An experimental investigation revealed that the d5mamic response of a given pressure tube depends not only on the mean local pressure and the frequency, but also on the pressure amplitude and the boundary-layer at the tube entrance. By analyzing the flow at the entrance of the tube, the parameters are determined, which possibly play a role in the description of the flow. For each series of unsteady pressure measurements, with the use of a parameter identification technique the relevant parameters, additional to mean local pressure and frequency, can be selected to describe a more accurate tube transfer function. This function is to be applied in the data reduction procedure. This report presents the principles of the method and the consequences for the results of previous NLR experiments in which tubes for the unsteady pressure measurements were used.