Direct-radiator loudspeakers have a problem of low frequency harmonic distortion in reproducing the input electrical signal. The problem is due to the nonlinearities of diaphragm suspension, magnetic flux, compression in box volume, etc. A polynomial model for diaphragm displacement is adapted to describe the driving and spring forces. Each of polynomial coefficients is evaluated by two methods : harmonic balance method and integral method. Calculated results are in good agreements with measured diaphragm displacement curves. Similarly, the oscillation in the speaker box is also modeled by the sum of polynomials of diaphragm displacement where the diaphragm is idealized by a moving piston. Adiabatic compression process is assumed to compare the coefficients with the experimental ones, and they agree very well. It is found that thethird harmonic component is predominant from the suspension and magnetic nonlinearities while the second harmonic is more effectively generated by the box nonlinearity.
