The sounds from certain types of gongs show a frequency shift, up or down, as the vibration decays. In order to understand this nonlinear behavior, we have studied the vibrations of a variety of flat and curved plates under varying amounts of radial tension or compression. Flat plates show a nonlinearity of a hardening type (falling frequency), whereas most curved plates show softening behavior (rising frequency). The degree of the nonlinearity appears to be sufficient to explain the observed frequency shift in the gongs studied.