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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.