Now that everyone seems too have got a clue about speakers impedanse.
Its time too spread some confusion
A speaker have not a linear impedanse.
A bass speaker in a closed box will have an impedanse varying from very high in high frequenzies too a minimum before it rises in a spike at the resonanse frequenzy at the speaker. (In box resonnanse)
A speaker in a ported box will be have much the same way but here it will be 2 spikes, one at the speakers resonnanse frequenzy and one at the ports resonnanse frequenzy.
Closed box. fb=50hz
Ported box fb=50hz fp=38hz
These curves will differ from speaker too speaker.
different boxes and ports will also affect the impedanse curve
Have fun
Ps i dont claim i are an expert in this.
i know it is soo but dont ask me too explane why
But i can tell you to not worry too much about this phenomen.
As long as you follow "mxmissile`s" advices and follow the ohm ratings on the speakers you will be just fine
