The paper describes a new subjective evaluation method of the acoustical quality produced by a sound system inside a car compartment. The method produces a single rating number, called IPA (Index of Performance Acoustic), defined as a weighted average of the subjective responses to a questionnaire, being compiled during listening tests conducted with the subject seating inside different cars. The paper describes the details of the subjective test, and focuses on the choice of questions in the questionnaire and of the weight to be employed. The principal innovation of the new method is the fact that the weights are changed according to the reliability of the subject (which is also inferred from the questionnaires), and thus the evaluation is very robust and almost immune from the inclusion in the panel of completely unreliable evaluators.
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