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European Drying Conference - EuroDrying'2011 Palma. Balearic Island, Spain, 26-28 October 2011 THE EFFECT OF DRYING ON THE ACOUSTIC ABSORPTION OF NOVEL GREEN NOISE INSULATION H.Benkreira1, K.V. Horoshenkov 1, A. Khan 1, A. Mandon2, R. Rohr2 1 School of Engineering, Design & Technology University of Bradford, Bradford, BD7 1DP, UK Tel.:+441274233721, E-mail: [email protected] 2 Canevaflor, 24 Rue du Docteur Guffon, 69170 Tarare, France Abstract: This paper presents acoustic absorption data of plant-soil systems that can be used as green noise insulation on building facades. They show that absorption varies significantly depending on the type of plant and soil which in turn depend on the drying they experience in a natural environment. Keywords: soil, plants, acoustic absorption, moisture. INTRODUCTION In recent years and with the advent of global warming, the world has awakened to the importance of green technology with the consequence that governments throughout the world are undertaking major plans to implement green initiatives particularly for energy generation (see Germany recent decision) and the control of CO2 emission (see the Montreal Protocol). In parallel with these developments, there are new initiatives, particularly in Europe of greening cities, i.e. embellishing them with as much greenery as it is possible and desirable to play a part in the sequestration of CO2 and the fight against global warming. Greening the cities initiative has given also an opportunity to explore the possibility of using plants on building facades as a means of absorbing noise from traffic and other sources that plague cities which are now increasingly densely built. These also provide a pleasing green contrast to build form, seasonal events such as spring leaves, flowers, autumn leaf fall, and the provision of shade in summer. The end gain then becomes one of controlling both CO2 and noise pollution but also providing pleasing scenery in an urban environment. This paper falls precisely within this ultimate objective by studying the acoustic absorption of plants, potted in various soils and arranged as insulation on building facades (see Figure 1). Fig.1: A typical green wall With regard to drying, the theme of this conference, this paper examines the effect of moisture content on the acoustic absorption of these systems (their soil and foliage). This is clearly an innovative study in term of the particular system but well rooted in the concept of porous media and how these media absorb and/or transmit noise depending on the porosity. With these green systems, the soils are clearly porous media whose porosity depends on the moisture trapped and which varies upon drying in time. The foliage can also be viewed as porous media in that void exist between the leaves, although of course the scales are much different but can be “normal” for certain types of plants (see Figure 2). and tortuosity were measured semi-theoretically. Two other soils were investigated, one clay based and typical of soil used by local authorities in the UK (on account they are plentiful and typical of the country) and one specially designed by our collaborating company Canevaflor to retain moisturea mix of perlite, coconut fibres and polymer gel. Table 1: Characteristics of the plants tested. Plant species Pieris Japonica Green Ivy Primrose Average thickness of single leaf (mm) Weight of single leaf (g) Area of single leaf (m2) Number of leafs on plant 0.41 0.4 0.0012 166 0.23 0.2 0.0021 44 0.74 1.9 0.004 17 Fig. 2: Leaves of the three plants studied. Although limited and predominantly based on the effect trees, there is supporting evidence from previous research [1-2] that vegetation provide acoustic absorption properties that depend on size and orientation of leaves. In this research we approach the problem by considering both the soil and plant as the acoustic absorption system. We however investigate separately the soil from the leaves in an effort to quantify the relative contribution and the effect of moisture which is the changing variable in time. Both the soils and the leaves are chosen to cover a range of soil types, leaf types, leaf sizes and foliage density. We carry out this quantification through acoustic measurements using standard techniques and underpin the measurements with theoretical models. EXPERIMENTAL The experimental programme was carried using three types of plants species which can be grown in a living wall system and three types of soil. These systems were fully characterized as shown below prior to acoustic absorption measurements. The Plants: These were Pieris Japonica, Green Ivy and Primrose, shown in Figure 2 and characterized using a number of parameters that describe the thickness, area and weight of their single leaf; the number of leaves and the height and equivalent volume occupied by the plants. Such parameters are presented in Table 1. The Soils: These were chosen according to the plants grown and were: Pieris Japonica soil, Green Ivy soil and Primrose soil. These soil-plant systems were used as received (from a typical garden centre) but as explained later; their soil porosity, air flow resistivity Compared to clay-based soil, little or no aggregation occurs in the low-density soil substratum which is able to retain water at least three times its own weight due to the presence of hydrophilic polymer gel. The texture of the two soils determines the pore size distribution which controls their acoustical properties. The presence of fibres, large particles of perlite and polymer gel in the substratum give rise to larger pores and, therefore have a major influence on its acoustic absorption coefficient. Table 2 shows the density and porosity of these soils, measured in the case of porosity using image analysis. Table 2: Characteristics of the soils used. Water added to soils (cm3) Substratum Clay soil Density (kg/m3) 250 1255 Porosity 0.76 0.39 Acoustic Absorption Measuring Technique: A standard technique was used- a Bruel and Kjaer impedance tube fitted with two microphones. The method is described in ISO 10534-2, 1998 [3] and illustrated in Figure 3. Fig. 3: Plant and Soil placed inside impedance tube holders 0.9 Absorption coefficient 0.8 0.7 0.6 0.5 Japonica - Soil + leaves 0.4 0.3 0.2 Soil 0.1 0 0 500 1000 Frequency (Hz) 1500 1 0.9 0.8 Absorption coefficient RESULTS & DISCUSSION Performance of Plant-Soil Systems: For ease of comparison, the acoustic absorption of the three plants systems are shown in Figure 4 which shows the acoustic absorption of the soil, the soil and plant and the leaves separately. The data show a significant difference in both the soils and the plants and indicate that the Primrose soil and plants are ideally suited as acoustic absorbers. Interestingly, with Primrose, the leaves contribution to acoustic absorption is very significant and this can be attributed to the larger surface area of their leaves which act as membrane. As indicated in Table 1, this large surface area (0.004 m2, compared to 0.0012 m2 for the Japonica and 0.0021m2 for the Ivy) coupled with a large the thickness (0.74mm, compared to 0.41 for the Japonica and 0.23mm for the Ivy) enable them to cushion and absorb more effectively noise (pressure) energy. In order to understand further the relative influence of soil on the measured absorption, it is desirable to measure the structural characteristics of the soil such as porosity, air flow resistivity, tortuosity and the standard deviation, the intrinsic material properties controlling absorption. Rather than measuring these directly, we employ here a well established model [4-5] that uses acoustic absorption data to infer them. Essentially the method is based on minimising the discrepancy between the measured absorption coefficient spectrum and the spectrum predicted by the model. Table 3 presents the four non-acoustical parameters for the three soils deduced from the optimisation analysis. On account of its high porosity, a tortuosity =1 and a relatively high air flow resistivity, Primrose soil is the better acoustic absorber. It is also the soil that exhibit a smaller standard deviation suggesting that in comparison with the other soil, the pore size distribution is less wider albeit that all these soils exhibit a wide pore size distribution. The difference in acoustic absorption and structural parameter reflect the fact that these soils sustain different moisture levels dictated by the plant s root system and moisture feeding characteristics. 1 Green Ivy - Soil + leaves 0.7 0.6 0.5 0.4 Soil 0.3 0.2 0.1 0 0 500 1000 1500 Frequency (Hz) 1 0.9 0.8 Absorption coefficient The spacing between the two microphones was 100mm which enabled to determine the acoustic absorption spectra in the frequency range between 50 and 1600 Hz. Using this technique, the soil and leaves acoustic absorptions were measured separately and this was followed by inserting a complete potted plant in the impedance tube and measuring the acoustic absorption of the whole system. Primrose - Soil + leaves 0.7 Soil 0.6 0.5 0.4 0.3 0.2 0.1 0 0 500 1000 1500 Frequency (Hz) Fig.4: Acoustic absorption of the 3 plant-soil systems with and without foliage. Table 3: Structural-Acoustic Parameters of the three soils tested. Plant species Pieris Japonica Green Ivy Primrose Flow Porosity, Tortuosity, Standard φ α∞ resistivity, deviation, 2 σ ,(Pa.s/m ) s 30,551 0.17 1.00 1.95 1,843 0.28 1.20 1.44 3,550 0.3 1.00 0.96 rubber granulates.”, Appl. Acoust. 62: 665-690 (2001). [5] K. V. Horoshenkov and M. J. Swift, “The acoustic properties of granular materials with pore size distribution close to log-normal.”, J. Acoust. Soc. Am. 104 (3): 1198-1208 (2001). Performance of the Clay and Substratum Soils: Figure 5 compares the measured acoustic absorption performance of bag, dry and saturated substratum with clay based soil. The absorption data presented suggest that low density bag and dry substratum are high acoustic absorbers whereas saturated substratum and clay based soils have a limited acoustic absorption. The effect of moisture saturation on acoustic performance is shown in Figure 6 for the two soils. These data indicate clearly the very good performance of the substratum. CONCLUSIONS Plants, if chosen appropriately, i.e. with the right porosity, air flow resistivity and porosity can be used as effective screens to absorb noise. Soil play an important part in the absorption process but the plants add to it and in some cases significantly. Performance depends crucially on soil types which in turns depend on how the soil absorbs moisture. The benefit of this work is that plants can be arranged on building facades to provide as well as noise absorption, also CO2 absorption and of course a pleasing natural environment in densely built urban cities. In the context of drying, the data show that performance is crucially related to the moisture content of the soil which in turns depends on the natural drying the plants experience. Fig.5: Acoustic absorption performance of the clay and substratum soils at different moisture levels. ACKNOWLEDGEMENTS We acknowledge the financial support from the European Commission under the EU Framework 7 project HOSANNA (http://www.greener-cities.eu/) and are grateful for the helpful support we receive from all our partners in this project. REFERENCES [1] S. H. Burns, “The absorption of sound by pine trees.”, J. Acoust. Soc. Am., 65(3):658–61 (1979). [2] M. J. M. Martens, “Foliage as a low pass filter: Experiments with model forests in an anechoic chamber.”, J. Acoust. Soc. Am., 67(1): 66-72 (1980). [3] ISO 10534-2. 1998. Determination of sound absorption coefficient and impedance in impedance tubes - Part 2: Transfer-function method. [4] K. V. Horoshenkov and M. J. Swift, “The effect of consolidation on the acoustic properties of loose Fig.6: Comparison on the effect of saturation levels on acoustic absorption performance of the soils.