A boatswain's whistle
An electronic boatswain's whistle
The boatswain's whistle (pronounced bosun's whistle) is a small whistle that was used aboard old sailing ships since the 15th century, usually to gain the attention of the crew, and was still used on starships up to the 24th century.
The USS Enterprise in the 23rd century used the sound of a boatswain's whistle at the beginning of inter-ship hails for the captain of the vessel. (Star Trek: The Original Series)
Aboard starships into the 24th century, it was normally used when an honorary guest comes aboard or at a crewmember's funeral, when their body was released into space. The shrill whistle was often played before a ship-wide or open frequency communication, usually made by the captain or another senior officer. (TNG: "Booby Trap", "Yesterday's Enterprise") The actual tone of the call has varied, as some captains preferred the distinctive notes of the electronic type over the more traditional metal whistle. (TNG: "Lower Decks")
In 2165 of an alternate timeline, a boatswain's whistle was blown by a crewman aboard Enterprise NX-01, announcing the arrival of Jonathan Archer, T'Pol and Phlox on the ship. (ENT: "Twilight")
When an Excalbian replica of Abraham Lincoln came aboard the USS Enterprise, Lieutenant Dickerson blew a boatswain's whistle. (TOS: "The Savage Curtain")
An electronic boatswain's whistle was blown when Admiral James T. Kirk set foot on the Enterprise in 2285. (Star Trek II: The Wrath of Khan)
Yeoman Burke blew a different electronic boatswain's whistle when the Klingon Chancellor Gorkon came aboard the USS Enterprise-A in 2293. (Star Trek VI: The Undiscovered Country).
The boatswain's whistle was blown in the shuttlebay when Captain Jean-Luc Picard took command of the USS Enterprise-D in early 2364. (TNG: "All Good Things...")
The boatswain's whistle was blown in 2372 during a memorial for Kurt Bendera, killed during a Kazon-Nistrim attack. (VOY: "Alliances")
In 2373, Captain Janeway hallucinated her own death and funeral aboard USS Voyager when a non-corporeal being tried to convince her she was dead, at which Crewman William McKenzie blew the whistle during her memorial. (VOY: "Coda")
In 2375, Nog blew the boatswain's whistle, at the wedding of Kasidy Yates and Benjamin Sisko, right before Kasidy entered the wardroom. Odo complimented him on his playing, despite Nog only having twenty minutes to learn it. (DS9: "'Til Death Do Us Part")
The boatswain's whistle was blown at the belated funeral of Lieutenant John Kelly. (VOY: "One Small Step")