Optimal design of sound absorbing systems with microperforated panels

As the development of technology makes economic prosperity and life more convenient, people now desire a higher quality of life. This quality of life is based not only on the convenience in their life but also on clean and eco-friendly environments. To meet that requirement, much research is being performed in many areas of eco-friendly technology, such as renewable energy, biodegradable content, and batteries for electronic vehicles. This tendency is also obvious in the acoustics area, where there are continuing attempts to replace fiber-glass sound absorbers with fiber-free materials. The combination of microperfoated panels (MPP) (one of the fiber-free sound absorbing materials), usually in the form of a thin panel with small holes, and an air backing may be one of the preferred solutions. These panels can be designed in many ways, and usually feature many small (sub-millimeter) holes and typically surface porosities on the order of 1 percent. The detailed acoustical properties of MPPs depend on their hole shape, the hole diameter, the thickness of the panel, the overall porosity of the perforated film, the film’s mass per unit area, and the depth of the backing air cavity. Together, these parameters control the absorption peak location and the magnitude of the absorption coefficient (and the magnitude of the transmission loss in barrier applications). By an appropriate choice of these parameters good absorption performance can be achieved in a frequency range one or two octaves wide. That kind of solution may be adequate when it is necessary to control sound only in a specified frequency range (in the speech interference range, for example). However, in order to provide appropriate noise control solutions over a broader range of frequencies, it is necessary to design systems featuring multiple-layers of MPPs, thus creating what amounts to a multi-degree-of-freedom system and so expanding the range over which good absorption can be obtained. In this research, three different situations were considered: one was studying the combination of microperforated panels with tapered holes and a specific depth of air backing space with a view to finding the trade-off between hole angle and surface porosity. Secondly, it was of interest to study the use of multiple-layer MPPs as functional absorbers. Finally, there is a study of the optimization of a multi-layer cylindrical duct liner that gives maximum axial attenuation. Note that “Functional Absorber” is the name given to a system that can be hung, in an industrial space, for example, to provide acoustic absorption. The duct applications of interest would be in HVAC systems, whether in buildings, automotive systems or personal ventilators. In both applications, the focus was on obtaining the best possible performance in the full speech interference range, which spans the range from 500 Hz to 4000 kHz. In each case, a transfer matrix method has been developed to calculate the transmission loss and absorption coefficients provided by the systems. Note finally that the design of an N multiple-layer MPP system depends on 5N-1 parameters, and so a general optimization becomes difficult in realistic cases when as many as ten layers might be used. Thus, the use of a genetic algorithm to optimize the system parameters has been adopted, since an algorithm of that sort can efficiently identify good solutions from a very large design space. The results, as presented in this thesis, show that it is possible to identify the best combination of MPP properties that improve the desired acoustic performance, whether absorption or transmission loss, in a prescribed frequency range

Rapid Recovery of Demolished Young-Dong Highway 205.4km Due to Heavy Rain

Yong-dong highway that has an important role to connect between the west end and the east resort areas of South Korea was demolished at 205.4km from the west end due to an unexpected heavy rain whose intensity is 62.4mm/h on July 15th, 2006. Foundations of concrete retaining walls contact on Walljeong stream that has a steep bed slope and small water volume except a rainy season were scoured, and the concrete retaining walls were overturned. The water came into the embankment, and the westbound lanes of the highway were destroyed. The highway was blocked to traffic just before a big holiday season, and Korea Expressway Corporation (KEC), an agency of South Korea government, changed the flow line away from the embankment to protect it from scour, and dropped a lot of huge rocks whose diameters are over 1m to recover the demolished embankment. KEC finished the recovery works only for two days and reopen the highway. To prevent the highway from the pavement deformation and the future scour, grouting under pavement and rock-socketed concrete retaining wall with earth anchors without a foot were made for four months after the rapid recover

Optimal design of sound absorbing systems with microperforated panels

As the development of technology makes economic prosperity and life more convenient, people now desire a higher quality of life. This quality of life is based not only on the convenience in their life but also on clean and eco-friendly environments. To meet that requirement, much research is being performed in many areas of eco-friendly technology, such as renewable energy, biodegradable content, and batteries for electronic vehicles. This tendency is also obvious in the acoustics area, where there are continuing attempts to replace fiber-glass sound absorbers with fiber-free materials. The combination of microperfoated panels (MPP) (one of the fiber-free sound absorbing materials), usually in the form of a thin panel with small holes, and an air backing may be one of the preferred solutions. These panels can be designed in many ways, and usually feature many small (sub-millimeter) holes and typically surface porosities on the order of 1 percent. The detailed acoustical properties of MPPs depend on their hole shape, the hole diameter, the thickness of the panel, the overall porosity of the perforated film, the film’s mass per unit area, and the depth of the backing air cavity. Together, these parameters control the absorption peak location and the magnitude of the absorption coefficient (and the magnitude of the transmission loss in barrier applications). By an appropriate choice of these parameters good absorption performance can be achieved in a frequency range one or two octaves wide. That kind of solution may be adequate when it is necessary to control sound only in a specified frequency range (in the speech interference range, for example). However, in order to provide appropriate noise control solutions over a broader range of frequencies, it is necessary to design systems featuring multiple-layers of MPPs, thus creating what amounts to a multi-degree-of-freedom system and so expanding the range over which good absorption can be obtained. In this research, three different situations were considered: one was studying the combination of microperforated panels with tapered holes and a specific depth of air backing space with a view to finding the trade-off between hole angle and surface porosity. Secondly, it was of interest to study the use of multiple-layer MPPs as functional absorbers. Finally, there is a study of the optimization of a multi-layer cylindrical duct liner that gives maximum axial attenuation. Note that “Functional Absorber” is the name given to a system that can be hung, in an industrial space, for example, to provide acoustic absorption. The duct applications of interest would be in HVAC systems, whether in buildings, automotive systems or personal ventilators. In both applications, the focus was on obtaining the best possible performance in the full speech interference range, which spans the range from 500 Hz to 4000 kHz. In each case, a transfer matrix method has been developed to calculate the transmission loss and absorption coefficients provided by the systems. Note finally that the design of an N multiple-layer MPP system depends on 5N-1 parameters, and so a general optimization becomes difficult in realistic cases when as many as ten layers might be used. Thus, the use of a genetic algorithm to optimize the system parameters has been adopted, since an algorithm of that sort can efficiently identify good solutions from a very large design space. The results, as presented in this thesis, show that it is possible to identify the best combination of MPP properties that improve the desired acoustic performance, whether absorption or transmission loss, in a prescribed frequency range

Numerical modeling of microperforated acoustical materials

As technology makes people\u27s lives more convenient and makes the quality of their life higher, people now desire clean and eco-friendly environments. This tendency is also shown in the acoustics area, where there is renewed interest in sound absorber design. Microperforated panels, one of these sound absorbing materials, can be designed in many ways. These panels feature many small (sub-millimeter) holes and typically small surface porosities on the order of 1 percent. The classical Maa theory, one of the most popular theories for microperforated materials, was initially formulated for constant diameter cylindrical holes. Since then, a number of ad hoc corrections have been suggested to account for different hole shapes, in particular, rounding of the aperture. These ad-hoc end corrections create a discrepancy with experimental results, especially in the low frequency range. In this thesis, more accurate end corrections have been calculated by using computational fluid dynamics (CFD). It is shown that the resistance and reactance of small apertures may be calculated using relatively simple CFD models in which a single hole is modeled. The fluid is assumed to be viscous but incompressible, and the hole geometry is assumed to be axisymmetric. It will be shown that this approach essentially reproduces the classical theory of Maa for circular apertures. However, it will also be shown that the empirical correction to the resistive end correction, in particular, exhibits a clear dependence on frequency and geometrical parameters that is not accounted for in conventional microperforated material models. In this thesis, a more convenient equation for the end correction, especially for the dynamic flow resistance, is formulated as a function of frequency and geometrical parameters. For the geometrical parameters, sharp-edged holes, round-edged holes and tapered holes were considered to create standard equations