The resonant frequency of audio speakers varies with size (as well as other factors. A 15" woofer will have a lower resonance frequency than, say, a 5" midrange. When removing the paper part (the cone) leaving only the spider and the coil the resonant frequency will also change.
Phase shift (current lag) is present in all inductors (including speaker voice-coils) I'm not sure you mean about the 180 degree shift. In an inductor the voltage will lead the current by 90 degrees if I remember my college days correctly (its been 27 years
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