Silence

Silence was the absence of sound.

On Xeros, Barbara Wright was the first to notice the silence, describing it as "the sort of silence you can almost hear." Ian Chesterton likened it to that of a graveyard. The silence was the result of the TARDIS having jumped a time track. (TV: The Space Museum)

On a doomed planet, the First Doctor was the first to notice the silence, which reminded him of Xeros. In this case, the silence was due to the complete lack of animal life. (TV: Galaxy 4)