A subsonic transmitter is a device that can transmit sounds below the threshold of the ear.

On stardate 3417.7, Commander Spock and Captain James T. Kirk constructed such a device and installed it in the communications console of the USS Enterprise so that the signal would be broadcast to every communicator on Omicron Ceti III. The signal could not be heard, but its assault upon the nerves could be felt, like "itching powder on the skin". This sensation induced anger and violence in those who heard it, destroying the Omicron spores that were affecting the crew of the Enterprise and the residents of Omicron colony. (TOS: "This Side of Paradise")