Variability of automotive interior noise from engine sources

A statistical analysis of a series of interior vehicle noise measurements on a set of 1106 automotive vehicles, both petrol and diesel versions, from one manufacturer is presented: a small hatchback model and a mid-sized family 5-door car. The engines were run at 50 rpm intervals over the range from 1000   up to 4000 and 5950 for the diesel and petrol variants respectively, with full load in second gear whilst the vehicle was on a dynamometer roller test rig, comprising a smooth rolling surface. The measured interior noise at four positions corresponding to passenger ear locations were subsequently analysed in third octave bands as well as the overall levels, with statistical tests being performed on the linear rms pressures. The normalised standard deviation of the linear data decreases with increasing frequency. It is typically less than 0.1 over the whole audio frequency range irrespective of vehicle type or engine. A lognormal distribution provides the best fit to the majority of the engine noise results and the overall engine noise could be described by a lognormal or gamma distribution. A lognormal distribution fit implies that the values in dB are normally distributed.