This paper discusses the application of coherence function techniques in developing semiempirical models of acoustical systems, with particular application to diesel engine noise. Pressure transducers, mounted in each of the combustion chambers, measured the cylinder pressures and a microphone mounted 1 m to one side of the engine measured the sound pressure. The frequency responses between each cylinder and the noise were computed. The frequency responses were used to predict the effect on the engine noise of varying combustion parameters such as injection timing. Previously, the effect on noise of combustion parameters has been determined by empirical means only. It is also shown that even though the inputs are highly coherent, it is still possible to determine the frequency responses for the case where the inputs are identical but delayed in time by fixed amounts, as is the case for multicylinder pumps, engines, and compressors.