Measurement of frequency responses and the multiple coherence function of the noise‐generation system of a diesel engine

A structural–acoustical system has been established to model theoretically the noise generation of a six‐cylinder diesel engine. In this system, six‐cylinder pressures have been treated as six mutually correlated inputs. The noise measured 3 ft away from the engine has been treated as the single output of the system. Six structural–acoustical frequency responses with respect to the six inputs were evaluated experimentally. The multiple coherence function between all the inputs and the single output has also been evaluated experimentally. Using the multiple coherence function, the engine noise which is coherent to the cylinder pressures was separated from the total engine noise. It has been found that, for the V‐6 diesel engine studied, the coherent noise is almost equal to the total noise. From the structural–acoustical frequency responses, the contributions of the individual cylinder pressures to the coherent engine noise were assessed.Subject Classification: 50.50; 60.20.