Summary: We propose mechanically linear and electrically nonlinear constitutive relations for auditory outer hair cell membrane (wall). These relationships are derived on the basis of dual thermodynamic potentials. The active forces and active strains generated by the cell are associated with mixed (piezoelectric) terms in dual thermodynamic potentials. By using experimental information from four independent experiments, where the cell was loaded mechanically or electrically, the free parameters of the model are estimated. In addition, the model is extended to a composite representation of cell membrane. In that representation, the outer layer is considered to be active (electroelastic), and the inner layer is considered to be passive (purely elastic). The layers are connected by elastic springs. We also present graphs for elastic energy stored in the passive layer versus cell wall potential.