The Chrysler Air Raid Sirens were the most powerful
warning devices ever made, producing a sound pressure level of 138
decibels (dB) at a distance of 100 feet. No other sirens, whistles, or
horns made before or since are so powerful. They were powered by
internal combustion engines instead of electric motors or steam, making
them mobile and self-contained.
Fig. 1. The original Bell Labs experimental high-output siren, nicknamed
Development of the Chrysler siren began around
January, 1942, when the E. D. Bullard Co. of San Francisco submitted an
engine-driven centrifugal siren to the Chrysler Division to see whether
the straight 6-cylinder Chrysler Royal engine could be adapted to it. The
output of this siren was inadequate, and it was concluded that
improvements would have to be made for it to be useful as an air raid
warning signal. Earlier it had been determined that a sound pressure
level of 120 to 140 dB at 100 feet was needed for an effective warning.
Despite considerable effort, the Bullard-type siren did not attain the
necessary output. The problem was still under consideration when the
Chrysler Corporation was invited to send representatives to a meeting
called by the Office of Civil Defense (OCD) in Washington, DC on
February 25, 1942, to inspect a new type of high output siren. This
device was the experimental result of new sound-producing principles
developed by the Bell Telephone Laboratories under the direction of Dr.
Harvey Fletcher. The Bell Labs siren (shown in Fig. 1) was brilliantly
engineered and very powerful, producing a level of 137 dB at 100 feet.
The following principles. quoted from Bell Labs' technical report,
governed the design:
(1). The source of pressurized air should be
connected directly to a horn or orifice by an opening whose area may
be varied periodically with time.
(2). The period during which the port is being
opened or closed should be short compared with the period during which
the port remains open or closed. The total efficiency of a siren whose
ports are either wide open or completely closed, and in which the
opening and closing are completed in zero time, is just twice that of
a siren whose port area is varied sinusoidally. This is typical of the
relation between a square wave and a sine wave.
(3). The 'Q' of the horn should be large compared
with unity. For an exponential horn, this means that the fundamental
frequency of the siren should lie well above the low-frequency cutoff
of the horn.
(4). The path of air through the siren should he as
direct as possible to avoid turbulence, and throttling of the air
through the port should be avoided.
In the Bell Labs design, the clearance between the
rotor and the port faces is only 0.010 to 0.015 inches. If smaller, the
chopper might rub against the ports, and if larger, the sound output
drops to an unacceptably low level. The experimental Bell Labs siren
used a compressor furnished by the American Locomotive Co. This blower
was driven by a 95 HP Ford engine and delivered air at 5 PSIG. Tests
showed that an additional 20 HP would be required to drive the chopper
rotor at 5,000 RPM, and this was provided by a separate 20 HP Wisconsin
engine. This siren produced a frequency of 440 Hz when the rotor was
driven at 4,400 RPM. A great deal of ingenuity, experimentation, and
thought went into the development of the high output Bell Labs siren.
Regrettably, this one-of-a-kind prototype seems to have been scrapped in
later years. It would have been a nice historical piece for a
museum or collector.
Fig. 2. Chrysler's first production siren, the 'CHRYSLER-BELL VICTORY
Note New York City mayor Fiorello LaGuardia in the operator's seat.
Three different production air raid sirens were
produced by Chrysler, based on the Bell Labs design. The first version
was called the 'Chrysler-Bell Victory Siren'. Its development was begun
in a meeting on March 7, 1942 between representatives from Bell Labs and
Chrysler. Fig. 2 shows this siren. The first production Victory siren
was completed and officially tested on June 4, 1942. These sirens sold
for $3,760, a substantial amount in 1942. 120 of these Victory sirens
were sold that year to 28 cities and locations in the U.S.A. and Canada.
New York City got ten, Detroit twenty, Chicago twenty, and Cincinnati
ten, to name a few. The Victory siren was powered by a Chrysler straight
eight-cylinder gasoline engine of 324 cu. in. and 140 HP. The blower was
a two-stage centrifugal unit producing 5 PSI for a sound output of 137
dB at a frequency of 430 Hz and a distance of 100 feet. I know of no
Victory sirens surviving today.
Fig. 3. The first version of the Chrysler air raid siren. Note the
seat for the operator.
The control panel is on the engine shroud, facing the seat.
Fig. 3 shows the first Chrysler Air Raid Siren to
carry that name. This version appears to be neater and more refined than
the Victory siren. It had the same sound output, the engine was the same
straight-eight, 324 cu. in. 140 hp gasoline power plant, and the blower
was the same two-stage unit. These sirens were built some time after the
Victory siren but before 1952. So far as I know, none of these survive
Fig. 4. The final version of the Chrysler air raid siren.
This was the three-stage V-8 powered model, built in quantity.
Fig. 4 shows the final version of the Chrysler Air
Raid Siren, introduced in 1952. This is the one that most of us are
familiar with. I believe that several hundred of these were built
between 1952 and 1957. They were the official Civil Defense sirens used
in large U.S. cities and other locations during the Cold War nuclear
attack threat era of the 1950's and 1960's. They had new three-stage
centrifugal blowers to compress the air to 6.95 PSI before it was
chopped by the rotor. These sirens were also fitted with Chrysler's new
industrial 331 cu. in. hemi-head 180 HP V-8 gasoline engine. This siren
produced an incredible 138 dB at 100 feet, and the frequency was 460 Hz
at 4.600 RPM. It could be remotely controlled from inside a building or
All four models had six horn-loaded ports. The three
production model Chryslers had engine block heaters and heated battery
cases, so that the sirens were ready to run down to -25°F in all weather
and climates. Because the sound output was highly directional, they
rotated at 1.5 RPM on a belt-driven turntable for full 360° coverage.
The Chrysler Bell Victory Siren and its successors had metal seats for
the operators, as shown. The operator rode the siren around rather like
I know of eight surviving sirens of the final
three-stage design. Horn & Whistle members account for four of them: I
have two, Jim Pritchard has one, and Jim Carruthers has one. As I was
growing up in Pittsburgh and attending school in the 1950's and 1960's,
a three-stage Chrysler siren was tested for at least fifteen minutes
every week. I remember its authoritative sound very well. That siren is
now in my possession.
The 'Mystery Siren' described on page 7 of
Horn & whistle issue No.
42 is an Army unit, perhaps one of a kind.
The siren portion is a two-stage Chrysler and the engine is a Waukesha.
Surely this combination was concocted by the U.S. Army. I have learned a
great deal about Chrysler sirens since this article appeared in 1990,
but I have never seen any documentation on this siren. It seems certain,
however, that Chrysler did not install a Waukesha engine in place of one
of their own.
For more reading on Chrysler sirens, see Horn
& Whistle issues Nos.
49. I have acquired much
additional material from the National Archives, the Henry Ford Museum,
owner's manuals, and other sources, and will be pleased to go into much
greater detail with those who are interested.