An audio lock was a solenoid switch which could only be activated by a particular sound. They became the preferred method of locking safes after standard combination locks became too easy to open due to burglars taking miniature computers with them on their expeditions.
The space pirates under the command of Maurice Caven utilised audio locks to seal the exits of the various prison cells within their hideout on the planet Ta. After the Second Doctor, Jamie McCrimmon, Zoe Heriot and Lieutenant Joe Sorba found themselves trapped within one of these prison cells, the Doctor attempted to use a tuning fork to find the correct sound that would deactivate the lock that was being used to seal them in. While the Doctor's efforts to discover the correct sound proved unsuccessful, Jamie inadvertently wound up producing the correct sound when he threw the tuning fork at the cell wall in frustration, at which point the audio lock deactivated. (TV: The Space Pirates [+]Robert Holmes, Doctor Who season 6 (BBC1, 1969).)