Loudspeaker Impedance, Series & Parallel Connection Basics

I am a big fan of "back to basics" type articles that about.com and others (such as Audioholics) have. They provide insight into reviews and articles you read elsewhere that are inundated with technical terms. Today's topic is a loudspeaker impedance and series and parallel connections. I know we all started out hooking up multiple speakers in a variety of ways not really knowing what the consequence was. I remember putting 5 totally different speakers in one big old enclosure.. 

From the article:

Since
we connected two identical speakers in series with our amplifier, each
speaker only sees half the voltage drop across it thus as a result will
see only 1/4 the power delivered to each speaker compared to a single
speaker connected to our amplifier.   The equivalent SPL now produced
by each speaker is 6dB lower than
if a single speaker were playing off the amplifier, for a combined
overall -3dB drop.  However, running two speakers effectively doubles
the volume displacement compared with that of one speaker. Thus
playback through the two drivers results in a 3dB gain. Adding this to
the 3dB drop previously mentioned and the net overall sound pressure level will remain unchanged.  Thus,
playing two identical speakers connected in series off of a common amp
(as opposed to playing just one speaker off that amplifier) results in
no level drop, when compared to the single speaker case.  This
analysis, of course, ignores mutual coupling and any room-induced
acoustical artifacts.  However, if the speakers connected in series are
not co-located and summing perfectly in the room, the net SPL would
likely be up to -3dB lower than playing a single speaker off the same
amplifier.  The net SPL product in this case has a dependent
relationship on distance between the speakers and frequencies they are
destructively interfering in the room.