Vibro-acoustical comfort in cars at idle : human perception of simulated sounds and vibrations from three and four cylinder diesel engines

This paper deals with comfort in diesel cars running at idle. A bench was used to reproduce the vertical vibrations of the seat, the vibrations of the steering wheel and the noise measured at the driver’s ears. Two paired comparison tests were carried out using stimuli recorded in seven cars equipped with 3 or 4 cylinder engines. In the first one, subjects were exposed to seat vertical vibrations only, and had to indicate the most comfortable stimulus within each pair. Results could be described by the unweighted root-mean-square values of the signal. The use of ISO 2631 weightings reduced the accuracy of the description. In the second experiment, subjects were exposed to all stimuli (vibrations and sounds). Their task consisted in evaluating the similarity between the two simulated situations, indicating which one was the most comfortable, and explaining the reasons for their choice, freely describing the similar and different features of the stimuli. Those free verbalizations were recorded and carefully analysed, providing information about the way the sound and vibrations were perceived and contributed to the evaluation of the overall comfort. Also, similarity evaluations were analysed through a multi-dimensional scaling technique to establish the perceptive space. Both analysis suggest the following results : the description of the vibrations was less complex than the description of the sounds (for which many features were mentioned by subjects) ; the verbalisations were helpful as regards to the understanding of the perceptive space; the preference was mainly related to the vibration of the seat and the pleasantness of noise.