The Thunderbolt was one of the more unique designs for an outdoor warning siren. Instead of using a large chopper (the part that makes the noise in mechanical sirens) the Thunderbolt uses a small chopper through which pressurized air is fed from a Roots blower at the base of the siren. Because the feed of air is constant from the blower, the siren would not lose volume as it raised and lowered in pitch, unlike conventional sirens. Despite having a chopper diameter of only a few inches across, the Thunderbolt was more powerful than most sirens of its era at ~127 decibels @ 100 feet. The only siren that could beat it in 1952 was the Chrysler Air Raid Siren, which was powered by a Hemi V8 engine.
Thunderbolts were produced by Federal Enterprises (later Federal Sign & Signal and then Federal Signal Corporation) from 1952 to 1990. Federal's main competition, Alerting Communicators of America, famously created their own Roots-blown siren called the Hurricane 130 to compete with the Thunderbolt with questionable results. The design, while high-performing, was also maintenance intensive as the rotation mechanism and blower had belts that needed to be replaced on occasion. It would be succeeded by Federal's 2001 siren, which is sold today as the 2001-130.
