Speaker

The Doctor inspects Billy's speaker. (DW: Delta and the Bannermen)

Speakers were a device for enhancing the volume of sound.

Billy built a speaker out of leftover parts from World War II; he used it when he performed with The Lorells. The Doctor and Billy adapted the speaker to project the singing of the Chimeron Princess, this resulted in the Bannermen becoming disorientated. (DW: Delta and the Bannermen)