Because of the simplifications used to obtain Eq. (5) some inaccuracy in the calculated Ret is introduced. At Mach 6 and lower, the values of Ret obtained by Eq. (5) are in general within 10% agreement. With the present knowledge of transition location, this accuracy seems acceptable. Because Eq. (1) is only valid above Mach 2.5, Eqs. (5) and (6) have this lower limit too. Another limitation is that the tunnel wall boundary layers have to be turbulent. From Eq. (5) it can be seen that the effect of the acoustic disturbances can now be described as a function of: 1) Reynolds number based on tunnel length, Rel . 2) Size of the tunnel test section. 3) "Aspect ratio" of the tunnel^see also Nagel Ref. 4): (c/L). 4) Mach number.