A study of low-frequency broadband noise control in ventilation ductwork systems based on Helmholtz resonator arrays

Abstract

The ventilation ductwork system is an essential and significant part of the building to maintain good indoor environmental quality. However, it is common to encounter noise problem in a ventilation ductwork system. The accompanied duct noise from the ventilation system could propagate into the occupied zones through the waveguide and could deteriorate human being's working or living environment quality. The aims of this thesis are to achieve a low-frequency broadband noise control in the ventilation ductwork system based on Helmholtz resonator arrays. Helmholtz resonator (HR hereafter) is one of the most basic acoustic models and has been widely used in engineering applications due to its simple, tunable and durable characteristics. In order to improve the noise attenuation performance of a HR, an extended neck or spiral neck taken the place of the traditional straight neck of a HR has been investigated. Based on the transmission loss index, the noise attenuation capacity index is first proposed in this thesis to evaluate the noise attenuation performance of a HR. Since a single HR is qualified as a narrow band silencer, an array of HRs is one possible way to obtain a broader noise attenuation band. A theoretical study of the acoustic performance of different HR configurations has been presented in this thesis. The proposed noise attenuation capacity is used to evaluate the acoustic performance of different HR array configurations. The predicted theoretical results fit well with the Finite Element Method simulation results. The dispersion characteristics of sound wave propagation in a periodic ducted HR system has also been investigated. The Bloch wave theory and transfer matrix method are adopted to investigate the wave propagation in a periodic ducted HR system. Owing to the coupling of Bragg reflection and HR's resonance, it is found that a periodic ducted HR system can provide a much broader noise attenuation band. However, the broader the noise attenuation band, the lower the peak attenuation amplitude. It is therefore that a noise control zone compromising the attenuation bandwidth or peak amplitude is proposed for noise control optimization. The transmission loss achieved by a periodic ducted HR system is depended on the structure and the number of HRs mounted on the duct. However, the number of HRs is restricted by the available space in longitudinal direction of the duct. Moreover, such system will occupy a large space and may have some spare space in the transverse direction of the duct. By adding HRs on the available space in the transverse direction, a modified ducted HR system is therefore proposed to improve the noise attenuation performance of the ducted HR system and fully utilizing the available space. The results indicate that both the noise attenuation band and peak amplitude are increased by adding HRs on arbitrary side of the cross-section of the duct. Aiming at broader noise attenuation bands for hybrid noise control at low frequencies, a periodic dual HR array is proposed and investigated. The dual HR which consists of two HRs connected in series (neck-cavity-neck-cavity) leads to two resonance frequencies. By analogy with a two degrees of freedom mechanical system, the resonance frequencies and transmission loss of a dual HR has been derived. The dual HR is also effective at its resonance pears with relative narrow bands. Owing to the coupling of Bragg reflection and dual HR's resonances, a periodic dual HR array can provide much broader noise attenuation bands at the designed resonance frequencies of the dual HR. It is hoped that the present work can advance the investigation of noise control method for the ventilation ductwork systems or other research areas in respect of HR.