6 Loudspeaker cabinets
6.5 Impedance
Impedance 
is much like resistance . The calculations given are also applicable to loudspeaker impedances. Connecting two cabinets to one amplifier (channel) always means parallel connection, unless explicitly mentioned. Connecting different cabs in series is generally not recommended, as it may give unpredictable results. Identical cabs can be series connected without any problems. A special "series-Y" cable may be needed.
Impedance describes the resistance of a load connected to AC (Alternating Current) instead of DC (Direct Current). Resistance should be seen as impedance at a frequency of 0 Hz (i.o.w. DC). Adding the variable of frequency complicates matters. If a loudspeaker were a simple resistor, its impedance would be a constant throughout its frequency range. But this is not the case. Its impedance changes with different frequencies. Around its resonance frequency the graph shows a high peak, then resturns to nominal, and with higher frequencies, the impedance gradually rises again. The specified impedance is really a nominal impedance: an averaged value over a certain frequency range. That's exactly why an ohmmeter will never show the real impedance of a cabinet. An ohmmeter measures (DC) resistance. Typically for loudspeakers, their DC resistance is about 60-80% of their rated impedance.
The standardised impedance for loudspeakers is 8 Ω. 4 Ω is less common, 2 Ω is rare, as are 16 Ω and 32 Ω.
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© Joris van den Heuvel 2001-2009