Chrysler Air Raid Siren - Photograph

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Photograph courtesy of Walter P. Chrysler Museum.

This is the siren chopper plate as seen with the front end of the siren/blower disassembled.  The siren is being restored for future display in the Walter P. Chrysler Museum in Auburn Hills, MI.

The blower (it's really a compressor) pulls air in like a fan. The compressor on the Cold War era Chrysler has three-stages. That's to say that it has three sets of vanes each with an incrementally smaller volume of internal area. As a result the same air is forced to fit into a smaller and smaller space. As the volume decreases the pressure increases. By the time the air gets to the siren chopper it is pressurized to 6.95 pounds per square inch.

[ Drawing of a two-stage compressor ]

All three sections of the compressor as well as the chopper are on the same propeller shaft and turn at the same speed. There is indeed a vane behind every chopper slot, although there is still a lot of clearance.

At full speed the compressor forces 2,610 cubic feet of air our through the chopper every minute. With six port openings on a chopper which is spinning at 4,600 RPM each port is open 1/920th of a second and closed 1/920th of a second. Each time a port opens 163.4 cubic inches of air is puffed out at 400 MPH (per horn). That's equal to about one 5-1/2 inch cube of air popping out each of the six projector horns 460 times per second. A combined total of 2,760 5-1/2 inch cubes per second.

The Chrysler Air Raid Siren is a single tone siren with an output of 460Hz at full speed (4,600 RPM). The chopper has six ports so air flows out six times every rotation. The openings cover 50% of the surface space that passes in front of the projector horn throats. That's 50% open and 50% closed. At 4,600 RPM the chopper opens and closes 27,600 times per minute (4,600 x 6) or 460 times per second (27,600/60). The result is a near square wave (isosceles trapezoid) at 460Hz.

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