Conversion of tones to soothing noise

Abstract

When pure-tone sound signal of frequency fs interacts with a boundary whose acoustic property varies at a frequency of fm, part of the incident sound energy is scattered to the sum and difference frequencies, fs ± n fm, where n is any integer but n = 1 dominates. When the modulation frequency fm has a bandwidth, spanning from fm1 to fm2, the resulting scattered sound carries the same bandwidth of fm2-fm1. In this manner, pure tone is partly converted to broadband noise and will be less annoying to human ears. This talk first introduces the initial experimental evidence of such frequency scattering using a realistic design (see reference paper with doi:10.1038/ s42005-021-00721-1). Then the latest results of numerical study on the maximization of the energy scattering is presented. An attempt is also made to examine the question of what room mode or eigen frequencies will be when part of the room boundaries has a time-dependent wall properties. Note that the time-varying wall remains passive in the sense that the property variation is independent of the incident sound and there is no energy input required.