The Sonic modulator was a Human-built sonic device probably reverse-engineered and based on the Doctor's sonic screwdriver.
More bulky than the sonic screwdriver, it could cause physical harm by emitting sonic waves and cause pain. Toshiko Sato, an employee of the Lodmoor Research Facility, stole plans plans for the device (or perhaps plans of the actual sonic screwdriver) and by order of a mysterious woman named Milton, made a working device from the plan's in exchange for the life of her kidnapped mother. However, as Jack Harkness stated, the plans wouldn't have actually worked except that, with her technical genius, Tosh re-figured the design so that it would work. Before her capture by UNIT, Milton tested the sonic modulator on Tosh's mother. (TW: Fragments)
- Other functions of the modulator, if any, remain unknown.