BBC Historical Sound Effects Library | Alarm Sounds | 1 Alarm Siren Sound Effects

Alarm Sounds

Air horn 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.

Air Horn Sound

The air-powered alarm siren (air horn siren) is easily identified by the horns mounted to the siren's head. Similar to a mechanical siren, the air horn alarm sirens create their signal by interrupting the air-stream cycle with the help of a perforated disc. The spinning disc inside the air horn siren's head is called a rotor; it is electrically powered. The air horn 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 air horn sound of the siren that is then directed to several air horns. The air-stream that is needed for the operation of the air horn is created by an compressed air canister underneath the air horn siren.

Benefits of Air Horn Sirens compared to Mechanical Sirens

The main benefit of the air horn siren is the extremely high sound level it creates. Air horn sirens can reach a sound pressure level of 130db (!) at a distance of 20 meters. The other benefit of the air horn is the use of pressurised air. With the help of compressed air from a tank, the air horn sirens can create a very high sound level almost immediately, which is very important for the proper operation of a siren. Air horn sirens also need less electrical power than mechanical sirens (but they do require more maintenance).

Many of these high-performance air horn 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).

Two-tone Air Horn Siren

Unlike mechanical sirens, air horn alarm sirens can create a two-tone sound. Pneumatic sirens with air horns are typically used on ships. The bigger the ship, the lower the sound of the air horn. The Sounddogs Archive features some very nice recordings of ship air horn sounds and pneumatic sirens.

Recording Air Horn Sound

Air horn sound can be recorded close to the air horn or in some distance to get some of the natural reverb - if you want that. Air horn sound like a big honk and can produce a very loud horn blast with high sound pressure levels. To avoid distortion when recording air horn sound, use a dynamic microphone like the legendary Electro Voice RE20 or the good old Sennheiser MD421.

Air horn sound are used as signal horns on vehicles like trucks, trains, busses or ships. Recording those air horns is pretty a lot of work. To get air horn sound effects from ships, find a good spot in harbours or large rivers where ship has to pass. Also ships uses their air horn sound to alert all other ships around, if they depart. The bigger the ship, the bigger the air horn sound, or the lower the air horn sound. So a supertanker has a very low air horn sound, while a sailing ship has a small air horn. This also makes sense, it would not be that comfortable to mount a huge air horn on your tiny boat.

Where to find Air Horn Sound

Field Recordist Rob Nokes recorded a great selection of air horn sound. He used hand held air horns and also recorded air horn sound from trucks and busses. The BBC Sound Effects Libraries contains also a nice selection of rare to find air horn sounds. The BBC engineers made some unique recordings from fog horns used on lighthouses. Their air horn sound different than the honking air horns. On lighthouses or even on sea traffic, Nautophone Horns are used. Nautophones works different than air horns. While air horn sound are created with compressed air, Nautophones creates their sound similar like alarm sirens or Klaxon siren. Nautophones uses a membran, activated or vibrated electrically while air horn sound are produced by slits or holes or with a rotating disc.