The effect of lowering the resonant frequency of the loudspeaker during impedance measurement as a function of the signal power

One of the main loudspeaker (woofer) characteristics is its voice coil impedance as a function of frequency. Its measurement is made using sinusoidal test signal (with very low harmonic content), which is feed from generator trough the power amplifier to the loudspeaker. The voltage across the voice coil is acquired directly and the current is measured as a voltage across the resistor with known value connected in series with the voice coil. As a standard a loudspeaker is driven with constant current so the supplied power varies with frequency, but should not overcome the value of 1 Watt. The authors propose to augment this procedure and to measure impedance characteristics at different power levels. In such a way the changes in resonance frequency of the loudspeaker can be observed, which as the experiments have shown, are quite noticeable and are correlated with nonlinear distortion level introduced by the loudspeaker. So the shift of the resonance frequency can be used as an assessment cue in a automatic quality testing of the transducer.