The most common effect related to ferroelectrets is the direct and inverse piezoelectricity, but in these materials, the effect occurs in a way different from the respective effect in ferroelectric polymers. In ferroelectric polymers, a stress in the 3-direction mainly decreases the distance between the molecular chains, due to the relatively weak van der Waals and electrostatic interactions between chains in comparison to the strong covalent bonds within the chain. The thickness decrease thus results in an increase of the dipole density and thus in an increase of the charges on the electrodes, yielding a negative d33 coefficient from dipole-density (or secondary) piezoelectricity. In cellular polymers (ferroelectrets), stress in the 3-direction also decreases the thickness of the sample. The thickness decrease occurs dominantly across the voids, the macroscopic dipole moments decrease, and so do the electrode charges, yielding a positive d33 (intrinsic or direct (quasi-)piezoelectricity).