Here’s an interesting little text file from 1983 that Jason Scott has in this vast archive of BBS files, backmask.txt, that delves into the history, technology, and social aspects of backmasking.
From the text file by William Poundstone:
TV programs such as PRAISE THE LORD and THE 700 CLUB have propagated rumors of a satanic plot in the recording industry, no less, in which various albums conceal “backward-masked” demonic murmurings. If THAT sounds too spacey to be taken seriously, consider that it was the fundamentalist groups who were behind House Resolution 6363, a bill introduced in the U.S. House of Representatives by Robert K. Dornan (R., Calif.) in 1982 to label all suspect records: “WARNING: THIS RECORD CONTAINS BACKWARD MASKING THAT MAKES A VERBAL STATEMENT WHICH IS AUDIBLE WHEN THIS RECORD IS PLAYED BACKWARD AND WHICH MAY BE PERCEPTIBLE AT A SUBLIMINAL LEVEL WHEN THIS RECORD IS PLAYED FORWARD.”
Many of the original rumours I heard about backmasking when I was a kid are in this file. It’s interesting to note that the claims of what exactly each songs says when played backwards has continued to evolve over the years.