The models were classified into three groups according to the and values: 1.3 and for Group A, 1.1 and for Group B, and 1.8 and for Group C. The and values were set to 0.1 and in all three groups. In Group A, was varied in the range of 0.013 to in layer #1, 0.011 to in layer #2, and 0.01 to in layer #4 separately. In Groups B and C, only and were varied and was set to . In all the models, was set to be larger than as the of the scalp is generally larger than that of the skull in adult heads,43–45 and was . The thicknesses of layers #1 and #2, () (mm), were set to (3, 5), (3, 7), (4, 7), (5, 7), (5, 5), or (5, 9) in Group A, (3, 7) or (5, 7) in Group B, and (5, 7) in Group C, whereas and were set as 1 and 90 mm, respectively. We mainly evaluated temporal variations in and differences between and the actual for the Group A models. The models in Groups B and C were used to investigate the effects of the variation in the upper layers on . The of the homogeneous reference was changed within the range of 1.1 to as described below, while the and the thickness were set to and 100 mm, respectively.