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Simulation and testing are often done by different engineers in different departments of a company. This can lead to disconnects and unrealistic predictions, especially if the person doing simulations does not have an experimental background. On the other hand, experimental results can also include errors that result in misleading answers. It is important for the engineer doing either testing or simulation to have a good understanding for what results are plausible and what results might be suspect. This paper will provide examples where error crept into testing or simulation that could have been caught and corrected early if a good feel for “reasonable” results had been in place. The importance of understanding how a software package is analyzing the data will be explained, since settings buried deep within a menu structure can drive misleading results. The paper will also make the case for simulation engineers to be provided with some experimental background, so that they have a better physical understanding of the structures or components being simulated. The ideal case is where the same person is responsible for both simulation and experimental validation.

Engines generate noise by directly radiating sound from engine surfaces and also by creating pressure pulses in the intake and exhaust systems. Forces generated by engine operation result in vibrations that can be transmitted into the vehicle structure or that result in sound radiation. Sources of vibration include unbalanced forces and moments and gas pressure forces. Noise radiated by engine surfaces is also a result of forces applied to the structure of the engine. These forces can be divided into combustion excitation and mechanical excitation. A number of design measures can be taken to reduce the sensitivity of the engine structure to excitation forces and to reduce the noise radiated by engine exterior surfaces. Radiation efficiency is a key concept to use in the reduction of noise radiated by the exterior surfaces of larger engines. There are also measures that can be taken to reduce the amplitude of forcing functions such as combustion excitation or gear rattle. Analytic techniques are available to predict the engine's structural response to forcing functions and to predict radiated noise that results from the engine's response to forcing functions. Keywords: diesel engine noise; spark ignited engine noise; combustion noise; forcing function; structural response; mechanical noise; radiation efficiency; noise prediction; NVH