Abstract: An exhaust control system is provided for a marine propulsion system used on a marine vessel. Several parameters are monitored by a controller and a controller uses the information provided by these sensors to control the position of a valve within an exhaust conduit assembly. Sound level is measured at a preselected position on the marine vessel and the degree of opening of a valve is controlled to limit the noise level emanating from the exhaust system. Some exhaust can be diverted directly to the atmosphere through the transom as long as the noise level does not exceed a preselected limit, which can typically be a state law regulation. If a noise level is exceeded, the controller forces the exhaust through an underwater discharge point, typically through the propeller hub of the marine propulsion system.