Improved effectiveness of acoustic absorbing materials by using air layers in a one-dimensional tube

Acoustic absorbing materials are used in various applications to reduce noise. In air ducts and pipes, they are used to attenuate noise from devices such as fans. When an absorbing material is installed across the entire cross-section of a duct, sound wave attenuation is large, but so is pressure loss in the flow. For greater effectiveness with less acoustic absorbing material in a one-dimensional tube system, we propose a method in which layers of acoustic absorbing material are separated by air layers and we derive the transmission loss in this method. A parametric study was performed to confirm the effectiveness of the air layers and investigate characteristics of the system. The results show that the method's effectiveness is due to the antiresonance properties of the air layers. An experiment with an acoustic tube was performed to assess the validity of the proposed method and the derived transmission loss. The experimental and numerical results for transmission loss agree well, indicating the proposed method is effective when acoustic absorbing material layers are used.