During World War II, Chrysler air raid sirens were used to dispel fog for aircraft to operate, causing the fog to condense into rain.
Manager of the Office of Civil Defense, Los Angeles, California, testing the Chrysler air raid siren in January 1954. Image: Gsjansen
The Chrysler air raid siren is as big as a car with a length of 3.7 m, a height of 1.8 m and a weight of about 2.7 tons. The giant horn is powered by a 180 horsepower eight-cylinder gasoline engine that drives a two-stage air compressor and rotary mower. The compressor pushes 73.9 m3 of air per minute at a pressure of nearly 0.5 kg/cm2 through the rotary cutter to create sound. Compressed air escapes through 6 giant horns at a speed of 644 km/h. This process creates extremely loud sound, reaching 138 dB, when measured at a position of 30 m from the horn. The loudness of the horn is unmatched by any other warning device.
The main purpose of the Chrysler whistle was to strongly warn people if the Soviet Union launched a nuclear attack during the Cold War (1947 – 1989). The horn was manufactured by Chrysler Automobile Company in cooperation with Bell Telephone Laboratories. Bell developed a new sound-generating design that used high-pressure air flow through the mower rotor, and Chrysler put that design into production.
Chrysler produced three types of air raid sirens based on Bell designs. The first generation was the Chrysler-Bell Victory horn, produced in the early 1940s and more than 100 units were sold throughout the United States. The original design used a 140 horsepower engine. An improved version, the Chrysler air raid siren with a 180 horsepower engine was introduced in 1952.
The Chrysler-Bell Victory Horn Test Team in New York. Image: Marie Hansen/The LIFE Picture Collection
The US government has supported law enforcement agencies of a number of states and counties in purchasing and installing horns at important locations in densely populated areas. For example, Los Angeles County purchased six, while another 10 were sold to other agencies in the state of California. They are nicknamed Big Red Sirens and only activate a few times during routine inspections. Their sound can still be heard at a distance of 40 km.
The Chrysler air raid siren was so powerful that it was used by the US Navy to dispel fog during World War II. Fog was a major aviation hazard in those days, when aircraft were less sophisticated and pilots relied more on vision than on-board instruments.
Both the US Navy and British Royal Air Force (RAF) engineers tested various techniques to dispel fog. Finally, the British used fire. Another method is to install a string of Chrysler air raid sirens about 100 feet apart along the path where the plane is about to land. The sound waves created by this series of large horns cause suspended fog particles to merge together and form rain.
The big disadvantage of the method of dispelling fog with sound waves is that the sound of the whistle causes a punctured eardrum, making the listener dizzy and nauseous. Airport workers wear cotton ear protectors covered with foam rubber, but that’s not enough to prevent the terrible changes in air pressure. Noise also harms animals and birds in the sky. A study published in the journal Mechanix Illustrated The 1940s suggested that ultrasound could produce a similar effect to fog without causing discomfort to humans and animals.
The last Chrysler air raid siren was produced in 1957. Previous units continued to operate into the 1970s, then deteriorated due to lack of use and regular maintenance. Many horns were removed from watchtowers and buildings to be reused as car engines. Others are sold as scrap. A few still exist in their original locations, but are rusted and beyond repair. For example, there is one on the roof of the Westin Poinsett Hotel in Greenville, South Carolina, and another on the roof of the Rochester Fire Department Maintenance Building in Rochester, New York.