Overview

Most natural mechanisms and man-made devices are nonlinear, although linearity is often a good approximation and has been the basis for most engineering developments. In many devices the effects of nonlinearity become apparent only under high drive conditions, while other devices are inherently nonlinear and exhibit nonlinear effects, such as frequency doubling, for the smallest of drives. In the latter cases approximate linearity can only be achieved by imposing a bias. Among the electroacoustic transducers only the piezoelectric and moving-coil mechanisms have a linear mechanical response to an applied field, and it remains linear only for small amplitudes.

The electrostrictive, magnetos trictive, electrostatic and variable reluctance mechanisms differ significantly from the piezoelectric and moving-coil mechanisms in that they have no region of linear operation, even for small amplitudes, unless linearization is imposed by applying a bias. In these cases the natural mechanical response, before linearization, is an even function of the applied electric or magnetic fields and therefore follows a square law for small amplitudes. The means of linearization is basically the same in all cases; a large electric or magnetic bias field is applied which establishes a polar axis that gives the material or device a one-way character. Then a superimposed alternating drive field that is smaller than the bias field can only increase and decrease the magnitude of the total field without changing its direction. The bias produces a linear component of the motion, but the nonlinear components are still present and become significant as the drive...