In 2152, Malcolm Reed began to develop a new security protocol, which he dubbed "tactical alert." In conjunction with the protocol, he came up with several possible alert signals, so designed to get the crew's attention. When he proposed each of two alert signals, a klaxon and a buzzer, with Charles Tucker, Tucker described them both as sounding "like a bag full of cats." (ENT: "Singularity")
While under the influence of DaiMon Bok's thought maker, in 2364, Captain Jean-Luc Picard began experiencing flashbacks of the time during his involvement in the Battle of Maxia. He explained to Doctor Beverly Crusher, that in these intense experiences, he found himself on the bridge of his old ship, the USS Stargazer, where he was surrounded by fire, thick smoke, and the sound of the klaxons, moments before giving the order to abandon ship. (TNG: "The Battle")
The klaxon was heard several times in many Original Series, Animated Series, and even in many Next Generation, Deep Space Nine, Voyager, and Enterprise episodes. This sounds off as a red light begins blinking throughout the ship or station. Examples include "The Doomsday Machine" and "The Best of Both Worlds", as the Captain orders a "red alert" in reaction to a hostile event, such as the ship or station being fired upon by a hostile ship.