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The use of a microphone array for acoustic measurements in wind tunnels is known to degrade the sound pressure levels and dynamic range of the acoustic image, due to the decorrelation between each pair of microphones, after the acoustic wave has crossed the turbulent boundary layer of the wind tunnel. Based on a large number of studies in the areas of radiophysics, underwater acoustics and atmospheric science, this paper describes research on a physical model of wave propagation in a turbulent medium and its stochastic solution, obtaining as a solution the statistical momentum of the wave signal after it has propagated through the turbulent medium. The parabolic equation for acoustic wave propagation is considered and the Markov approximation is assumed for the stochastic solution of the wave. The signal coherence between each pair of microphones is the amount considered responsible for the decorrelation of the signals.

The two main noise sources of an engine are the combustion process and mechanical impacts. Of the latter, generally the most important is piston slap, which is produced by impacts between the piston and cylinder wall. These impacts cause the engine outer surfaces to vibrate and consequently radiate noise. In this paper the time history of a piston slap impact force model was developed. This model takes into account the influence of the oil film on the impact behaviour, which was found to be an important factor. Test rigs were built to study these effects on the impact phenomenon and extensive tests were carried out.