Delta wave

A delta wave is a form of brainwave associated with sleep. Delta wave augmenters were devices used to aid sleep. (DW: Kinda)

Certain species have specific delta wave frequencies, for example the Daleks who posessed a higher frequency than normal.

A delta wave could also refer to an energy wave that would fry the brains of all sentient life within a given radius, although these could be calibrated to be species-specific. The Doctor constructed a delta wave generator on the Game Station to destroy the Daleks, but was unable to bring himself to use it. (DW: The Parting of the Ways)