Sound barrier

Abstract

A sound barrier made from recycled concrete aggregate, the sound barrier being divided into at least two or more sections, each said section including: an inner layer for supporting the barrier, and an outer layer, said outer layer having a predetermined number of voids for absorbing sound energy at a particular frequency, wherein said respective outer layers of said at least two or more sections have different predetermined numbers of voids so as to absorb sound energy at different frequencies. A method of making a sound barrier is also provided.

Description

[0001] This invention relates to sound barriers. In particular, the invention relates to concrete sound barriers for highways and other road surfaces carrying large volumes of motorised traffic. Another application of the invention is the use of the sound barrier for internal or external cladding in buildings.

[0002] City communities are more aware of significant noise coming from road and rail traffic. Traffic noise is recognised as serious environmental problem and consequently relevant traffic and government authorities have defined residential noise limits for newly constructed and in (some instances) existing roads. In Denmark for example, the legal limit is 55 dB(A) day and night, and in Netherlands the night limit is even lowered to 45 dB(A). In the state of Victoria in Australia, the road traffic noise limit is set to 63 dB(A) L10(18 hour). There is often a difference in the sound pressure level generated on the road surface and what affected residents are willing to accept.

[0003] To reduce the sound pressure level that reaches nearby residents, a wide range of options are available to city authorities to consider, such as earth mounds, wider road reserves, etc. However, space limitations and financial considerations dictate in most cases the use of the noise barriers. The maximum practical reduction that can be achieved with the use of a vertical barrier is usually 15 dB(A). In general, the lower the noise level to be achieved, a higher and thicker sound barrier is required. Because of the limited distance between barriers and/or source and receiver, the use of reflective barriers is considered to be satisfactory. Consequently, this approach creates more visual obstruction as well as causing extensive use of materials and substantial construction costs. Moreover, these types of barriers, which reflect or disperse noise, in many cases do not properly satisfactorily solve traffic noise problems. To lessen the visual impact of the barriers and to achieve the target noise reduction, the use of quiet road surfaces and sound absorptive barriers have been observed as an effective alternative.

[0004] In addition to the increased nuisance related to the traffic noise, another environmental problem that needs immediate attention of the community is the growing amount of waste generated and requiring disposal at local landfill sites. The construction and demolition (C&D) industry, with its waste, is the major contributor to the total waste stream, and accounts for up to 70% of the total. The major component of C&D waste is concrete, which amounts for approximately 1.5 million tonnes per year in the Metropolitan Melbourne area of Victoria, Australia alone. Approximately 50% of concrete waste can be recycled and reused in new road infrastructure projects mainly as a substitute for natural crushed aggregate in bound and unbound pavement sub-base layers. This recycled concrete waste may be used as an alternative construction material and is often referred to as recycled concrete aggregate or RCA. RCA materials may be manufactured in quality assured processes to set specifications, and subsequently have well defined basic engineering properties. Concrete products made from RCA are also referred to as recycled aggregate concrete or RAC. However, RCA and RAC products are often under utilised.

[0005] According to one aspect of the invention there is provided a sound barrier made from recycled concrete aggregate, the sound barrier being divided into at least two or more sections, each said section including:

[0006] an inner layer for supporting the barrier, and

[0007] an outer layer, said outer layer having a predetermined number of voids for absorbing sound energy at a particular frequency,

[0008] wherein said respective outer layers of said at least two or more sections have different predetermined numbers of voids so as to absorb sound energy at different frequencies.

[0009] There is also provided according to another aspect of the invention, a method of making a sound barrier divided into at least two or more sections from a concrete mix comprising recycled concrete aggregate, the method comprising the steps of:

[0010] (a) casting the concrete mix;

[0011] (b) compacting the concrete mix to form a plurality of voids in the concrete mix;

[0012] (c) curing the concrete mix to form one said section such that an outer layer of said section has a predetermined number of said voids for absorbing sound energy at a predetermined frequency;

[0013] (d) repeating said steps (a)-(c) for one or more further sections wherein the number of voids in each respective outer layer differs so as to vary the frequency of sound energy absorbed by each respective outer layer, and

[0014] (e) connecting said section and said one or more further sections to form said sound barrier to absorb sound energy at different frequencies.

[0015] As the outer layer of each sound barrier section has a porous structure due to the voids, sound energy is absorbed by each section by channelling sound energy through the voids, causing the sound to dissipate or disperse in the structure of each respective outer layer. Accordingly, by varying the number of voids in each sound barrier section, sound at different frequencies is absorbed by the sections of the sound barrier. Therefore, this improves the sound absorption capacity and allows the sound barrier and method according to the invention to have an overall smaller thickness and be less visible than existing sound barriers. In addition, the sound barrier and method according to the invention reduces the consumption of building materials by utilising RCA.

[0016] Preferably, the number of voids is determined by varying the thickness of the outer layer. The thickness of the outer layers may be up to 125 mm. Preferably, the thickness of the outer layer is no more than 85 mm. In a preferred embodiment, the thickness of the outer layer is between 15 mm and 60 mm.

[0017] The outer layers preferably absorb sound energy of frequencies in the range of between 100 Hz and 2000 Hz.

[0018] It is preferred that there is a transition zone between the inner layer and the outer layer. The transition zone preferably reflects any residual sound energy that travels through the outer layer.

[0019] The inner layer preferably is denser than the outer layer.

[0020] The sound barrier preferably has the three sections arranged sequentially from top to base relative to the ground in ascending order of each frequency absorbed or reflected. In a preferred embodiment the sound barrier of the invention has a top section or panel for absorbing sound at 400 Hz, a middle panel for absorbing sound at 600 Hz, and a base panel for absorbing sound at 800 Hz.

[0021] It is preferable that the outer layer has a higher thickness at a top section of the sound barrier than the outer layer at the base of the sound barrier, relative to the ground. The top section outer layer preferably has large interconnected air channels. The thickness of the outer layer may gradually increase from the base of each sound barrier section to the top portion. Preferably, the top section of the sound barrier includes up to 15% of the total volume of the sound barrier.

[0022] Preferably, the voids may include one or more of the following types or a combination of the following types: gel-like pores; capillary-like pores; interconnected permeable voids; isolated air pockets or channels, and interconnected air pockets or channels. The voids may range in size between 100 Angstroms and 5 mm in diameter.

[0023] The outer layer of each section is preferably covered with an additional layer, the additional layer having perforations or discontinuous portions to facilitate the absorption of sound energy. The perforations or discontinuous portions preferably extend into the additional layer to a predetermined depth. The perforations or discontinuous portions may extend into the additional layer to a depth such that portions of the outer layer are exposed. Preferably, the perforations or discontinuous portions are formed over at least 30% of the outer surface of the additional layer. The perforations or discontinuous portions can be mechanically induced in the additional layer.

[0024] The perforations or discontinuous portions can be formed in a regular pattern. One embodiment has 9 mm holes spaced at 20-25 mm centres. Irregular patterns of perforations or discontinuous portions can be used.

[0025] The additional layer may be composed of a perforated material. In one embodiment, the additional layer is made from a mortar including RCA fines, cement, water and an air entraining agent. Other perforated material may include plasterboard or render.

[0026] The additional layer is preferably thin relative to the thickness of the inner layer and the outer layer. It is preferred that the thickness of the additional layer is at least 10 mm. It is expected that the maximum thickness is at most 25 mm.

[0027] The concrete mix preferably includes one or more of the following: general purpose cement; fly ash; concrete sand and water. Preferably, the concrete mix comprises between 45 and 60% of RCA.

[0028] Preferably, an inner layer is formed with the outer layer. The inner and outer layers of each sound barrier section can be formed in a single pour of concrete mix.

[0029] It is preferable that the recycled concrete aggregate has a lower density than normal concrete. The recycled concrete aggregate may have a density that is 10% lower than the density of normal concrete.

[0030] Preferably, the casting step is performed on a horizontal casting bed.

[0031] It is preferred that the compacting step includes vibrating the concrete mix at a predetermined frequency and time. The concrete mix is preferably vibrated at a frequency of 50 Hz for 3 to 6 minutes, more preferably 3.3 to 5.5 minutes.

[0032] The curing step is preferably performed by steam curing or by introducing a curing compound to the concrete mix.

[0033] While the perforations or discontinuous portions can be formed in the additional layer before or after the additional layer is joined to the outer layer of the second barrier, it is preferred that the perforations/discontinuous portions are formed after the additional layer is joined to the outer layer.

[0034] It is preferred that the total thickness of the sound barrier is 150 mm.

[0035] To assist in the understanding of the invention, preferred embodiments of the invention will now be described with reference to the drawings, of which:

[0036] FIG. 1 is front view of a sound barrier made according to one embodiment of the invention;

[0037] FIG. 2 is a sectional side view of the sound barrier of FIG. 1 along the line A-A;

[0038] FIG. 3 is a side view of a section of the sound barrier of FIG. 1;

[0039] FIG. 4 is a schematic drawing showing the sound absorbing mechanisms employed by the sound barrier of FIG. 1.

[0040] FIG. 5 is a schematic side view of a section of a sound barrier in accordance with another embodiment of the invention.

[0041] FIG. 6 is a chart comparing the sound absorbing characteristics of embodiments of the invention.

[0042] FIG. 1 illustrates a sound barrier 10 in accordance with a preferred embodiment of the invention. Sound barrier 10 comprises three separate sections or panels 13, 14, 15 mounted on rigid supports 12. The supports 12 are affixed to the ground via footing members 11 made from RAC. Each of the panels 13, 14, 15 respectively has an outer layer (13a, 14a or 15a) and an inner layer (13b, 14b or 15b). The outer layers 13a, 14a, 15a each have voids for absorbing sound energy. The thicknesses of the outer layers 13a, 14a, 15a increase from the base of the sound barrier 10 to the top, as can be seen more clearly in FIG. 2.

[0043] FIG. 3 shows a side view of section 13 of sound barrier 10 of FIGS. 1 and 2 in more detail. Sections 14, 15 have the same structure as section 13 except in relation to the thicknesses of their respective inner and outer layers. Section 13 has an inner dense layer 13b (also indicated as "RL"--reflective layer) and outer porous layer 13a (also indicated as "SA"--sound absorbent layer). The inner dense layer 13b provides structural support for 13 as well as for reflecting sound energy at the transition zone 20 between the inner layer 13b and the outer layer 13a. The outer layer 13a has a number of voids in the form of interconnected air channels 40. The outer porous layer 13a is an absorptive layer for channelling sound energy through voids or air channels 40 to disperse or dissipate sound energy generated by traffic noise.

[0044] It has been found that the greater the porosity of the outer layer of the sound barrier, the lower the frequency of the sound absorbed by the sound barrier. This means that the number of voids in the outer layer determines the frequencies at which sound is absorbed. Accordingly, particular sound frequencies may be absorbed according to the thickness of the outer porous layer of the sound barrier as this determines the number of voids.

[0045] Thus, each panel or section 13, 14, 15 will absorb and reflect different frequencies of sound. For example, panel 13 has an outer layer 13a of approximately 60 mm thickness and absorbs sound frequencies of 400 Hz. Panel 14 has an outer layer 14a of approximately 40 mm thickness and absorbs frequencies of 600 Hz. Panel 15 has an outer layer 15a of approximately 30 mm thickness and absorbs frequencies of 800 Hz.

[0046] It has been found that sound barriers made in accordance with the invention may absorb sound having frequencies in the range of between 100 Hz and 2000 Hz. In particular, the most effective range of frequencies absorbed is between 400 Hz and 800 Hz. The thickness of the outer layer is preferably no more than 85 mm, although thicknesses of up to 125 mm may be used.

[0047] By dividing the sound barrier into sections or panels, each panel having an outer layer of a different thickness, the sound barrier according to this preferred embodiment of the invention may absorb a broader range of frequencies than a sound barrier having an outer porous layer of uniform thickness.

[0048] The sound absorbing mechanisms used by each section 13, 14, 15 are illustrated in FIG. 4. Incident sound generated from passing traffic is absorbed or reflected by the sound barrier in two ways--flow resistivity and resonance. Firstly, an incident sound wave may be simply reflected from the outer porous layer 13a. Secondly, an incident sound wave may penetrate and be absorbed by the outer layer 13a. In this case, the incident sound wave travels through the outer porous layer 13a via voids or air channels 40, dissipating most, if not all, of its energy. Any residual sound energy that reaches transition zone 20 is reflected back through the outer porous layer 13a due to the denser nature of the inner layer 13b. The reflected residual sound energy will dissipate as it travels back through the outer layer 13a. Therefore, each sound barrier section 13, 14, 15 both absorbs and reflects sound energy generated by traffic noises.

[0049] It has been discovered that the sound absorption properties of the RAC sound barrier are proportional to the porosity of the RCA that is used to make up the RAC. This means that the overall porosity of the sound barrier (that is, the concrete density and the number of voids per volume (also known as void volume)) determines the effectiveness of the sound barrier. Table 1 indicates examples of this relationship between porosity of the RCA and noise reduction, the Noise Reduction Coefficient being an arithmetic mean of Sound Absorption Coefficient measured at 250 Hz, 500 Hz, 1000 Hz and 2000 Hz.

1TABLE 1 Acoustic characteristics of sound barrier panels Concrete Noise Density % Reduction Panel [kg/m.sup.3] Porosity Coefficient Alpha-400 1,650 22.0 0.42 Alpha-600 1,745 18.0 0.27 Alpha-800 1,865 13 0.21

[0050] FIG. 5 schematically shows one section 50 of a sound barrier according to another embodiment of the invention. The sound barrier section 50 of this embodiment has an inner structural layer 51, a porous outer layer 52 and an additional or second outer layer 54 covering outer layer 52. The inner layer 51 and outer layer 52 are respectively the same as the inner layer 13a, 14a, 15b and outer layer 13b, 14b, 15b of the sound barrier 10 of FIG. 1. The only difference between the two embodiments is the provision of additional layer 54.

[0051] The additional layer 54 has a number of perforations 56 formed over at least 30% of its outer surface 58 to assist in the absorption of sound energy. The perforations 56 extend from outer surface 58 right through additional layer 54 to expose portions of outer layer 52. It has been found that this additional layer 54 assists in improving the sound absorbing characteristics of the sound barrier section 50, particularly at lower frequencies of sound energy.

[0052] In the preferred embodiment, the perforations 56 are mechanically induced in the additional layer 54 after the additional layer is joined to the outer layer 52. The perforations 56 are formed in a regular pattern of 9 mm holes spaced at 20-25 mm centres. The perforations may be substituted with discrete discontinuous portions. Furthermore, an irregular pattern can be used with either perforations or discontinuous portions. For example, discrete discontinuous portions can be formed in an irregular pattern to the same effect as a regular pattern of perforations, so long as the discontinuous portions are in at least 30% of the outer surface.

[0053] Generally, as the additional layer 54 increases in thickness, the sound barrier has more improved sound absorbing characteristics. The minimum thickness of the additional layer 54 in this embodiment is 10 mm. The maximum thickness of the additional layer 54 will depend on the sound frequencies being targeted by the sound barrier, although it is expected that a maximum thickness is at about 25 mm.

[0054] The sound barrier of this embodiment has a density of about 1650-1865 kg/m.sup.3. The outer layer 52 of the sound barrier has a density of 1200-1700 kg/m.sup.3 whereas the additional layer 54 has a density of about 840-1200 kg/m.sup.3.

[0055] Reverberation chamber tests were conducted using three examples of embodiments of the invention to compare their sound absorption characteristics. Two of the examples (labelled S1 and S2) were structurally the same as the sound barrier of FIG. 1; ie. having an inner and outer layer. The third example (labelled S3) was a sound barrier made according to FIG. 5, having an inner layer, an outer layer and an additional layer, the additional layer being made from perforated plasterboard material.

[0056] FIG. 6 is a chart showing the results of the tests for each of the above sound barriers. As can be seen in FIG. 6, example S3 generally has a higher sound absorption coefficient than examples S1 and S2 over the frequency range, especially between about 400 Hz to 1400 Hz, due to the presence of the perforations in the additional layer.

[0057] A method of making the sound barrier 10 in accordance to another aspect of the invention is also provided. A concrete mix is formed primarily of RCA, the proportion of RCA varying between 45 and 60% of the concrete mix. Other materials can be included, such as general purpose (GP) cement, fly ash, concrete sand or water. The GP cement and fly ash is generally used as a binder for the section or panel. It has been determined that the best results use a combination of the above materials. Various proportions of RCA and other materials in different concrete mixes are shown in Table 2 below for a panel with a compressive strength of 25 MPa.

[0058] The concrete mix design is classified as a combination of no-fines and gap-graded concrete and is described as "less-fines concrete".

2TABLE 2 Concret mix design Coarse RC Aggregate Fine RC Water Binder Bulk Aggregate Concrete Vibration V Mass V Mass V Mass Air Mass V Density time Panel Binder [m.sup.3] [kg] [m.sup.3] [kg] [m.sup.3] [kg] V [m.sup.3] [kg] [m.sup.3] [kg/m.sup.3] [min] Alpha- GP+ 0.150 150 0.08 240.0 0.452 950 0.22 250 0.10 1,650 5.5 400 Fly 0.02 60.0 Ash Alpha- GP+ 0.150 150 0.08 240.0 0.474 995 0.18 300 0.11 1,745 5.5 600 Fly 0.02 60.0 Ash Alpha- GP+ 0.150 150 0.08 240.0 0.507 1065 0.13 350 0.13 1,865 5.5 800 Fly 0.02 60.0 Ash

[0059] The concrete mix designs in Table 2 are based on the following characteristics:

[0060] GP cement having a specific gravity of 3.15;

[0061] fly ash having a specific gravity of 2.4;

[0062] coarse aggregate concrete with a specific gravity of 2.1 and a bulk density of 1600 kg/m.sup.3;

[0063] fine aggregate concrete with a specific gravity of 2.58;

[0064] a slump of concrete of 50 mm (ie. the workability of fresh concrete into moulds to create the final shape of the set concrete);

[0065] a water/cement ratio of 0.55 (used in the mix and relates to the final compressive strength of the mix), and

[0066] fineness modulus (FM) of fine aggregate of 3.

[0067] FM is an aggregate property, which is the sum of cumulative ratios retained on 4.75 to 0.075 mm sieves. FM does not describe a particle size distribution of the aggregate.

[0068] One manner in which the method according to the invention may be performed is described below.

[0069] Once the appropriate concrete mix is selected, the mix is placed on a horizontal casting bed. The cast concrete mix is then compacted, preferably by using a vibrating table to promote the creation of voids in the outer layer. The vibrating table is usually run at 50 Hz for 3.3 to 5.5 minutes with amplitude of approximately 2 mm. An immersed vibrator may also be used to promote the creation of voids in the outer layer.

[0070] After compaction, the concrete mix is then cured to set the concrete mix as the sound barrier section. Curing may be performed using either steam curing or introducing a curing compound to the concrete mix. Typical curing compounds include Duro-Seel, made by Ability Building Chemicals.

[0071] Once the desired number of sections or panels have been produced, the sections are then joined or connected to each other to form sound barrier 10. This can be done by sliding the sections into H-shape posts or attached to posts with bolts. Other methods can be used to connect the sound barrier sections using standard structural engineering design and construction methods.

[0072] Table 3 shows several sound barriers made using according to the above method using different concrete mixes, the manner of performing the method only varying in the use of steam curing or a curing compound.

3TABLE 3 Concrete mix design and manufacturing method Concrete mix design Recycled Concrete Aggregate General 14/10 or Manufacturing method Panel Purpose 10 mm Curing/ type (GP) Pulverised Concrete Class 2 Placing/ Compaction/ (SC) steam frequency [Hz]/ Cement Fuel Ash Sand Industry Horizontal Vibrating curing/ compressive AS 1315 AS3582.1 AS 2758 Standard Water casting table - 50 Hz (CC) curing strength [MPa] [kg] [kg] [kg] [kg] [l] bed (3.3-5.5 min) compound .alpha. -400/25 MPa 240 60 250 950 150 Yes Yes SC .alpha. -600/25 MPa 240 60 300 995 150 Yes Yes SC .alpha. -800/25 MPa 240 60 350 1065 150 Yes Yes SC .alpha. -400/32 MPa 330 80 250 950 200 Yes Yes CC .alpha. -600/32 MPa 330 80 300 995 200 Yes Yes CC .alpha. -800/32 MPa 330 80 350 1065 200 Yes Yes CC

[0073] In another example, about 57% RCA was used in the concrete mix to create a sound barrier having a compressive strength of 15 MPa, which can absorb frequencies of 400 Hz. Other sound barriers of compressive strengths in the range of 25 MPa to 40 MPa were also be made.

[0074] For the embodiment described at FIG. 5, after the concrete mix is set to form the inner and outer layers, the additional layer is joined to the outer layer. While the perforations or discontinuous portions can be mechanically induced into the additional layer prior to joining, it is preferred that the perforations or discontinuous portions are mechanically induced in the additional layer after joining the additional layer to the outer layer.

[0075] While the above example describes one particular manner of performing the method according to the invention, it is readily apparent to the person skilled in the art that other means may be employed to perform the casting, compacting and curing steps of the method according to the invention.

[0076] The above preferred embodiments and examples relate to a concrete sound barrier for absorbing traffic noise generated along road surfaces. However, the sound barrier of the invention may also be applied to other sound reducing applications, such as for internal or external cladding in buildings.

[0077] It is understood that various modifications, alterations, variations and additions to the construction and arrangement of the embodiment described herein are considered as falling within the ambit and scope of the present invention.

Claims

The claims defining the invention are as follows:

1. A sound barrier made from recycled concrete aggregate, the sound barrier being divided into at least two or more sections, each said section including: an inner layer for supporting the barrier, and an outer layer, said outer layer having a predetermined number of voids for absorbing sound energy at a particular frequency, wherein said respective outer layers of said at least two or more sections have different predetermined numbers of voids so as to absorb sound energy at different frequencies.

2. A sound barrier according to claim 1, wherein the outer layers have different thicknesses to determine the number of voids in each section.

3. A sound barrier according to claim 2, wherein the thickness of said outer layers is up to 125 mm.

4. A sound barrier according to claim 3, wherein said thickness is no more than 85 mm.

5. A sound barrier according to claim 4, wherein said thickness is between 15 mm and 60 mm.

6. A sound barrier according to claim 1, wherein said outer layers absorb frequencies in the range of between 100 Hz and 2000 Hz.

7. A sound barrier according to claim 6, wherein said outer layers absorb frequencies between 400 Hz and 800 Hz.

8. A sound barrier according to claim 1, wherein there is a transition zone between the inner layer and the outer layer to reflect residual sound energy that travels through the outer layer.

9. A sound barrier according to claim 8, wherein said inner layer is denser than said outer layer.

10. A sound barrier according to claim 1, wherein the sound barrier has three sections arranged sequentially from top to base relative to the ground in ascending order of each frequency absorbed.

11. A sound barrier according to claim 1, wherein the voids include one or more of the following types or a combination of the following types: gel-like pores; capillary-like pores; interconnected permeable voids; isolated air pockets or channels, and interconnected air pockets or channels.

12. A sound barrier according to claim 11, wherein the voids range in size between 100 Angstroms and 5 mm in diameter.

13. A sound barrier according to claim 1, wherein the outer layer is covered with an additional layer, said additional layer including perforations or discontinuous portions to facilitate the absorption of sound energy.

14. A sound barrier according to claim 13, wherein the perforations or discontinuous portions extend into the additional layer to a predetermined depth.

15. A sound barrier according to claim 14, wherein the perforations or discontinuous portions extend into the additional layer to a depth such that portions of the outer layer are exposed.

16. A sound barrier according to claim 13, wherein the perforations or discontinuous portions are formed over at least 30% of the outer surface of the additional layer.

17. A sound barrier according to claim 13, wherein the additional layer is composed of a perforated material.

18. A sound barrier according to claim 17, wherein the perforated material includes a mortar including RCA fines, cement, water and an air entraining agent.

19. A sound barrier according to claim 17, wherein the perforated material includes plasterboard or render.

20. A sound barrier according to claim 13, wherein the thickness of the additional layer is at least about 10 mm.

21. A sound barrier according to claim 13, wherein the thickness of the additional layer is at most about 25 mm.

22. A sound barrier according to claim 13, wherein the perforations or discontinuous portions are formed in a regular pattern.

23. A sound barrier according to claim 13, wherein the perforations or discontinuous portions are mechanically induced in the additional layer.

24. A method of making a sound barrier divided into at least two or more sections from a concrete mix comprising recycled concrete aggregate, the method comprising the steps of: (a) casting the concrete mix; (b) compacting the concrete mix to form a plurality of voids in the concrete mix; (c) curing the concrete mix to form one said section such that an outer layer of said section has a predetermined number of said voids for absorbing sound energy at a predetermined frequency; (d) repeating said steps (a)-(c) for one or more further sections wherein the number of voids in each respective outer layer differs so as to vary the frequency of sound energy absorbed by each respective outer layer, and (e) connecting said section and said one or more further sections to form said sound barrier to absorb sound energy at different frequencies.

25. A method according to claim 24, wherein said number of voids in each respective outer layer is determined by varying the thickness of each said respective outer layer.

26. A method according to claim 24, wherein the inner layer is formed with the outer layer of each section in a single pour of concrete mix.

27. A method according to claim 24, wherein the concrete mix includes 45 to 60% of recycled concrete aggregate.

28. A method according to claim 24, wherein the recycled concrete aggregate has a lower density than normal concrete.

29. A method according to claim 24, wherein the concrete mix includes one or more of the following: general purpose cement; fly ash; concrete sand and water.

30. A method according to claim 24, wherein said casting step is performed on a horizontal casting bed.

31. A method according to claim 24, wherein the compacting step includes vibrating the concrete mix at a predetermined frequency and time.

32. A method according to claim 31, wherein said predetermined frequency is about 50 Hz and said predetermined time is between 3 to 6 minutes.

33. A method according to claim 24, wherein said curing step is performed by steam curing or by introducing a curing compound to the concrete mix.

34. A method according to claim 24, wherein each respective outer layer is covered by an additional layer including perforations or discontinuous portions to facilitate the absorption of sound energy.

35. A method according to claim 34, wherein the perforations or discontinuous portions are mechanically induced in the additional layer.