Bell Labs "Big Bertha" protototype

[acoustics101]

http://www.victorysiren.com/x/articles/SNL42/SNL42.htm
Since this siren was supercharged and had a large horn with a low cutoff frequency it would have maintained its output well into the lower frequency range, just as a Federal Signal Thunderbolt or ACA Hurricane.

Even though at 95 HP for the blower it would have had a lower rated output at 100 feet than the 180 HP Chrysler, it would have equalled or outperformed the Chrysler from a distance of two miles or more, due to its ability to maintain its output at the important lower frequencies, where atmospheric absorption loss is at a minimum and progation and penetration are at maximum. The Chrysler siren was not supercharged and lost output dramatically as the shaft RPM and frequency decreased, as the blower was on a common shaft with the chopper.

As for being able to hear it from indoors at a distance the "Big Bertha" prototype is most likely the best siren ever built. It's a shame the supercharging principle was only applied to smaller, less powerful production versions.

[Jack Schroll]

I am not certain that I concur with the assumption that this siren would maintain full power at lower RPM's. The output of a supercharger is determined by the engine speed. I have a Mustang shop and we install a lot of modern superchargers. They do NOT put out much in the way of pressure or volume until the reach an optimum operating speed. I would love the opportunity to put this theory to the test. I would actually be willing to take my Chrysler and do a "heads up" side by side test. It would be glorious.

Jack

[EL1998P71]

I don't know if I'm linking this photo correctly, but if you look up on Virtual Motor City from Wayne State Univ. There's a photo of that siren being tested in Detroit back in the 1940's.

You can try below,

http://dlxs.lib.wayne.edu/cgi/i/image/i ... id=18292_2

Or search for image number 18292_2 by copy and pasting it to the link below( you must have the underscore line between the 2's).

http://dlxs.lib.wayne.edu/cgi/i/image/i ... ndex;c=vmc

[SirenMadness]

I am not certain that I concur with the assumption that this siren would maintain full power at lower RPM's. The output of a supercharger is determined by the engine speed.

It said that the chopper and the blower on the siren have their own engine, so it should out-compete the Victory siren at low chopper RPM. Not to mention twenty-five kilowatts of sound power; most sirens don't even have that much motor power!

[Adam Pollak]

Yes, the original Big Bertha did run separate engines for the compressor and chopper assemblies, giving it true constant output (in terms of CFM at least) over a range of pitches. The Chrysler Air Raid Sirens and Chrysler Victory Sirens have one engine for both, so there was decreased output when not at full speed. Of course there is both a cut-off and a sweet-spot frequency with the horn setups. Once you drop below the cut-off pitch, you are definitely going to lose sound output. When you are right around the resonant sweet spot, you are gaining a lot more sound output. I'm not the acoustical engineer though, so take that lightly.

[acoustics101]

In an exponential horn, the on axis output is fairly independent of the frequency above the flare cutoff frequency, provided that the drive power to the throat of the horn is constant. In the case of a separate blower and independent chopper motor this would be the case. This large of a horn would have a fairly low cutoff frequency.

[acoustics101]

The supercharger you are talking about is a belt driven blower on the engine of some high performance cars to increase the boost pressure to the engine. In this case the boost pressure would increase as the RPM increases.

Superchargers on sirens are really external blowers which maintain a constant speed and pressure to the chopper, which is turned by a separate motor. In this case the output of the chopper is independent of the frequency, unlike choppers that share a common shaft with the blower, as do most sirens.

I am not certain that I concur with the assumption that this siren would maintain full power at lower RPM's. The output of a supercharger is determined by the engine speed. I have a Mustang shop and we install a lot of modern superchargers. They do NOT put out much in the way of pressure or volume until the reach an optimum operating speed. I would love the opportunity to put this theory to the test. I would actually be willing to take my Chrysler and do a "heads up" side by side test. It would be glorious.

Jack

[acoustics101]

http://tinyurl.com/yjklmts
Here is a link to Big Bertha's patent. You can easily see the separate blower and chopper. The blower motor runs at a constant speed while the chopper motor's speed is varied.