Time-domain simulation of acoustic impedance tubes

Abstract

The two-microphone impedance tube method is widely used to measure the absorption and reflection characteristics of acoustic materials under normal incidence. The design of an impedance tube requires a proper choice of the tube dimensions, microphone positions, type of the excitation signal, and loudspeaker response. Although there are ASTM and ISO standards to guide the development and construction of impedance tubes, some issues would benefit from further studies, such as the relation between the signal-to-noise ratio, the spacing between microphones, and the amount of damping of the test specimen. To shed light on this issue and to provide the designer with an additional tool, this paper presents a time-domain numerical procedure based on the finite element method that can be used to simulate many steps related to the two-microphone method. The sound pressure signals that would be measured in practice are numerically evaluated and polluted with random noise to mimic a given signal-to-noise ratio. Then, the same signal processing procedures that would be used in a real experiment are applied to the simulated signals, so that the desired absorption properties are estimated. The simulation procedure is validated through numerical examples, which also investigate the effects of the spacing between microphones, the signal-to-noise ratio, and the amount of damping on the tube performance.