MORE ON LOUDEST SOUNDS

by James A. Carruthers, Decibel, Inc.

On page 19 of issue #7 (July - August, 1983) there was a reference by Ed Fagen to the loudest man-made signaling device, which was the air-raid siren mounted atop the RCA building in New York's Rockefeller Center during World War II. However, technical data was not known at the time of the reference cited. Consequently, I became interested and was able to copy some literature on the siren from a library a few days ago.

The enclosed literature describes the developmental history of a large-scale air-raid warning signal more powerful than any other signal built up to that time. The earlier version of this signal, apparently, was the one used on top of the RCA building. Later on, improved versions of this signal were installed in large cities across the country to be used for warning of a nuclear bomb attack. The latest version(s), built around 1952 was, and still is, to my knowledge the loudest sound signaling device ever built in the world, at least in decibel output in one direction. A possible exception would be specially built high intensity acoustic noise generators (HIANG) developed by Northrop Corn, Norair Div. around mid 60's. Since these Hiang's were not used for general signaling purposes, the siren described above, would be the most powerful signal ever built.

I did make a tape recording, around late 1968 or so, of one of these sirens in Los Angeles, CA. I was about 150 ft. from the base of a 60 ft. tower on top of which was mounted the siren. As the siren would be very directional, the decibel level in the horizontal plane at the same level as the siren would be considerably greater than where I was recording. I would get about 10 db more at 0 degrees on axis, at the same level as the siren (and at the same distance) than I did while recording it from ground level as it rotated around in my direction. To my recollection of actually hearing it, I do confirm that the siren is the loudest sound signal I have ever heard, at least in one direction. In total sound produced (overall acoustic power) the three giant steam horns on the RMS QUEEN MARY may actually produce more total sound. These are Kockums 575's with heavy duty diaphragms and large diameter horn bells using dry steam at maximum pressure.

With all the recent trends to electronics and electronic related devices (radio, satellites, lasers, etc.), the acoustic signal seems to be all but forgotten. However, the greatly increased awareness of nuclear war may prompt renewed interest in high powered acoustic warning signals in some areas or circles of thought. Whether or not this will justify any real, practical developmental work on new or existing signals, remains to be seen.

Tim Bjareby and I are currently interested in doing some research and development on existing signals, and if time and opportunity permits, on a new powerful type of long range signal, similar to the above described Bell-Chrysler air raid siren. If one of these Bell-Chrysler sirens are still available for purchase (might be hard to find nowadays as I understand that they were scrapped a few years ago), one could restore, modify, and improve it in such' a way as to increase it's versatility, slightly increase it's sound output, and improve its reliability. I think that one of the reasons they were put out of service is that they had reliability problems. An air cooled diesel or gas turbine engine of similar power and torque characteristics as the original engine, independent rotor-chopper drive, and horn extension, along with a complete overhaul and dimension tolerance check and balancing of moving parts, would bring it up to date. Using 3 or 4 air horn diaphragm assemblies merged into a common horn is another way of developing a high power signal. The HIANG type valve is similar to an intake or exhaust valve on an engine. They could be modified and used in groups to power large horn-array configurations and would be the most powerful signal yet.

At maximum RPM, with short acoustic-coupler (horn-extension) the Bell-Chrysler air-raid siren generates 138 dbC (approximately 134 dbA) at 100 ft. or 30 meters, 0 degrees on axis, in anechoic, full space environment, and an estimated 30,000 watts of overall acoustic power. The three Kockums steam horns mentioned above would generate around 132 dbC (approximately 126 dbA) at 100 ft., 0 degrees on axis, in anechoic, full-space and about 50,000 watts of overall acoustic power.

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MISC. LIB NOTES ON AIR RAID SIGNALS (copied Dec. 1983)

Siren on top of RCA bldg: Produces 170 db, loudest sustained sound ever produced by man. Physicists think sound can't be much louder than 190 db, beyond that would be continuous vibrations or explosions. 12 ft. from 12" rifle has peak of 225 db. Siren designed by Dr. Harvey Fletcher of the Bell Telephone Labs at Army's request (possible use to deafen and scare enemy as well as air raid alert) powered by 140 hp Chrysler automobile engine. Consists of cylindrical chamber into which air is pumped at a pressure of 5 psig. Then the air blows through 6 holes at other end of cylinder, the holes being alternately opened and closed by a six bladed fan like chopper. A trained human voice, which can touch 124 decibels, and, in opera, often does.

From THE NEW YORKER, August 1, 1942.

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A SIX BLADED ROTARY CHOPPER interrupted a flow of air delivered by a blower at a speed of something over 375 mph.

THE NEW YORKER, May 19, 1943.

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CHOPPED AIR: The world's loudest siren. "We guarantee 138 db at 100 ft. from throat", he said. "Engine is 180 hp Chrysler V-8 industrial. The American Blower Corp. supplied the parts for the compressor and Chrysler machined it. We guarantee 189 db at the throat. We guarantee a radius of 2 miles (4 mile radius in favorable conditions)". Siren weighs 8,000 lbs and costs $5,500.

Quoted from article in THE NEW YORKER, September 20, 1952.