Shown here, on the cover of Radio Craft, October 1941, are RCA President David Sarnoff (left) and New York Mayor Fiorello LaGuardia (right), acting in his role as National Director of Civilian Defense. War was still two months off for America, but the nation was on a defense footing, and LaGuardia is being shown an air-raid alert which could be used with existing broadcast radios. In this test, a signal was sent out from station WJZ which would sound a bell or turn on a light on the receiver. This was accomplished by sub-audible tones (30 Hz for On, 24 Hz for Off), which would trigger the alarm.

I’m not aware of this system being put into production, but the modern equivalent is the modern weather alert radio, which we have previously featured.