edge barriers would be advantageous. It may be possible to obtain further insertion loss gains by detailed changes to the size, angle and spacing of the panels.

Conclusions

The following conclusions can be drawn from the results of the above tests.

1. The average improvement of insertion loss for 2m high multiple-edged barriers over a simple plane reflective barrier of identical overall height range from 2.4 to 2.7dB(A) depending on detailed design.

2. Multiple-edge barriers give consistent insertion loss gains compared with a simple plane reflective barrier over a wide area.

3. Multiple-edge barriers could be constructed simply and appear to have the greatest potential for cost-effective app1ications.

Further test results and results of analytical and numerical modelling, for noise barriers falling within the scope of the present invention, are detailed in:

G R WATTS 1992 Proceedings of the Eurosymposium in

Nantes: The Mitigation of Traffic Noise in Urban Areas, 223-250; "The acoustic performance of novel shaped barriers - a state-of-the-art review"; and

D C HOTHERSALL 1992 Proceedings of the Eurosymposium in

Nantes: The Mitigation of Traffic Noise in Urban Areas, 251-262; "Modelling the performance of noise barriers".

Table I

Average Noise Levels Behind Barriers and Relative Insertion Losses at Normal Incidence