  Mixtures of gas and dust are pervasive in the universe, from AGN and
molecular clouds to proto-planetary discs. When the two species drift relative
to each other, a large class of instabilities can arise, called resonant drag
instabilities (RDIs). The most famous RDI is the streaming instability, which
plays an important role in planet formation. On the other hand, acoustic RDIs,
the simplest kind, feature in the winds of cool stars, AGN, or starburst
regions. Unfortunately, owing to the complicated dynamics of two coupled fluids
(gas and dust), the underlying physics of most RDIs is mysterious. In this
paper, we develop a clear physical picture of how the acoustic RDI arises and
support this explanation with transparent mathematics. We find that the
acoustic RDI is built on two coupled mechanisms. In the first, the converging
flows of a sound wave concentrate dust. In the second, a drifting dust clump
excites sound waves. These processes feed into each other at resonance, thereby
closing an unstable feedback loop. This physical picture helps decide where and
when RDIs are most likely to happen, and what can suppress them. Additionally,
we find that the acoustic RDI remains strong far from resonance. This second
result suggests that one can simulate RDIs without having to fine-tune the
dimensions of the numerical domain.
