The problem of evaluating the acoustical comfort inside vehicles is not completely resolved yet. Some references, especially for railway-carriages, are generically related to L{sub}A levels. The procedure elaborated in our laboratory for the evaluation of the acoustical comfort inside buses for urban transportation takes into account the A weighted sound level and the speech interference level. In order to show both these aspects in a meaningful way, a two-dimensional plotting diagram has been used. On the two axes are reported the A sound level and a parameter SI (Spectrum Index). So it is possible to plot on the diagram three boundary lines corresponding to the speech level of 69 dB, 75 dB and 81 dB. These lines separate two zones of acceptable and unacceptable acoustical comfort. In order to validate the method, it is necessary to perform subjective tests, which could allow to plot equal contour lines of comfort level. In this first step of the research, only few tests were performed in order to verify the congruence of the calculated boundary lines.
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