The bell-shape profile (Figure 2a,c) is at- tested to tubes with high outer to inner (r) diameter ratios (R/r) where the internal cavity does not signifi- cantly affect the E-field distribution and resulting pi- ezosignal (i.e., penetration depth of the E-field is com- parable or smaller than the wall thickness). To avoid the influence of any geometrical constraints on the cantile- ver torsion and IP signal detection, we always used the maximum contrast value for the analysis (arrow on Fig- ure 1d). In some cases, we observed a two-hump pro- file (Figure 2b,d) where the piezosignal dropped to about half of its maximum value. This situation reflects the distortion of the applied E-field due to the presence of the cavity