Anechoic termination for acoustic plane wave suppression

The design of a low-reflecting duct termination which performs in the low frequency range (20-800 Hz) is proposed, constructed, and tested. The core element of the termination consists of a "stack of plates" separated by thin spacers. The construction of the termination is compact, light, cheap, and it can be readily produced using conventional manufacturing technologies and standard materials. The design is described in detail which allows reproduction of the termination in any acoustic laboratory. The systematic parametric study of the termination is performed both experimentally and numerically by using finite element simulation software, COMSOL. On the basis of the conducted study, the optimal set of parameters is outlined. Some design rules are also proposed. The physical mechanisms governing the processes providing the performance of the termination are determined. For this purpose, a range of numerical experiments aiming to elucidate the relative role of different phenomena is conducted. It is found that the main relevant effects include: viscous losses in the thin slots between the plates; expansion of the wave front in the inter-plate
space; reflection from the periphery of the stack. The roles of plates' material, thickness, and acoustic radiation from the outer edge of the stack plates are found to be of secondary order of importance. The particular design of the invented device allows us to reach reflection coefficients less than 0.1 in the frequency range [20-800 Hz].