"Introduction This report summarizes Phase II of a research program conducted by the American Petroleum Institute (API) to evaluate the performance of technologies that can be used to detect leaks in the floors of aboveground storage tanks. During Phase I@ an analytical assessment of the performance four leak detection technologies was investigated [1@ 2]. The four technologies included: (1) passive-acoustic sensing systems@ (2) volumetric systems@ especially differential pressure (or ""mass"") measurement systems@ (3) advanced inventory reconciliation methods@ and (4) tracers methods. During Phase IT@ field tests were conducted on an aboveground storage tank to make an engineering assessment of the performance of two of these technologies@ passive-acoustic sensing systems and volumetric detection systems. This report describes the engineering assessment of the acoustic systems that were examined; the engineering assessment of volumetric systems is described in a separate report [3]. The specific objectives of the Phase II research in the area of acoustics were to: ?assess the current state of AST leak detection technology ?determine the nature of the leak signal and the ambient acoustic noise in an AST ?perform field experiments on a full-scale AST ?recommend ways to improve existing AST detection systems The field tests were conducted at the Mobil Oil Refinery in Beaumont@ Texas@ on a 50@000-bbl@ 114-ft-diameter AST containing a heavy naphtha petroleum product. The experiments focused on identifying and quantifying the acoustic leak signal and its source mechanisms@ and on formulating the strategies necessary to detect the leak signal."