Speaking tube

A speaking tube or voicepipe is a device based on two cones connected by an air pipe through which speech can be transmitted over an extended distance. While its most common use was in intra-ship communications, the principle was also used in affluent homes and offices of the 19th century, as well as expensiveautomobiles, military aircraft, and even locomotives. For most purposes, the device was outmoded by the telephone and its widespread adoption. This device was also known as a "megaphone", but that use has since become superseded.

Early voicepipes consisted of two cones, of wood or metal, one end shaped to fit the speaker's mouth, connected to the other which was flared to amplify the sound (specifically to match the impedance of the tube to that of the open room). Later designs of the voicepipe inserted a removable cork-mounted whistle, which could be sounded by blowing into the tube from the other end. On naval vessels, this created a distinctive sound associated with urgent intra-ship communication on old warships. The sound of the whistle would summon the listener, who would remove the whistle and answer the call.

Voice pipes could be used over distances as long as 300 feet (90 m). However, very long speaking tubes might use an electrical signalling device to indicate a call, as the large volume of air in the pipe would make it difficult to blow with enough pressure to sound a whistle at the far end. Despite this, a pipe with a larger internal diameter was desirable for longer runs as the signal loss is inversely proportional to the pipe's cross-sectional area.

Voicepipes have no switching mechanism and so, to provide multiple destinations, separate voicepipes with dedicated transit pipes have to be provided between all pairs of desired endpoints.

The technology continues to be used into the electronic age due to its reliability and low cost. Voice pipes are unaffected by a complete electrical power loss or by an electromagnetic pulse. Warships built as late as the 1950s continued to incorporate voicepipes alongside more advanced technology.

...
Wikipedia