ABSTRACT
It is a generally accepted principle that 'control at source' is the most desirable form of noise control measure. Indeed, the European Community directives on machinery noise control specifically exhort those responsible for their implementation to make this target a priority. Airborne noise is often the end product of a chain of events and mechanisms of which the origin is some mechanical or fluid process such as gear meshing or combustion. Sound intensity measurement is most frequently applied at the point in the chain where solid surface vibration generates sound waves in a contiguous fluid (usually air), or where airborne sound which is generated within a system emerges into the open, as in ventilation systems. In this context we may restrict our definition of the 'source' which we hope to identify to actions occurring at this interface. We may also wish to study the structure ofa sound intensity field as an indicator of the form of the mechanism creating the sound, for the purpose of source characterisation.
