Radiation by a subwavelength‐sized acoustic monopole: Energy considerations and application to subwavelength time‐reversal focusing.

Abstract

In this work, the radiation of pulsed acoustic waves by an acoustic monopole is theoretically studied based on energy consideration. Specifically, the instantaneous acoustic intensity and the instantaneous acoustic energy contained in a spherically symmetrical pulsed radiated field are calculated. In the case of a subwavelength‐sized acoustic monopole, it is shown that the radiation of a given amount of energy involves energy exchanges between the field and the source that are much larger than the radiated energy. It is also shown that even after the source has been turned off, the instantaneous acoustic intensity may be directed inwards, although the field cannot exchange energy with the source anymore. By time‐reversing the radiated field, we demonstrate that subwavelength refocusing requires a subwavelength energy exchanger, which is not only needed to absorb the incoming energy as well‐known, but importantly also to transiently provide energy to the field before reabsorbing it. In other words, we demonstrate in a homogeneous medium that the acoustic sink needed to perform subwavelength time‐reversal must transiently be an active source of energy.