Ship sirens use compressed air horns for alarms and signals. Unlike mechanical sirens, pneumatic alarm sirens can create a two-tone sound. Pneumatic sirens with horns are typically used on ships. The bigger the ship, the lower the sound of the horn. The Sounddogs Archive features some very nice recordings of ship horn sounds and pneumatic sirens. Below you will find an impressive example of a big ship\'s signal horn.
Ship Sirens And Horn Siren Sounds
Pneumatic alarm sirens can create enormous sound pressure and spread it across big distances thanks to the horns mounted on the top of the siren. The construction of pneumatic alarm sirens is different from mechanical sirens. While the mechanical siren creates the airstream with the help of centrifugal forces, the pneumatic alarm siren uses a compressed air canister to pump the airstream into the siren head.
Alarm Horn Siren
The air-powered alarm siren (pneumatic siren) is easily identified by the horns mounted to the siren's head. Similar to a mechanical siren, the air-powered alarm sirens create their signal by interrupting the air-stream cycle with the help of a perforated disc. The spinning disc inside the siren's head is called a rotor; it is electrically powered. The air-stream is directed through the holes in the rotor and external vents. The spinning motion of the rotor either opens or closes the holes, which creates the sound of the siren that is then directed to several horns. The air-stream that is needed for the operation of the siren is created by an Aerosol canister underneath the siren.
The main benefit of this type of siren is the extremely high sound level it creates. Pneumatic sirens can reach a sound pressure level of 130db (!) at a distance of 20 meters. The other benefit is the use of pressurised air. With the help of compressed air from a tank, these sirens can create a very high sound level almost immediately, which is very important for the proper operation of a siren. Pneumatic sirens also need less electrical power than mechanical sirens (but they do require more maintenance). Many of these high-performance sirens were de-installed after the Cold War because they were no longer used. You can still see the pylons that those big old sirens used to be attached to (although nowadays they are more likely to house a mobile antenna than a siren).