A slow-waveguide filter as an acoustic interference controlling device

Abstract

Using the fact that low-frequency sound waves are diffracted by any kind of obstacle, a ‘‘sound velocity reducer’’ waveguide has been designed so as to create a 180° phase lag between the diffracted part of the incident noise and that transmitted through the duct of the guide. Further, this waveguide, being a low-pass filter, insures that the acoustic pressure amplitudes diffracted and transmitted are of the same order of magnitude. For a low-frequency band of noise, this results in the formation of coherent dipole line sources and, thus, of destructive interferences, at a distance from the acoustically transparent obstacle. The goal of this paper is to show how we can calculate this phase lag and the relative amplitudes of the components, and how they compare to the actual measurements. The results for the model investigated show good agreement between calculation and measurement, within the precision expected.