This article clipping courtesy of Harry Barry.

'Big Bertha,' Air Raid Siren, Undergoes a Voice Test

Placed on Manhattan Bridge in New York, and Pointed
Toward City Hall, It Was Heard Above Mid-Day Traffic

A FEW eastern cities recently have heard the roar of a new Siren whose banshee wail is the loudest sustained sound ever produced by man.

When the siren opens up, its cry is equal to the sound intensity of 1,000 large symphony orchestras all playing at once.

The siren demonstrations were conducted for the Office of Civilian Defense. The machine itself, a 95-horsepower Big Bertha, was developed by the Bell Telephone Laboratories for the National Defense Research Committee in cooperation with the U. S. Army. The OCD believes the siren will prove loud enough for air told warnings. It will be commercially produced by the Chrysler Corporation and others.

Placed recently on Manhattan Bridge and pointed towards the City Hall in New York its sound was heard plainly above mid-day traffic in most of the downtown skyscraper area. The siren has also been used in Detroit and Washington to Study the transmission of loud sounds in other congested areas, and so to afford some idea as to the number and size of sirens which will be required to give adequate coverage for air raid warnings.

Last summer the problem of developing a powerful air signal came to the attention of Dr. E. C. Wente of Bell Telephone Laboratories and his collaborators. For many years Dr. Wente has been interested in the development of loud speakers for motion picture theaters and public address systems. The air signal problem had to be approached from a different angle, however, because a much more powerful sound is required. The solution was an adaptation of the previously known principle of producing sound by chopping a stream of air with a rapidly rotating slotted disk.

The air stream is generated in the siren by a blower driven by an automobile engine.  A rotating disk of sheet steel with slots in it, which is driven by a second gasoline engine interrupts this air stream 440 times per second producing a steady tone. The tone can be "wailed." however, by varying the speed of the chopper. To concentrate the sound largely in one direction it emerges through a horn. More than 25 kilowatts of acoustic power are projected by the siren and this is unquestionably the loudest sound that has ever been continuously generated by a single source.

When the siren was tested in New York, engineers of Bell Telephone Laboratories were stationed with sound meters at nine points in the area covered.  They observed sound levels as much as 15 decibels above the background of the city noise; which satisfied them that a wide area can be covered by one of these powerful sirens if It is well above nearby rooftops.