The klaxon is an audio signal designed to notify personnel located on a planetary body, or aboard a starship or starbase that they are under red alert.

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")

Different ships had different klaxons that sounded during alerts. (TOS; TAS; TNG; DS9; VOY)