Acoustic Attenuation of Periodic Helmholtz Resonator and Their Use in Designing Periodic Scatterers for Outdoor Noise Control

Abstract

Present thesis deals with the study of various configurations of Helmholtz resonators, combined with various acoustic elements such as perforated plates, micro perforated plates and porous materials. It also emphasizes the noise control and metamaterials perspective including designing periodic scatterer made with array of Helmholtz resonators along its length for outdoor noise control in low frequency regime. The research starts with studying a Helmholtz resonator from acoustic metamaterial perspective. Helmholtz resonator is a single negative metamaterial where the effective bulk modulus is negative. The said fact has been re-established analytically and verified by results obtained from simulations. First the scattering parameters have been calculated from transfer matrix which have been used to calculate the effective acoustic properties with an assumption that the material under investigation is homogeneous, and width of the sample is much smaller compared to the wavelength corresponding to maximum frequency of interest. Additionally, the experimental results are presented which support the said observations. Next, various configurations of Helmholtz resonators have been studied to attain double negative effective properties. The various configurations are Helmholtz resonators in series, parallel, and the dual Helmholtz resonators in series and parallel. The said configurations are also studied with the similar and dissimilar resonators. Out of seven configurations, it has been shown that with five configurations attain double negative properties. Additionally, with an array of Helmholtz resonators, it has been demonstrated that the effective bulk modulus and the effective density are getting negative at resonance frequency regime having elevated frequency bandwidth. Following, the acoustic elements such as perforated plate (PP) and micro perforated plate (MPP) have been combined with Helmholtz resonator and have been studied to attain double negative effective properties in broadband. Total eight different configurations have been conceptualized and corresponding analytical solutions supported with experimental measurements have been presented. Afterwards, with an intention to broaden the bandwidth of negative effective properties, the porous material has been installed in Helmholtz resonator cavity and have been studied. On installing the porous material inside the resonator, the peak amplitude of transmission loss around resonance frequency regime is getting dampened and widened. The detailed study discovers that the resonator filled completely with porous material is practically not of interest because of its very low amplitude. However, with array configurations it has been shown that the resonance peak amplitude is getting significantly elevated in broadband which can address many practical problems. From metamaterial perspective, it also has been demonstrated that on installing porous material in resonator cavity, the hybrid Helmholtz resonator is turning into a double negative metamaterial which is also observed with finite array of Helmholtz resonators. Eventually, the noise of outdoor unit of the split type air conditioners (AC) have been studied. The experimental measurements show that the noise of outdoor unit carries dominant low frequency components with high amplitude typically below 200 Hz. The tuneable periodic scatterer has been suggested as a noise barrier where each scatterer is an array of Helmholtz resonators along its length. A detailed design and laboratory level characterization agreeing the results obtained from simulations have been presented. After deployment of said designed noise barrier in front of three outdoor units of split type air-conditioners, the overall noise reduction have been presented.