<P> The Federal Signal Thunderbolt series creates a distinct tone using a separate blower to force air through the rotor . Horns having an exponential profile amplify the sound . Within the Thunderbolt product line, three different configurations were offered . The Thunderbolt 1000 is a single - tone siren, and the Thunderbolt 1000T is a dual - tone siren . The Thunderbolt 1003 is essentially the same as the Thunderbolt 1000T, except that it employs solenoid - actuated slide valves to create a "hi - lo" signal primarily used as a fire signal . Another example of a siren that has a separate blower is the Alerting Communicators of America (ACA) Hurricane . </P> <P> A variation on the electromechanical siren is the pneumatic Hochleistungssirene (HLS), produced by the German firm Pintsch - Bamag, and later by the German firm Hörmann . Soon afterward, Hörmann improved on the design to create the HLS 273, which did away with the massive siren head of the original in favor of a more compact head and cast aluminum exponential - profile horns . These sirens stored a reservoir of compressed air, recharged periodically by a diesel engine - driven compressor in a vault in the base of the massive siren unit . The later HLS 273 located the large (6,000 liter) air tank underground beside the machinery vault, instead of in the mast itself as in the earlier HLS units . </P> <Dl> <Dd> See also: EOWS </Dd> </Dl> <Dd> See also: EOWS </Dd>

What did the air raid siren sound like