Combining subjective and objective assessments to improve acoustic comfort evaluation of motor cars

A study leading to the establishment of the acoustic comfort index in motor cars is described. Five new light to medium hatch cars of different makes have been tested for two different conditions: public roads, and restricted test roads. Additional tests have also been carried inside an acoustic chamber using a roll dynamometer. Both subjective (jury) and objective with instrumental measurement have been carried out for the public road procedure, which involved three different road conditions, namely: high-speed highways, city centre roads and small gravel covered roads. Testing in the restricted circuit only involved objective measurements for two road conditions which were rough asphalt and smooth asphalt surfaces, where standard tests of speed and acceleration have been carried out. The universe of testers was composed of experienced drivers, all having some kind of experience in car engineering the remainder being NVH experts. They were asked to rank each car and provide a brief technical description of what they could hear during the test. The results were then compared with the measurements and treated using Statistics, in order to establish a series of comfort indexes, which have been developed and presented for each test condition. The idea was not only to establish a comfort index equation, but also mainly to provide alternatives to what is usually an expensive and time consuming procedure. It has been found that standard testing in closed test roads is able to characterise the comfort index, provided that the test conditions are well controlled. However further testing is still needed to improve these conditions so that the quality of results is compared with those obtained in the more usual way.