“It was not until 10 years ago, however, that the Sound Effects Man really achieved professional status; in 1929 the National Broadcasting Company created a Sound Effects Department and put in charge the man who still heads this department as its Chief Technician – N. Ray Kelly.
“Until then sound effects had been produced, when provided at all, by a snare drummer, the traditional sound effects man of the old theatre.
“Despite the considerable degree of candor with which sound effects men discuss their art, still, it reeks of “trade secrets”; and employs all the wizardry expert technicians are able to muster in order that such “scenery” will supply broadcast programs with the proper acoustical backdrop.
“One of the most “exclusive” occupations in the world, less than 100 men are professionally employed in the trade, one network estimates.”
Image and excerpts froma 1939 Radio Craft magazine story via http://www.rfcafe.com/references/radio-craft/unique-sound-effects-february-1939-radio-craft.htm .
The article describes the more difficult effects achieved at Hollywood and New York radio broacast studios.
Watch a 3-1/2 minute movie clip showing the sound effects being produced for a 1939 radio show: https://youtu.be/Qy-OJVnS2jg .