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

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

These delta wave could be used to "fry" the brains of all sentient life within a given radius of the relevant delta wave generator, although they could also be calibrated to be species-specific according to the brainwave frequency of the species. The Ninth 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, meaning that all humans on Earth would have died too. (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)

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