The technology behind sound masking came out of the realization by scientists and engineers that "oral privacy" is actually a simple matter of making speech unintelligible. That is, if each of us can't understand what the other is saying, we have effectively established oral privacy - even though we may still be able to see and, to some extent, hear each other.

http://www.speechprivacysystems.com/pages/What_is_White_Noise