An electrodynamic loudspeaker was analyzed in three steps, (a) as a device supplied by the market, (b) removed upper suspension and (c) dismantled assembly consisting only of vibrating spider and voice coil. In three steps, resonant frequency and stiffness were measured dynamically for driving currents up to 100 mA, whereas stiffness was also measured quasi-statically by the use of calibrated masses. It was found that widely quoted effect of decreasing resonant frequency, as plotted against driving current, comes from the residual strain in the vibrating material, and significant contribution is associated with the spider. When driving current increases residual strain is gradually compensated, giving rise to the minimum of stiffness, and further increase of resonant frequency is attributed to a common nonlinearity in the forced vibrating system.
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