The title is a little bit misleading, this is a case of an <i>intentional covert channel</i> (both communicating parties are controlled by the attacker, and one intentionally creates acoustic radiation that the other listens for), and not a <i>side channel</i> (where the attacker listens for <i>unintentional</i> acoustic emanations from a device they do not control, like <a href="https://www.tau.ac.il/~tromer/acoustic/" rel="nofollow">https://www.tau.ac.il/~tromer/acoustic/</a>).