Tradescantia Zebrina as New Creeping Plant for Good Sound Absorption Performance

The increasing need to address the environmental consequences of noise and visual pollution caused by infrastructure and industrial projects in residential regions has led to the implementation of regulations. These regulations aim to minimize such impacts and have resulted in a rising demand for acoustic barriers and visual screening solutions. Presently, green plants are being employed globally not only for creating a sustainable environment within green buildings but also for their sound absorption capabilities. The choice of plants suitable for green walls primarily focuses on tropical and annual species that demonstrate effortless growth capabilities. This research examines the capability of sound absorption of chosen creeping plants namely Tradescantia Zebrina (TZ) that potentially be used for green facades. The physical properties including morphology and acoustical properties by measuring the sound absorption coefficient (SAC) using two microphone impedance tubes. The plant with the noise reduction coefficient (NRC) was investigated further for the effect of the thickness of the plant on the sound absorption performance of a wall. The results showed 0.23, 0.35, and 0.52 NRC values respectively obtained from the different thicknesses of TZ There was an increment of sound absorption as the plants were attached to concrete brick as a green facade on its surface. On this basis, the capability to absorb sound by TZ as the green facade can be considered when designing the environmental structure.


Introduction
The acoustic output of noise barriers can be influenced by factors such as material, location, dimensions, and shape.Synthetic materials commonly used in acoustic absorbers on the market not only contribute to pollution and global warming but also have adverse effects on health [1].In urban areas, residents frequently experience the negative health impacts of traffic noise.Construction noise barriers typically utilize materials like concrete, wood, earth berms, and metals.In the case of the noise barriers installed by the Malaysian Public Works Department (JKR) in front of a primary school near Skudai-Johor Bahru Highway, specifically at Sekolah Kebangsaan Kampong Pasir Johor Bahru, there are concerns about their safety and condition.Over the years, the physical performance of these barriers has been compromised due to a lack of maintenance.The barriers display small holes and cracks on the surface.In 2016, heavy rain and strong winds led to the collapse of wall panels as trees fell.Despite reporting the issue to authorities, no action was taken.Subsequent maintenance was carried out without proper guidelines thus the barriers were replaced with plastered concrete blocks.This technique is often applied without adequate design, making it challenging to maintain quality control when using in situ cast concrete [2].The construction of noise barriers should be less compressive since they only need to support their weight and withstand wind forces.
Currently, green plants are used to create a beautiful and sustainable green environment in urban areas.In addition to being planted on the side of the road, the method of planting on the edge of the wall using climbing trees or creeping trees attached to the surface of the wall has been widely implemented [3].One of the creeping plants that can be used is Tradescantia Zebrine, a perennial herb with long and thin leaves that look like a bladder [4].The top side of the TZ leaf ranges from green to purple and features two broad, silvery-white stripes, whereas the underside of the leaf is consistently deep magenta in color [5].In previous research by [6], other creeping plants of Ficus Pumila and Vernonia Elliptica produced encouraging results in absorbing sound.Vernonia Elliptica and Ficus pumila produced maximum sound absorption at 800 Hz with 0.78 and 0.73, respectively.10 mm to 50 mm thickness of Vernonia elliptica was found to produce a SAC value greater than 0.35 [7].In the previous study [7], fern trees placed on the floor produced SAC at low (315 Hz), medium (1250 Hz), and high frequencies (5000 Hz) were 0.80, 1.00, and 1.00, respectively.Theoretically, plants could reduce noise by refracting, reflecting, dispersing as well as absorbing sound [11].There are three main mechanisms responsible for sound absorption by living plants consisting of leaves, branches, or trunks [8].First, the heat dissipation mechanism occurs below the audible frequency range of 100-400 Hz.Second, the viscous dissipation mechanism that occurs at medium frequencies (400-2000 Hz) where the acoustic wavelength is still greater than the characteristic leaf dimensions (e.g.15-250 mm for normal plants) and thirdly, the dissipation of leaf vibration and multiple scattering that occurs in the higher frequency range (eg above 1-2 kHz) where the acoustic wavelength becomes comparable to or smaller than the characteristic leaf dimensions.In addition, destructive interference also occurs through the substrate medium [14], [18].
Plants attenuate noise more effectively than solid concrete surfaces [8].Plants with soil/substrate medium used in a vertical wall add more sound absorption [7].It was found that larger leaf size, density, and orientation angle are key morphological characteristics that may predict plant flow resistance and tangles improve noise attenuation [21], [22], [23], [10].Larger leaves exhibit superior performance compared to those with smaller leaves [26].For example, Pometia Pinnata plant species that have thicker leaves and larger surfaces in terms of width, length, and surface area absorb more sound [24].Likewise, for plants with a porosity between 96-99%, high leaf area density, and angled leaves increase sound absorption [9], [10].[9] uses 95-97% porosity as a basis because this value is the value of the natural gradient of porosity of plants related to illumination.However, there is a study stating that acoustic attenuation is mostly obtained from leaves rather than trunks [17].[8] and [9] relate the variation of sound absorption to the variation of the surface impedance modulus.Sound absorption reaches a minimum value related to the resonance wavelength of the sample which depends on the thickness of the plant specimen [9].These criteria encourage the use of appropriate construction technology for the implementation of vertical facade systems as an architectural element for enhancing the acoustics of interior spaces or urban squares [12].For that, the use of green façade and green walls has been studied.A green façade is a wall propagated by plants that can produce an acoustic reduction index of 15 dB, and the weighted SAC is 0.40 [13].For the green wall, which is the use of plants with a medium substrate or air gap attached to the wall, the SAC value is equivalent to or higher than other building materials, and its impacts were highly important on low frequencies because its properties were superior to other low-frequency sound absorbents [14].Sound absorption is found to be above 0.5 [15].However, the number of research on green facades and green walls as sound absorbers is few, and the procedures utilized were extremely varied.It must be recognized that the ultimate goal of architectural acoustics is to reduce the noise humans are exposed to within buildings.The use of TZ as a plant for green facades or green walls has not yet been explored, therefore this study examines the capability of sound absorption of TZ and its potential usage in the construction of green facades.The leaf characteristics, morphology, and sound absorption properties of TZ with three thicknesses that represent reality were measured using two microphone impedance tubes.The capability of TZ as a green facade was then tested and compared with the capability of normal walls for building construction.The results of the study can determine the recommendation for the use of TZ as a new plant to be used either for a green facade or a green wall.

Experimental Method
The study begins by identifying its physical properties including characteristics and morphology.A ruler was used to measure the length and width of leaves (cm) while a digital vernier caliper was used to measure the thickness of the leaf (mm) as shown in Figure 1. Figure 2 illustrates the graph paper method for calculating the average leaf area obtained using 5 leaves as suggested in [11], [12].Firstly, the outline of the five (5) leaves was traced using a pencil on graph paper.The surface area was determined by multiplying the number of full squares by 1 cm 2 and the number of partial squares by 0.5 cm 2 then adding them together to obtain the total surface area in cm 2 .Next, a scanning electron microscope (SEM) was used to observe the morphology of the upper and lower surfaces of the leaves.Specimens are prepared to test sound absorption using specimens with a thickness of 10 mm, 25 mm, and 50 mm.For the preparation of plant specimens, TZ is arranged in a specimen holder made of PVC pipe with a diameter of 98mm and thickness; h = 10mm, 25mm, and 50mm (Figure 3).Mesh fabric is used to cover one end of the hole, serving as a way to keep the plants inside.Plants were cut and arranged in layers in a holder to fit in an impedance tube to simulate real conditions.Additional precautions are taken to ensure a uniform weight measurement in proportion to the thickness, to obtain a porosity close to 95% to represent the actual condition of the plant.For that purpose, each specimen is ensured to have almost the same density value.Porosity was calculated after going through a process involving the immersion of a weighed branch in a 1L beaker, which allowed measurement of the resulting increase in water level [19].
The sound absorption, reflection, and surface impedance of TZ specimens with different thicknesses were measured using a Brüel & Kjaer Type 4206-A two-microphone impedance tube according to the specifications outlined in Standard ASTM E1050-98 (1998).The range of frequency measurement was 63 Hz to 1600 Hz.These specimens are placed in front of the rigid backing layer.Further investigation was then carried out on 150 mm thick concrete brick (CB) attached to the plant specimens to represent the green facade.The green facade specimen was tested with the CB surface placed in front of the rigid backing layer.CB was made of sand and cement, simulating an actual wall scenario as shown in Figure 4.The findings were compared with results obtained from previous research that used other plants for the purpose of green facades and green walls, as well as sound absorption by walls using other building materials that are currently used in the construction industry.

Characteristics of plants
TZ has leaves measuring an average of 8.16 cm x 3.52 cm x 0.04 cm thick and a 20.1 cm 2 surface area.SEM images of both the upper and lower surfaces of the TZ leaf are shown in Figure 5.The lower part of the leaf, reveals the TZ has a pore structure with openings ranging from 5 to 8 μm in length and height ranging from 35 to 38 μm while the upper surface of the TZ does not have any pore openings.These holes are very small compared to the wavelength of the sound in this study.Therefore, these pores do not have a major influence on sound absorption in the frequency range of less than 2000 Hz.However, physical properties such as width and length as well as "succulent" type leaves produce a high density of leaves per m 2 and potentially contribute to improving acoustic characteristics through sound dissipation due to friction as also found by [15].The porosity of the TZ specimen for the thickness variation studied is shown in Table 1.It was found that porosity has almost similar values between thicknesses because the density is almost the same and the covariance is less than 1.44%.According to [13], a covariance of less than 12% is acceptable and considered uniform for all specimens and accepted in this study to fulfill the purpose of determining the effect on the reduction of SAC due to the thickness of plant specimens.It was found that the resulting porosity was also almost similar and was in the range of the actual porosity of the plant as obtained by [9] and [10].The porosity of the material representing the actual state of the plant is used to accurately predict the flow resistance and the tortuosity of the plant which improves the noise attenuation.

Acoustic performance
With the same porosity for each thickness of 10, 25 and 50 mm TZ specimen, the reflection coefficient, sound absorption coefficient, and surface impedance modulus of the specimen are shown in Figure 6 can be analyzed.The reflection coefficient is the inverse of SAC, R=1-SAC.This reflection shows that the surface impedance matches the reflection trend.Variation at 10 mm thickness, high impedance modulus at less than 1000 Hz frequency, due to the low sound absorption coefficient of less than 0.35.The variation of the surface impedance modulus versus the frequency showing the minimum value is related to the quarter wavelength of the sample resonance at 1250 Hz for a sound speed of 347 m/s at a temperature of 27°c.The minimum value of this surface impedance changes to three-quarters of the resonance wavelength at 1000 Hz and 520 Hz when the thickness is reduced to 25 mm and 50 mm, respectively.For the h=10mm specimen, the acoustic absorption for h=10mm remains relatively low (< 0.35) in the entire frequency range of less than 1000 Hz due to the high surface impedance in that frequency range.The trend of low sound absorption at frequencies below 400 Hz, results in a similar curve as the trend of sound absorption on shrubs studied by [10].
Because this study limits the frequency to less than 2000 Hz and leaves have a maximum length of 10 mm, the sound absorption mechanisms that occur are thermal dissipation and viscous dissipation.At 1250 Hz, SAC falls which can be explained by the three-quater-wavelength resonance in the 50 mm thickness which is due to and reflection of sound by the wall specimen.Overall, the highest sound absorption coefficients 7th International Conference on Noise, Vibration and Comfort (NVC 2023) were obtained near the three-quarter resonance wavelength for samples with a better match between the surface impedance of the sample and the characteristic impedance of the air.A study by Attal also found a similar phenomenon for spindle plant parts with a thickness of 80 mm and 160 mm with a rigid backing layer behind the specimen [9].In this study, this situation occurs on a sample h=25 mm at 1000 Hz with a sound absorption coefficient of 0.88.This value is higher than the previous measurement value published by [6] for samples of plants such as Ficus pumila and Vernonia Eliptica with similar thicknesses due to the characteristics of thicker and wider leaves.
Furthermore, the average value of sound absorption between 250-1250 Hz and the average value of SAC at 250, 500, 1000, and 1600 Hz or the NRC noise reduction coefficient for thickness variations increase in direct proportion to the thickness of the sample (R 2 >0.9).It is observed that the peak of the sound absorption coefficient shifts to the lower frequency zone when the thickness of the specimen increases.The relationship between thickness, peak frequency, and sound velocity from a thickness of 25 mm to 50 mm can be expressed in Equation 1.This relationship shows that the plant sample has a low density and is porous, meeting the concept of porous material absorption mechanism [14], [15] as shown by previous researchers.
Figure 7 compares the values obtained for the sound absorption coefficient of TZ with a thickness of 10mm, 25 mm, and 50 mm and the Japanese spindle with a thickness of 80mm and 160 mm [9] but with the same porosity.TZ with a thickness of 50 mm provides better sound absorption characteristics than the Japanese spindle at the frequency of 400 -800 Hz even though it is thinner.This is due to the characteristics of TZ which have better sound-absorbing leaves such as thicker leaves and a larger surface and width compared to Japanese spindles even though the porosity is more or less the same.

Sound absorption performance of a green facade
Without TZ, the CB specimen has an NRC value of less than 0.2, indicating that it is a sound-reflective material.Figure 8 shows the sound behavior when the wall is covered by TZ with a thickness of 10mm, 25mm, and 50mm, containing two peaks as well as other 2-layer material curves [13] and [16].Wall specimens with TZ 25 mm and TZ 50 mm produced SAC above 0.3 at frequencies above 400 Hz.This frequency range is within the sound frequency range from the reflection of traffic noise [17].At 1250 Hz, SAC falls which can be explained by the three-quarter-wavelength resonance in the 200 mm thickness due to diffraction of sound by the edge wall specimen or by the transmission through the gaps between plant and wall or between wall and hard barking.The drop in SAC at 600-800 Hz is caused by the coherent scattering of sound by the plant leaves in the direction of the acoustic intensity probe, as also occurred in the study by [8] Furthermore, the NRC and average SAC values for TZ 25mm m and TZ 50 mm exceed 0.40 obtained using porous contre in work by [16].Figure 8(d) compares the values obtained for the sound absorption coefficient of green walls studied by [9] using walls attached with a perlite layer and Spinde 80 mm.Compared to others, TZ 50mm above the concrete wall provides the same or better sound absorption characteristics than the wall covered by perlite and Spinde 80 mm at a frequency of 400-600 Hz.In addition, concrete wall specimens with TZ have better acoustic properties than common building materials such as tile, brick, glass windows, and block concrete [18].Even TZ 25mm and 50 mm exceed the sound properties specimen of coarse block building material and green wall developed by [18] at frequencies above 400 Hz.

Conclusion
In conclusion, TZ has good characteristics to absorb sound, for example, the leaves are wide and long and are succulent.TZ absorbs sound well when the thickness reaches 25 mm and above.Additionally, TZ reveals a pore structure with openings ranging from 5 to 8 μm in length and height ranging from 35 to 38 μm.There is an increase in sound absorption when TZ is attached to the concrete brick on its surface as a green facade and produces NRC 0.37, 0.48, and 0.52 for TZ thicknesses of 10mm, 25mm, and 50mm, respectively.The value of the green facade with TZ 25mm and 50mm has a performance as good as the performance of a green wall made using a layer of perlite and spindle plants.A green facade with TZ 25mm and 50mm has a better sound absorption value than the common building materials.This study shows that TZ can be considered as one of the creeping plants that can be used to create a green facade in order to reduce noise pollution.

Figure 1 .
Measurement of (a) length, (b) width, and (c) thickness of the plant.

Figure 2 .
Figure 2. Measurement of the surface area of the plant using a graph paper method.

Figure 3 .
Figure 3. Preparation, physical, and acoustical measurement of the plant.

Figure 4 .
Figure 4. Specimen setup in impedance tube represents the green facade.

Figure 6 .
Figure 6.Result of impedance tube test and sound absorption coefficient: (a) reflection coefficient vs frequency; (b) variation of surface impedance vs frequency; (c) sound absorption vs frequency; (d) average SAC and NRC.

Figure 7 .
Figure 7.Comparison with previous research [9] on Spindle plant thickness of 80 mm and 160 mm.

Figure 8 .
Figure 8. Result of impedance tube test and analysis on green façade specimen: (a) Sound absorption of green facade with TZ; (b) Average SAC and NRC of green facade with TZ; (c) Comparison with green wall with Spindle plant and perlite layer obtained from previous research [9]; (d) Comparison with other building materials obtained from previous research[18]

Table 1 .
Density and porosity of specimens.