• 3D-Printed Blocks: Thermal Performance Analysis and Opportunities for Insulating Materials

      Tullio de Rubeis (MDPI AG, 2022-01-01)
      The building energy balance is strongly influenced by the heat transmission losses through the envelope. This justifies the growing effort to search for innovative and high-performance insulating materials. The 3D printing process, also known as additive manufacturing, is already used in various industrial applications thanks to its ability to realize complex structures with high accuracy. It also represents an emerging and still poorly explored field in the world of “building physics”. The aim of this work is to present the design, realization, and analysis phases of a 3D-printed thermal insulating block. The performance analysis of the block was performed via theoretical and experimental approaches. The testing phase was conducted using a Hot Box specially built for this purpose, which allowed to have known, repeatable, and steady thermal conditions. The experimental phase, based on the infrared thermography technique and heat flow meter method, allowed a preliminary evaluation of the 3D-printed block performance. Moreover, to implement the concept of circular economy, the internal cavities of the block were filled with different recovered waste materials: polystyrene and wool. The results obtained have shown, although preliminarily, the potential of additive manufacturing in the field of insulating materials.
    • 3D-Printed Programmable Mechanical Metamaterials for Vibration Isolation and Buckling Control

      Ali Zolfagharian; Mahdi Bodaghi; Ramin Hamzehei; Liam Parr; Mohammad Fard; Bernard F. Rolfe (MDPI AG, 2022-06-01)
      Vibration isolation performance at low-frequency ranges before resonance is a vital characteristic that conventional springs cannot exhibit. This paper introduces a novel zero Poisson’s ratio graded cylindrical metamaterial to fulfill two main goals: (1) vibration isolation performance in low-frequency bands prior to resonance and (2) global buckling control of a long cylindrical tube. For this purpose, “soft and stiff” re-entrant unit cells with varying stiffness were developed. The cylindrical metamaterials were then fabricated using a multi-jet fusion HP three-dimensional (3D) printer. The finite element analyses (FEA) and experimental results demonstrate that the simultaneous existence of multi-stiffness unit cells leads to quasi-zero stiffness (QZS) regions in the force-displacement relationship of a cylindrical metamaterial under compression. They possess significant vibration isolation performance at frequency ranges between 10 and 30 Hz. The proposed multi-stiffness re-entrant unit cells also offer global buckling control of long cylindrical tubes (with a length to diameter ratio of 3.7). The simultaneous existence of multi-stiffness re-entrant unit cells provides a feature for designers to adjust and control the deformation patterns and unit cells’ densification throughout cylindrical tubes.
    • 3DPortCityMeasure: Methodology for the Comparative Study of Good Practices in Port–City Integration

      María J. Andrade; João Pedro Costa; José Blasco López (MDPI AG, 2020-01-01)
      Most port cities have a long history of investment in the waterfront, adapting these spaces to improve the quality of life of its inhabitants and increase the tourist interest of the city, in a 50-year process of waterfront regeneration that started in the late 1960s. Even though one of the drivers of development in today’s port cities continues to be the transfer of knowledge and experiences between different cases, not all these cities have achieved their goals, nor have all done so in a sustainable way. This article exposes a new methodology, motivated by the need to carry out a comparative study of good practices of port−city integration for twelve specific cases. To enable a comparison of intangible realities such as port−city integration, it is mandatory to have a common benchmark to quantify features of cities from different cases. The 3DPortCityMeasure methodology is intended to provide a framework for analysing port-city integration, with results that supply an immediate understanding of each case. This tool enables direct comparative evaluation and provides support for land use planning and urban design approaches. The results show that the proposed approach for measuring intangible factors in the field of the port−city relationship is a very useful tool, novel in this discipline, and fully applicable to other cases and other urban issues.
    • 4D Printing: Materials, Technologies, and Future Applications in the Biomedical Field

      Ankur Bajpai; Anna Baigent; Sakshika Raghav; Conchúr Ó. Brádaigh; Vasileios Koutsos; Norbert Radacsi (MDPI AG, 2020-12-01)
      4D printing can be defined as the fabrication of structures using smart materials that allow the final object to change its shape, properties, or function in response to an external stimulus such as light, heat, or moisture. The available technologies, materials, and applications have evolved significantly since their first development in 2013, with prospective applications within the aerospace, manufacturing, and soft robotic industries. This review focuses on the printing technologies and smart materials currently available for fabricating these structures. The applications of 4D printing within biomedicine are explored with a focus on tissue engineering, drug delivery, and artificial organs. Finally, some ideas for potential uses are proposed. 4D printing is making its mark with seemingly unlimited potential applications, however, its use in mainstream medical treatments relies on further developments and extensive research investments.
    • 4E Analyses of a Hybrid Waste-Driven CHP–ORC Plant with Flue Gas Condensation

      Hossein Nami; Amjad Anvari-Moghaddam; Ahmad Arabkoohsar; Amir Reza Razmi (MDPI AG, 2020-11-01)
      The combination of a waste-driven hybrid heat and power plant with a small organic Rankine cycle unit was recently proposed and investigated from a thermodynamic perspective. The present study provides a more comprehensive assessment from system operation through considering the energy, exergy, exergoeconomic, and exergoenvironmental (4E) aspects in a revised design of this concept to obtain a bigger picture of the system’s technical, economic, and environmental effects on existing and future energy systems. The revised design includes a flue gas condensation unit and alternative friendly organic working fluids. For this, the hybrid plant is modeled for its thermal, economic, and environmental performances. Then, the exergy losses and environmental effects of the system are scrutinized, the cost of losses and pollutions are predicted, and lastly, sorts of solutions are introduced to improve the exergoeconomic and exergoenvironmental performances of the system. The results indicate that the highest share of exergy destruction relates to the incineration (equipped with a steam generator) with a levelized cost of approximately USD 71/h for a power plant with almost 3.3 megawatt electricity output capacity. The hybridization proposal with the flue gas condensation unit increases the sustainability index of the system from 1.264 to 1.28.
    • 5 Key Challenges and Solutions for Governing Complex Adaptive (Food) Systems

      Mollie Chapman; Susanna Klassen; Maayan Kreitzman; Adrian Semmelink; Kelly Sharp; Gerald Singh; Kai M. A. Chan (MDPI AG, 2017-09-01)
      There is increasing recognition in academic circles of the importance of adaptive governance for the sustainability of social-ecological systems, but little examination of specific implications for the 34% of land-use where human activities are pervasive but potentially commensurate with functioning ecosystems: agricultural production systems. In this paper, we argue for the need to view food systems and agro-ecosystems as multi-scalar complex adaptive systems and identify five key challenging characteristics of such systems: multi-causality; cumulative impacts; regime shifts; teleconnections and mismatch of scales. These characteristics are necessary features of multi-scalar adaptive systems, and apply equally to social and natural subsystems. We discuss the implications of these characteristics for agricultural production systems and consider how governance can rise to these challenges. We present five case studies that highlight these issues: pollinator declines; payments for ecosystem services; pest control and pesticide resistance; downstream aquatic systems in Tasman Bay, New Zealand; and riparian buffers in Puget Sound, USA. From these case studies we derive recommendations for managing agricultural systems, both specific and general. Ultimately, adaptive governance of agro-ecosystems will likely hinge upon three paradigm shifts: viewing farmers and ranchers not only as food producers but also as land and water managers; seeking not yield maximization but rather resilient management of food ecosystems; and critically, as it transcends the production-system literature, engaging broad audiences not only as consumers but also citizens.
    • 50 Shades of Green: Insights into Personal Values and Worldviews as Drivers of Green Purchasing Intention, Behaviour, and Experience

      Marjolein C. J. Caniëls; Wim Lambrechts; Johannes (Joost) Platje; Anna Motylska-Kuźma; Bartosz Fortuński (MDPI AG, 2021-04-01)
      Despite the booming interest in determinants of green (i.e., sustainable) consumption, the psychological factors that influence pro-environmental consumption patterns are not yet fully understood. To answer this call, we developed and analysed a model that offers an integrative approach to sustainable consumption patterns by addressing the full palette of consumers’ personal value orientations. Specifically, we linked consumers’ egoistic, altruistic, and biospheric values at the personal level to pro-environmental purchasing intentions, behaviours, and experiences. Furthermore, we examined whether implicit beliefs about the balance between humanity and nature (i.e., worldviews) moderate these relationships. To support the theoretical propositions in our model, we drew on the key premises of the theory of planned behaviour. Data from 291 Polish respondents were analysed by using multiple linear regression analysis, and the moderating effect of worldviews was investigated with simple slope analysis. While controlling for cultural values, the findings show that personal values, specifically biospheric values, predict sustainable consumer patterns and that consumers’ worldview moderates this relationship. Our study offers a novel holistic approach to analyse sustainable consumption patterns, which will assist environmental management scholars and practitioners who seek to understand and stimulate pro-environmental consumer behaviour. Our findings may help practitioners to develop strategies to influence consumer intentions and behaviours concerning green products.
    • 51 Flavors: Regional Resource Configurations and Foreign Multinational Market Entry in the U.S. Biopharmaceutical Industry

      Roberto S. Santos; Denise R. Dunlap (MDPI AG, 2021-08-01)
      Creating a sustainable regional economy requires not only attracting new local ventures, but also foreign multinationals. In this regard, understanding which resources are influential in market entry decisions is crucial given that there are different resource needs between developed (DMNE) and emerging market (EMNE) multinationals. Answering calls for more neo-configurational studies in the literature, our study uses a fuzzy-set qualitative comparative analysis (fsQCA) approach to examine foreign multinational entry decisions in 51 regions of the U.S. We constructed a novel dataset comprised of 3287 foreign firms from 61 countries and territories operating in the biopharmaceutical industry. We find that there are substantial differences in the configuration of resources that attract DMNEs and EMNEs to regions. The resource configurations in our models account for over 80% of the factors influencing DMNE and EMNE market entry location decisions. Some resources played a more important role in these decisions, such as FDI stocks, cluster size, and manufacturing intensity. Our findings show that EMNEs seek out regions with a greater abundance of different resources than DMNEs. This study provides practical implications for firms entering foreign markets as well as for policy makers who want to attract these firms to bolster their regional economic development.
    • 5G Mobile Services and Scenarios: Challenges and Solutions

      Yousaf Bin Zikria; Sung Won Kim; Muhammad Khalil Afzal; Haoxiang Wang; Mubashir Husain Rehmani (MDPI AG, 2018-10-01)
      The Fifth generation (5G) network is projected to support large amount of data traffic and massive number of wireless connections. Different data traffic has different Quality of Service (QoS) requirements. 5G mobile network aims to address the limitations of previous cellular standards (i.e., 2G/3G/4G) and be a prospective key enabler for future Internet of Things (IoT). 5G networks support a wide range of applications such as smart home, autonomous driving, drone operations, health and mission critical applications, Industrial IoT (IIoT), and entertainment and multimedia. Based on end users’ experience, several 5G services are categorized into immersive 5G services, intelligent 5G services, omnipresent 5G services, autonomous 5G services, and public 5G services. In this paper, we present a brief overview of 5G technical scenarios. We then provide a brief overview of accepted papers in our Special Issue on 5G mobile services and scenarios. Finally, we conclude this paper.
    • 6+: A Novel Approach for Building Extraction from a Medium Resolution Multi-Spectral Satellite

      Mayank Dixit; Kuldeep Chaurasia; Vipul Kumar Mishra; Dilbag Singh; Heung-No Lee (MDPI AG, 2022-01-01)
      For smart, sustainable cities and urban planning, building extraction through satellite images becomes a crucial activity. It is challenging in the medium spatial resolution. This work proposes a novel methodology named ‘6+’ for improving building extraction in 10 m medium spatial resolution multispectral satellite images. Data resources used are Sentinel-2A satellite images and OpenStreetMap (OSM). The proposed methodology merges the available high-resolution bands, super-resolved Short-Wave InfraRed (SWIR) bands, and an Enhanced Normalized Difference Impervious Surface Index (ENDISI) built-up index-based image to produce enhanced multispectral satellite images that contain additional information on impervious surfaces for improving building extraction results. The proposed methodology produces a novel building extraction dataset named ‘6+’. Another dataset named ‘6 band’ is also prepared for comparison by merging super-resolved bands 11 and 12 along with all the highest spatial resolution bands. The building ground truths are prepared using OSM shapefiles. The models specific for extracting buildings, i.e., BRRNet, JointNet, SegUnet, Dilated-ResUnet, and other Unet based encoder-decoder models with a backbone of various state-of-art image segmentation algorithms, are applied on both datasets. The comparative analyses of all models applied to the ‘6+’ dataset achieve a better performance in terms of F1-Score and Intersection over Union (IoU) than the ‘6 band’ dataset.
    • 7S Model for Technology Protection of Organizations

      Hyunae Park; Youngcheon Yoo; Hwansoo Lee (MDPI AG, 2021-06-01)
      Given the importance of technologies to organizations, technology leakages can cause considerable financial losses and threaten the survival of firms. Although organizations use technology protection diagnostic models to prevent such leakages, most diagnostic models focus on cybersecurity, and the evaluation system is complex, making it difficult for SMEs to use it. This makes them unsuitable for the general technology protection diagnosis of companies. Hence, this study proposes a diagnostic model that assesses these technology protection capabilities of organizations from personnel and administrative perspectives. Drawing upon the individual elements of the 7S model—shared values, strategy, structure, systems, staff, style, and skills—our model analyzes the influence of the elements on the technology protection capabilities of organizations. To determine this influence, the study conducts a questionnaire survey among 435 employees from large, larger medium-sized, and small and medium enterprises. Using the partial least squares and the artificial neural network methods, the study determines the ranking of the relative importance of the 7s elements. The results show that the shared values element most significantly influences these capabilities. The remaining elements influence the technology protection capabilities in the following order from the greatest to the least effect: staff, strategy, structure, systems, style, and skills. These findings highlight the significance of developing an awareness of the necessity of technology protection among all the members of an organization.
    • 90 Days of COVID-19 Social Distancing and Its Impacts on Air Quality and Health in Sao Paulo, Brazil

      Daniela Debone; Mariana V. da Costa; Simone G. E. K. Miraglia (MDPI AG, 2020-09-01)
      The COVID-19 pandemic has imposed a unique situation for humanity, reaching up to 5623 deaths in Sao Paulo city during the analyzed period of this study. Due to the measures for social distancing, an improvement of air quality was observed worldwide. In view of this scenario, we investigated the air quality improvement related to PM<sub>10</sub>, PM<sub>2.5</sub>, and NO<sub>2</sub> concentrations during 90 days of quarantine compared to an equivalent period in 2019. We found a significant drop in air pollution of 45% of PM<sub>10</sub>, 46% of PM<sub>2.5</sub>, and 58% of NO<sub>2</sub>, and using a relative-risk function, we estimated that this significant air quality improvement avoided, respectively, 78, 337, and 387 premature deaths, respectively, and prevented approximately US$ 720 million on health costs. Moreover, we estimated that 5,623 deaths by COVID-19 represent an economic health loss of US$ 10.5 billion. Both health and economic gains associated with air pollution reductions give a positive perspective of the efforts towards keeping air pollution reduced even after the pandemic, highlighting the importance of improving the strategies of air pollution mitigation actions, as well as the crucial role of adopting efficient measures to protect human health both during and after the COVID-19 global health crisis.
    • A <i>p</i>-Robust Green Supply Chain Network Design Model under Uncertain Carbon Price and Demand

      Ruozhen Qiu; Shunpeng Shi; Yue Sun (MDPI AG, 2019-10-01)
      The problem of designing a multi-product, multi-period green supply chain network under uncertainties in carbon price and customer demand is studied in this paper. The purpose of this study is to develop a robust green supply chain network design model to minimize the total cost and to effectively cope with uncertainties. A scenario tree method is applied to model the uncertainty, and a green supply chain network design model is developed under the <i>p</i>-robustness criterion. Furthermore, the solution method for determining the lower and upper bounds of the relative regret limit is introduced, which is convenient for decision-makers to choose the corresponding supply chain network structure through the tradeoff between risk and cost performance. In particular, to overcome the large scale of the model caused by a high number of uncertain scenarios and reduce the computational difficulty, a scenario reduction technique is applied to filter the scenarios. Numerical calculations are executed to analyze the influence of relevant parameters on the performance of the designed green supply chain network. The results show that the proposed <i>p</i>-robust green supply chain network design model can effectively deal with carbon and demand uncertainties while ensuring cost performance, and can offer more choices for decision-makers with different risk preferences.
    • A 10-year Analysis on the Reduction of Particulate Matter at the Green Buffer of the Sihwa Industrial Complex

      Sin-Yee Yoo; Sumin Choi; Namin Koo; Taehee Kim; Chan-Ryul Park; Wan-Hyeok Park (MDPI AG, 2021-05-01)
      Green buffer (GB) zones are designed to prevent the spread of air pollutants and odors from industrial complexes (ICs) to residential areas (RAs). We analyzed changes in the concentration of particulate matter (PM) and the number of high PM pollution days for 10 years after the GB was implemented, using the National Atmospheric Environmental Research Stations 2001–2018 dataset. We also performed field measurements of PM<sub>10</sub> and PM<sub>2.5</sub> from February 2018 to January 2019 to analyze the PM concentrations at human breathing height throughout the GB. Before GB implementation (2001–2006), PM<sub>10</sub> in the RA was 9% higher than that in the IC. After GB zone implementation (2013–2018), PM<sub>10</sub> in the RA was 11% lower than that of the IC. Furthermore, the PM concentration in the RA (slope = ∆Concentration/∆Time, −2.09) rapidly decreased compared to that in the IC (slope = −1.02) and the western coastal area (WCA) (slope = −1.55) over the 10-year period. At PM concentrations at human breathing height, PM<sub>10</sub> and PM<sub>2.5</sub> in the RA were lower than those in the IC by 27% and 26%, respectively. After GB implementation, the wind speed was positively correlated but SOx was negatively correlated with the PM reduction rate at a local scale. These results show that there was a reduction of PM during and after GB implementation, implying the need for proper management of GBs and continuous measure of pollutant sources at the green buffers of industrial complexes.
    • A 10-Year Statistical Analysis of Heavy Metals in River and Sediment in Hengyang Segment, Xiangjiang River Basin, China

      Jingwen Tang; Liyuan Chai; Huan Li; Zhihui Yang; Weichun Yang (MDPI AG, 2018-04-01)
      Heavy metal elements in water and surface sediments were characterized in Hengyang river segment in Xiangjiang River basin, one of China’s most important heavy metal control and treatment region. Data of heavy metal monitoring results in water and sediment for 10 years were acquired from an environmental monitoring program in the main channel of the studied area. Descriptive and exploratory statistical procedures were performed to reveal the characteristics of the sample distributions of heavy metal elements. The sample distributions of heavy metal elements were largely skewed right. Data censoring and too severe rounding in the water monitoring data were identified to have caused discretization in the sample distributions. Temporal and spatial characteristics of the data sets were addressed. The chromium (Cr) in the sediment possessed unique behavior, and this could be caused by a rapid deposition and releasing process.
    • A 3D-FE Model for the Rutting Prediction in Geogrid Reinforced Flexible Pavements

      Giovanni Leonardi; Federica Suraci (MDPI AG, 2022-03-01)
      Permanent deformation (rutting) is an important disturbing failure on flexible road pavements. This phenomenon appears on the flexible pavement as longitudinal depressions, and it is a consequence of the degradation of materials under high traffic loading based on consolidation/densification, surface wear, plastic/shear flow, and mechanical deformation. Hence, the rutting phenomenon depends on the accumulation of permanent deformations on pavement surfaces subjected to repeated wheel loads. In recent years, several studies have confirmed that the service life of asphalt pavements can be increased by using geosynthetics between or within layers because of the improved mechanical properties. The aim of this paper is to present the results of the 3D-finite element (FE) simulations and the development of the rutting phenomenon in a traditional flexible pavement and a reinforced one, both subjected to a cyclic load. Through Abaqus/CAE software, a road section reinforced by a geogrid was analyzed and compared with a traditional road section to investigate the advantages given by the geosynthetic completely embedded at two-thirds of the asphalt concrete layer (AC) in terms of permanent deformations. The results show the capability of the proposed FE study, that uses the plasticity model of Drucker-Prager for unbound materials combined with the simple creep law to model HMA layers to predict the permanent deformation distribution.
    • A 5S Lean Strategy for a Sustainable Welding Process

      Carlos Manzanares-Cañizares; Alberto Sánchez-Lite; Víctor F. Rosales-Prieto; José Luis Fuentes-Bargues; Cristina González-Gaya (MDPI AG, 2022-05-01)
      The correct performance of the welding processes used to join metal parts or structural elements is considered to be of vital importance to guarantee the reliability of these products during their useful lives. Adequate workstation design ensures a safe environment in which an operator can perform these processes correctly. In order to guarantee the quality of welding seams, which are used to join metal parts and structural elements, a series of standards have been developed; these standards establish requirements to guarantee the correct performance of welding processes, and the inspection of the metal welds obtained. The 5S methodology has proven to be a valid tool for improving workplaces in industrial and service activities; it is a capstone methodology when companies implement lean production approaches. This work presents a framework for applying the 5S methodology in metal welding workplaces. It defines an index to evaluate the degree of implementation of the 5S methodology, and the application of the important performance analysis methodology (IPA). Fuzzy logic is used to treat the uncertainty in evaluating the different evaluation indicators proposed. This framework is applied to a real practical case, to provide an example of its use in establishing good manufacturing practices that guarantee compliance with the requirements of welding standards, and guaranteeing the correct handling and storage of the materials and tools used in welding processes for the manufacture of welded parts and structural elements.
    • A Balancing Method of Mixed-model Disassembly Line in Random Working Environment

      Xuhui Xia; Wei Liu; Zelin Zhang; Lei Wang; Jianhua Cao; Xiang Liu (MDPI AG, 2019-04-01)
      Disassembly is a necessary link in reverse supply chain and plays a significant role in green manufacturing and sustainable development. However, the mixed-model disassembly of multiple types of retired mechanical products is hard to be implemented by random influence factors such as service time of retired products, degree of wear and tear, proficiency level of workers and structural differences between products in the actual production process. Therefore, this paper presented a balancing method of mixed-model disassembly line in a random working environment. The random influence of structure similarity of multiple products on the disassembly line balance was considered and the workstation number, load balancing index, prior disassembly of high demand parts and cost minimization of invalid operations were taken as targets for the balancing model establishment of the mixed-model disassembly line. An improved algorithm, adaptive simulated annealing genetic algorithm (ASAGA), was adopted to solve the balancing model and the local and global optimization ability were enhanced obviously. Finally, we took the mixed-model disassembly of multi-engine products as an example and verified the practicability and effectiveness of the proposed model and algorithm through comparison with genetic algorithm (GA) and simulated annealing algorithm (SA).
    • A Bamboo Treatment Procedure: Effects on the Durability and Mechanical Performance

      Quoc-Bao Bui; Anne-Cécile Grillet; Hoang-Duy Tran (MDPI AG, 2017-08-01)
      Bamboo is a natural material having a fast reproduction and high mechanical strengths. However, when a bio-based material in general, and bamboo in particular are expected to be a construction material, their sensitivity to moisture and their durability are usually questionable. Indeed, it is well known that these materials do not possess the same performance in the long-term, when compared to industrial materials. Sustainable solutions for the bamboo treatment still need to be investigated. The present study explores the oil-heated treatment with different types of oils, like flax or sunflower oils. The present investigation concentrates on mechanical properties and durability of treated bamboos to assess the effectiveness of these kinds of treatment. First, bamboo specimens were treated to decrease their sensitivity to moisture and improve their durability. Different conditions of treatment were tested: treatment at 100 °C or 180 °C; with flax oil, sunflower oil, or without oil; treatment durations of 1 h, 2 h, or 3 h; and, different cooling methods and cooling durations. Then, mechanical and durability tests were carried out on untreated and treated bamboos: uniaxial compression tests, 3 points bending tests, water immersion tests, and humidity tests. The results showed that some tested treatment methods could increase both the durability and the compressive strength of treated specimens, compared to untreated bamboo. The best results were observed on specimens treated at 180 °C during 1 h or 2 h without oil, and then cooled in 20 °C sunflower oil.
    • A Basic Study on a Rectangular Plane Space Sound Absorber Using Permeable Membranes

      Kimihiro Sakagami; Takeshi Okuzono; Yu Somatomo; Kota Funahashi; Masahiro Toyoda (MDPI AG, 2019-04-01)
      In this communication, the sound absorption characteristics of rectangular-shaped plane space sound absorbers without any backing structure using permeable membranes (PMs) are measured by reverberation room method. First, three types of PMs, in this study woven fabrics, are selected with different flow resistances and surface densities. They are prepared in the plane rectangular-shaped space absorbers of two different sizes. The measured results are discussed through comparison with the existing theoretical and measured results for absorbers of the other shapes or configurations. The present results and discussion demonstrate that the reverberation absorption coefficients of the proposed absorbers are low at low frequencies and converge to a moderately high value at high frequencies. Especially, ones with higher flow resistance than the air impedance converge to a value greater than 0.5, which is a theoretically estimated maximum absorption coefficient of infinite single-leaf PM. This is inferred to be attributed mainly to area effect. From these results the proposed absorbers can be used effectively despite of their very simple structure. Also it is found that the proposed absorber can offer higher sound absorption than permeable membrane absorbers of other shapes or configuration. Regarding the effect of the size, the absorbers of smaller size offer higher absorption coefficients regardless of material properties of the PMs used in the experiments.