Document presents considerations on factor x used in the Sound Expectation Model to split constant speed test results into tyre rolling sound and power train mechanic references. Factor x values are concentrated between 80 and 99, indicating tyre dominance. Small variations in measurements cause disproportional variability in the minor sound source when one source dominates. Two options are proposed: Option 1 requires factor x determined during type approval to be reported and used for subsequent assessments, with any redetermination not exceeding the type approval value; Option 2 requires the power train reference from type approval to be used for all further assessments regardless of measurement variations.