Definition and background:
Aerophone, Free Aerophone, Percussion instrument, Auxiliary Percussion A percussion instrument that creates the stylized sound of a police siren. The Acme siren is a hand-held mouth-blown instrument that can be made out of metal or plastic. It is comprised of a cylinder shaped body with a fan-like blade mounted inside. As the performer blows into the cylinder, the blades are set into motion creating a high-pitched whistle effect that raises or lowers in pitch determined by the speed of the blades. This is a small version of early police and fire sirens that were mounted on vehicles and cranked by hand to rotate the blades. This is a sound effect that is normally used for humorous effects in compositions and to add extra comic effect for sight gags on stage and in cartoons. The sound of the Acme siren starts low and gets higher as the blades turn faster. The faster the blades turn, the higher the pitch and the slower the blades turn, the lower the pitch. Since this is a mouth-blown instrument, duration of sound and different sounds can be produced by the amount of breath control capable by the performer. The Acme siren has no way of altering or adjusting accurate pitches, so there is no range, nor is there any specific pitch or set of pitches associated with the instrument. A composer can provide direction for higher or lower pitch as well as the duration of sound. Also [Eng.] Acme siren; [Eng.] siren; [Fr.] sirene; [Ger.] Sirene; [It.] sirena.
For more information on the music term "Acme siren" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary