This paper was originally written in 1967 and then delivered at the University of Hawaii ih 1969 in a series of three lectures. Minor modifications of the original paper have been made, in the form of deletions of now well-known subjects. Scientific advances since 1969 are not discussed. The author's matched-signal (''MESS'') technique and his detailed 1965-67 environmental measurements have so far been made known only to a limited audience. A patent [A. Parvulescu, U.S. Patent No. 3,939, 461 (19 November 1962)] was granted to the author in 1962 and declassified in 1976. The paper is being published now, because the MESS technique continues to be significant in its own right, and because it is useful for making environmental and operational measurements. Certain comments, for example, those addressing the practical limitations in performing correlations, while no longer true, were previously driving forces and are mentioned as such. Similarly, it may be noted that the agreement between models and measurements has improved over the years. (C) 1995 Acoustical Society of America.