Delta wave

A delta wave was a form of brainwave. Different species had different delta wave frequencies. (TV: The Parting of the Ways)

Delta waves were associated with sleep. The delta wave augmenters could be used to augment sleep. (TV: Kinda)

These delta wave could be used to fry the brains of all sentient life within a given radius with a delta wave generator, 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 as there wasn't enough time to allow him to calibrate it for Daleks alone. (TV: The Parting of the Ways)

The sky fish were fond of singing, since it resonated the crystal fog they lived in and caused pleasing delta waves. (TV: A Christmas Carol)