A subaudible tone is audible; however, it is usually at a low level that is not noticeable to the average listener at normal volumes.
These tones are included in the audible main portion of audio in the case of satellite; on tape, these often are filtered.
With the advent of computers and digital satellite, these tones are relegated to triggering commercial announcements and legal IDs on a dwindling number of radio networks, as tones in the audio have been supplanted by external data channels sent independent of audio on digital satellite feeds for radio.
These trigger relay closure terminals on the satellite receiver itself (Starguide being a prominent system).
Previously, the phonographic record or audio cassette accompanying a filmstrip to provide its soundtrack would have an audible tone to signal the person operating the projector to advance the film to the next frame.