AbstractNoise and vibration produced by mechanical systems will usually reach the environment by a variety of paths. For example, noise from a ship’s diesel engine may be transmitted to the ship’s structure via the engine mounts, the air intake, the exhaust, the cooling water system, the air in the engine compartment, and possibly by other mechanisms.Methods for noise reduction usually involve attempting to identify the most important path or paths, and then incorporating some form of vibration isolation: for example, standing an engine on rubber blocks to reduce transmission via the mounts. Frequently, such measures in fact give little reduction in noise. This may be because the remaining noise paths are responsible for a large part of the noise transmission; alternatively some noise paths may interfere destructively, so that blocking one can actually lead to an increase in noise transmission. It is therefore of great value to be able to characterise the contributions of the paths before planning noise reduction measures.In this paper we present a method of determining the contributions of individual paths to the transmitted noise. The method is non-intrusive: no changes to the system under investigation are required. We have christened it‘MPSD’(Multiple Path Signature Decomposition). We present an analysis of the principles of operation of the method and of the predictions which may be made from them regarding noise control. The method has been proven on practical systems. We present results from an experiment on an air compressor.