Hi msa,
Just as every speaker has a different frequency response, so do they have a different impedence across the frequency spectrum. Look at the Fostex FE-166 graph:
http://www.fostexinternational.com/docs/speaker_comp/pdf/fe166erev.pdfThe top curve is frequency response, the lower curve is the impedence at each frequency. Note the peak in impedence at 50 Hz ... right at Fs. Also note that the nominal impedence is also the lowest across the chart and falls in the span between 300-600 Hz.
From the Fostex overview page:
Quote:IMPEDANCE
This figure represents the input impedance measured at the driver terminals. It needs to be noted that impedance is input frequency-dependent, except for some technologies such as RP system tweeters. Nominal impedance shown in catalogs is the impedance of the point where the frequency response hits the lowest point above the 'fo' frequency. The unit is 'Ħ' (ohms).
Hope this helps.
9anda1f