History of Radio Drama Research 

Useful web page detailing how sound effects were produced in 1920's.

http://www.irdp.co.uk/britrad.htm

Useful website showing a timeline involving the key points throughout the evolution of radio production.

http://www.longwharf.org/sister-george-history-radio

More detailed timeline of the history of radio.

http://www3.northern.edu/wild/th100/radio.htm