Subjective Comparison of Different Car Audio Systems by the Auralization Technique

This paper reports the results of a subjective evaluation experiment, based on the collection of questionnaires compiled by volunteers after listening to sound fields reconstructed by the auralization technique. Each synthetic sound field included both the car’s interior noise and the transfer function of the sound system coupled with the passenger’s compartment. 0. Introduction The evaluation of car sound systems is usually done both by means of objective measurements (frequency response, distortion, etc.) and by listening tests. The latter are particularly long and difficult, because it is necessary that the subject seat on each car and listen to a pre-defined music sample, played though the sound system. Furthermore, it is difficult to take into account the noise due to the engine and tyres, because it would be required to conduct the tests with the car running on a test track or inside a specially equipped laboratory. Actually the large number of subjective tests are made with the engine not running. By employing the auralization technique, it is possible to prepare sound samples for making comparative subjective tests of the sound system of different cars: the sound tracks used for the subjective tests are not recorded inside the car compartments: instead they are reconstructed by convolving the original signal (a music sample taken from a commercial CD) with the binaural impulse responses previously measured for each channel of the sound system, and adding the car’s noise, also synthesized on the basis of experimental measurement of the average noise spectrum. The new technique is very fast to implement, does not require expensive instruments or tools, and makes it possible to conduct the listening tests everywhere requiring simply a notebook computer: this way a reasonable number of significant results were collected in a very little time, with minimum cost, and with the certainty that the results are not biased by the knowledge of the car’s maker or by non-acoustical effects due to the furniture of the car or to other confort-related topics. The main goal of this paper is to detail the technique, regarding both the measurements inside cars and the auralization system employed for the listening tests: a comprehensive statistical analysis of the subjective judgements will be presented elsewhere [1]. In the following chapters the measurements made inside 9 car compartments are first described. Then the auralization/reproduction system is described, and a special subjective test