With NAP2626's comments I think we are getting close. This is helping me to remember now from acoustics school:
Piezoelectricity gives a linear relationship between stress and voltage.
Whereas Ferroelectricity gives fundamentally a quadratic relationship.
That is why ferroelectric...
Longitudinal and shear waves are subject to the continuity conditions of infinite media. However, a surface imposes a different elastic condition. The key condition on a surface is that the stress component perpendicular to the surface must tend to zero as one approaches the surface (otherwise...