Sound absorption performance test of grass compression board to promote resource circulation

. Grass resources such as miscanthus sinensis, reeds, and rushes have been used as materials for roofs and floors in Japan since ancient times. These materials are the most eco-friendly and sustainable building materials from the viewpoint of resource recycling, but their use has been declining in modern times due to the spread of high-performance building materials created in the process of modern industrialization. This research is to find a way to utilize such grass resources as sound absorbing materials in interior spaces as one possibility to utilize them again in modern life, and to explore their practical possibilities. As a result of measuring the sound absorption performance of compression-molded boards made from grass resources at different densities and thicknesses, it was confirmed that boards made from Miscanthus sinensis at a density of 0.35 g/cm 3 showed a peak sound absorption at around 900 Hz, indicating high sound absorption performance in the sound range of human speech. COVID-19 has caused the problem of noisy talking voices from online meetings in offices, and demand for this material can be expected as one of the means to solve this problem.


Introduction
Grass resources such as miscanthus sinensis, reed, and rush have been widely used as raw materials for building materials in Japan since ancient times, as typified by thatched roofs, earthen walls of straw mats, and tatami mat floors. These grasses emit very little carbon dioxide during the process of use as building materials and can be returned to the soil as compost after use, making them highly recyclable resources. Among these, thatch (a general term for when awn and reeds are cut after the above-ground parts have withered and used as roofing material) is collected at thatched fields scattered in villages and regions throughout Japan, and is used not only as roofing material for houses, but also as daily necessities such as baskets and brooms, and is recognized as an indispensable material for daily life. However, since the 1950s, the use of thatch has been on the decline due to several factors, including the spread of high-performance building materials created by modern industrialization and the decline of mutual support in local communities. Currently, few new houses with thatched roofs are built in Japan, and the use of thatch as roofing material for cultural heritage buildings accounts for about 80% of its demand. [1] In recent years, as awareness of global environmental issues has increased, the recyclability and environmental performance of building materials made from grass resources have once again come under the spotlight. In the Netherlands and Denmark, new thatched buildings are being constructed while maintaining traditional thatching, taking advantage of the environmental performance of thatch, and attempts are being made to use thatch as a material for modern buildings such as buildings with energy-saving performance. [2,3] In Japan, there have been studies on thatch construction methods and thatch procurement [4], as well as previous studies showing insulation performance [5] and environmental impact assessment [6], and there have been attempts, albeit on a small scale, to introduce thatch into the roofs and walls of modern buildings. [7] In order to solve problems such as the decline of thatch farms and the lack of bearers and to restore their cyclical nature, it is essential to increase the use of thatch. However, in order to use thatch as a building material in modern urban areas, it is difficult to ensure its fire resistance performance as required by law, which is a major hurdle for its widespread use. This study is one possibility to reutilize such grass resources in daily life, finding a way to use it as a soundabsorbing material in interior spaces and practically seeking to promote its use. The sound absorption performance of materials created by crushing, chipping, and compression molding grass resources will be measured, and a way to effectively use them in daily life will be verified within the limits of what is possible to utilize even under modern regulations. We believe that it is an important research issue to investigate effective ways to promote the use of grass resources as building materials in order to address global environmental issues and realize a sustainable society.

Sound absorption performance test
To verify the sound absorption performance of compression molded plates, sound absorption performance tests were conducted using the vertical incidence method with acoustic tubes, referring to JIS A 1405. [8] The configuration of the test apparatus is shown in Figure 3, and a photograph of the acoustic tube is shown in Figure 4. A plane wave is generated in the tube by a sound source, and a wall-mounted microphone is used to measure the complex sound pressure transmission function between two fixed positions to resolve the incident wave component and the reflected wave component. From both components, the vertical incident sound absorption coefficient is calculated.

About the compression molded board
The compression-molded boards to be tested were made from Miscanthus sinensis, which was created by a company (Fuji Yuwa Sangyo) that harvests and sells thatch, and by a company (S wood) that manufactures compression-molded boards made from various natural materials. The boards are made using only natural materials, including adhesives, and can be returned to the soil after use as a building material. Material 1 was a thin compression-molded board that was crimped onto a plywood board, and materials 2 and 3 were boards made by varying the density of the compression. Photographs of the materials used and the density of compression are shown in Table 1.

About Test Materials
In conducting the sound absorption performance test, several materials were cut to a size of 98 mm in diameter to be attached to the acoustic tube. A list of the test materials is shown in Table 2. Table 2. Types of test materials Materials 1, 2, and 3 are the above-mentioned compression-molded boards made from Miscanthus sinensis. 1 is a thin compression-molded board crimped to a plywood board, and materils 2 and 3 are boards made only from Miscanthus sinensis, with different compression densities. Test material 4 is a wood board and material 5 is a urethane foam. The sound absorption coefficient of each material was measured by the vertical incidence method using an acoustic tube to compare and verify their sound absorption characteristics. Considering the individual differences in density and thickness of the part to be cut as a test piece, two test pieces were made for each of test pieces 1 through 4, and sound absorption measurements were made from the front and back surfaces, and the average value of the four data obtained was calculated for comparison and verification.

Results
The sound absorption coefficient for each sound range (frequency band) of each test materials obtained from the sound absorption performance test is shown in Fig 5. Compression molded board which were crimped to veneer 1, wood board 4, and urethane foam 5, showed higher sound absorption coefficients as the sound range became higher, whereas boards 2 and 3, which were compression molded only with awn, showed higher sound absorption in the low and middle frequency range of 500 to 1000 Hz, close to the sound range band of human speech. Comparing boards 2 and 3 with different compressive densities, the sound absorption coefficient of board 3, which was compression molded at a density of 0.35 g/cm 3 , was higher overall than that of board 2, which was compression molded at a density of 0.55 g/cm 3 , especially in the range of 900 to 1000 Hz, where board 3 showed about twice the sound absorption of board 2. In the low frequency range below 500 Hz, all specimens showed a low sound absorption coefficient of 0.2 or less.

Conclusion
Conventional porous sound-absorbing materials, such as glass wool and urethane foam, exhibit sound absorption properties due to the viscous resistance of air when sound waves pass through the voids in the material. These materials show high sound absorption coefficient in the high frequency range, but the sound absorption coefficient in the low and mid frequency range is insufficient. The results of this experiment showed that boards made by chipping grass resources and compression-molding them at low density exhibited high sound absorption properties, and that the sound absorptive sound bands varied depending on the density and thickness of the boards. The COVID-19 has caused a problem in offices, where the noise from online meetings has become a problem.
To solve this problem, demand can be expected for this material, which shows a peak in sound absorption at 500-1000 Hz, the sound bandwidth band of the human voice. In addition, there are reports that in shelters set up in times of disaster, the reverberant noise of talking voices and footsteps during walking has a stressful effect on the shelters as noise, and demand for this material is expected to solve such problems as well. [9] In order to use the low-density compression-molded plates used in this study as sound-absorbing materials in actual indoor spaces, it is necessary to verify them from various angles, such as strength, light weight, and surface smoothness. We also consider it a future task to seek a composition of composite sound-absorbing materials with effective sound-absorbing properties for a specific sound range by overlapping multiple materials with different densities and thicknesses. In order to widely promote the use of sound-absorbing materials made from grass resources as modern building materials, it is necessary to consider their use in urban areas under strict legal restrictions. In addition to the possibility of using the material for interior walls and ceilings, it should be used for furniture, fittings, art panels, and other interior spaces on a smaller scale, within the scope of no legal interior restrictions. We would like to continue to explore the possibilities for uncovering new demand for grass resources and restoring its cyclical nature in our daily lives once again.