Originally Posted by racineboxer
So I ASSume these designs are wired in series which is why the impedance drops them to 4 ohm rated?
Wrong way 'round.
Paralleled impedance is always lower, while series impedance is the additive sum.
Note, however, that two (or more) speakers of different types may not follow the basic DC resistance equations WRT the total impedance.
To simply say that two 4 Ohm speakers in series will equal an 8 Ohm load is a bit of an oversimplification, and it may not even be true. The actual impedance of any speaker varies with frequency; the "rated" impedance is generally an average or "nominal" value. Thus, the sum (or inverse sum of inverses) will also vary with frequency. In fact, the two may either compensate for each other for a flatter impedance curve, or may in fact create steep peaks or dips that can really mess up the overall performance.
Here's my point: A nominal 4 Ohm multi-driver speaker system may have an impedance minima at some frequency which results in a lower value. Say, for example, the speaker system has a 3 Ohm point at 35 Hz. Now, if a 100W amp is designed to operate right at its limit at 4 Ohms, the maximum current would be 5 Amps (I*I*R=P); this leaves us with a drive Voltage of 20 Volts. Now, if the load is actually 3 Ohms at 35 Hz, an attempt for the amp to drive to that 20 Volt drive level would result in an output current of 6.33 Amps, which is a significant over-current.
This is one of the reasons why some speakers are referred to as being "easier" to drive than others.