A subspace modulator was a piece of subspace technology.

In 2372, Admiral Leyton ordered Lieutenant Arriaga to secretly attach a subspace modulator to the wormhole relay station. The modulator caused the Bajoran wormhole to open at random, making it appear as though a cloaked Dominion fleet could be traveling through it. This deception was part of Leyton's plan to impose Starfleet martial law over Earth to address the changeling threat. (DS9: "Paradise Lost")