Team organization may help swarms of flies to become invisible in closed waveguides

TL;DR

This paper investigates how flies modeled as sound-soft obstacles can organize within a waveguide to achieve invisibility to acoustic waves, analyzing conditions for perfect invisibility and limitations at certain frequencies.

Contribution

It introduces a method for flies to arrange themselves for acoustic invisibility and demonstrates fundamental limitations at specific wavenumbers.

Findings

Flies can be organized to become acoustically invisible in waveguides.

Complete invisibility is impossible at certain low wavenumbers due to persistent scattered fields.

Embedded sound-soft obstacles always produce some scattered field at infinity for small enough wavenumbers.

Abstract

We are interested in a time harmonic acoustic problem in a waveguide containing flies. The flies are modelled by small sound soft obstacles. We explain how they should arrange to become invisible to an observer sending waves from $-\infty$ and measuring the resulting scattered field at the same position. We assume that the flies can control their position and/or their size. Both monomodal and multimodal regimes are considered. On the other hand, we show that any sound soft obstacle (non necessarily small) embedded in the waveguide always produces some non exponentially decaying scattered field at $+\infty$ for wavenumbers smaller than a constant that we explicit. As a consequence, for such wavenumbers, the flies cannot be made completely invisible to an observer equipped with a measurement device located at $+\infty$.