The acoustic analysis and design of duct lining for low-frequency noise control

Abstract

Noise pollution, such as industry noise, traffic noise and so on, have raised more and more concern with the development of society and technology. The absorption of noise in low-frequency range is still a big challenge. This study introduces the numerical tools for the duct lining and investigates practical approaches to improve the acoustic performance of lined ducts of silencers in low-frequency range.

Firstly, a spectral method of Chebyshev collocation (ChC) method is introduced for duct acoustics. The convergence of ChC is validated by a 1D and a 2D problem with analytical solutions and the accuracy and computation speed are compared to the Finite Element Method (FEM).

Based on ChC method, the transmission loss of the lined duct is calculated. The influence of the basic parameters on the acoustic performance, flow resistivity and lining thickness, are investigated by analyzing the acoustic impedance of the lined duct, especially for the low-frequency range. When an MPP is covered on the surface of lining, the acoustic impedance is changed which moves the peak of transmission loss to low frequency and improves the performance in low-frequency range. By adjusting the perforation ratio of MPP, the acoustic impedance can be changed which influences the position of the peak and the bandwidth of transmission loss. Increase of lining thickness also enhances the performance in low-frequency range, but the pressure loss of the duct is increased. Considering the constrain of pressure loss, the calculation shows increasing lining thickness can enhance performance in low-frequency range. The effectiveness of above two approaches is analyzed by regarding the sound wave in lined duct as oblique incidence approximately and analyzing the acoustic impedance of the lining material.

The performance of asymmetrical silencer is studied based on the previous calculation. The transmission loss of silencers with different symmetrical and asymmetrical design are calculated and compared. It is found that odd order modes are excited in the asymmetrical silencer and the direction of sound intensity is changed, which influences the transmission loss the silencer.

For the lined duct of finite length, Dirichlet to Newmann (DtN) boundary condition is introduced as the out-going wave boundary condition. The accuracy of DtN boundary condition is checked by an expansion chamber example. As an application of above calculation, a silencer is designed using the combination of the two approaches for improving performance in low-frequency range to reduce the noise from a cooling tower. The experiment result shows that the design effectively enhance the transmission loss in low-frequency range.

The ChC method is also applied in 2D cases. The sound propagation in a tunnel is investigated and the influence of some parameters is studied. Some approaches, including the increase of thickness of absorbing material and adding of MPP, are investigated based on ChC method.