McCoy using white sound device on Kirk

The white-sound device in use

A white-sound device was an instrument in use aboard Starfleet vessels that was able to mask individual sounds from sensor devices.

It was employed on the bridge of the USS Enterprise in the year 2267, during testimony in the court martial of Captain James T. Kirk. Doctor Leonard McCoy used the device to mask the sound of the remaining crew members' and court personnel's heartbeats, allowing the sensors to detect that of Lieutenant Commander Benjamin Finney, who was previously thought dead by Kirk's actions; he was ultimately found to be hiding in engineering. (TOS: "Court Martial")