IMPLEMENTATION OF SOFT BOUNDARY CONDITION ON MICRO-PERFORATED PANEL TO ENHANCE THE ABSORPTION PERFORMANCE AT LOW FREQUENCY

• Main Author: Ulfah Pratiwi, Radhiyah
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/80019
• Institution: Institut Teknologi Bandung

Sound absorption at low frequencies (<250 Hz) remains challenging because the acoustic material is only effective at high particle velocities, which allows for greater viscous and heat dissipation mechanisms. This condition generally satisfies when the absorber thickness equals 1/4 of the wavelength. In short, a material requires minimum thickness to achieve the specified absorption target. This relationship is formulated through the causality principle and hard boundary condition, with the maximum particle velocity obtained at the material's interface. When the absorption target is related to low-frequency waves, this causes the absorber system's dimensions to become thick, which then causes technical issues in practical applications. This study implements the soft boundary condition concept in the Micro Perforated Panel (MPP) to improve absorption performance at low frequencies. When a material uses a soft boundary condition, the maximum particle velocity is found at the boundary condition plane rather than the material surface. Consequently, this can improve absorption performance regardless of the material's thickness. With these properties, high absorption performance at lower frequencies can be achieved with the same material thickness when a hard boundary condition is used. The development of the material in this study concentrated on combining these two boundary conditions to achieve bandwidth absorption from low frequencies to higher frequencies. Thus, analytical modeling and simulation use the Maa formulation and matrix transfer approach for MPP. We verified the results through experimental validation. The obtained results demonstrate that the ideal soft boundary condition in MPP results in improved absorption performance at frequencies below 100 Hz with a thickness of less than a quarter of the wavelength (?/4), and there is agreement between simulation and experimental results. We also developed and experimentally validated the open gap to realize the idea of soft boundary conditions in MPP. The obtained results demonstrate that the characteristics of the resulting absorption performance are close to those of ideal soft boundary conditions. Further development was carried out to enable practical application of the soft boundary condition through a parallel combination structure of a hard-soft boundary condition and open gap on the front and sides of the MPP. MPP with both hard and soft boundaries combined in parallel yields an absorption coefficient (?) of 0.9 at a frequency of 58 Hz. The combination of MPP with open-gap at the back of the cavity depth and on the side and front sides of the MPP yields an ? at a frequency of 32 Hz of 0.6-0.8. Finally, a parallel combination of MPP with a hard boundary condition and MPP with an open gap at the front, side, and back results in an ? of 0.7 at a frequency of 30 Hz. Despite its shortcomings, several experimental studies show that the proposed design has absorption performance close to ideal soft boundary conditions. Furthermore, an enhancement in bandwidth absorption at low frequencies can be obtained by adjusting the cavity depth of MPP. Therefore, this research's findings contribute to realizing a soft boundary condition design that can be implemented in an MPP absorber system. ?