Found some answers while searching the net.
Just figured that I'd share what I found with you guys...
Why does speaker impedance change with frequency?
The function of a speaker is to produce a constant sound pressure level for a constant drive voltage across a defined frequency range. Power is calculated as voltage multiplied by current. If the loudspeaker impedance drops and the amplifier voltage is held constant, then the current in the loudspeaker increases (current = voltage ÷ impedance). If the current increases, then doesn’t the power consumed increase? Yes! That is exactly what happens. If the power in the loudspeaker increases, then why doesn’t it play louder? Because the speaker efficiency is changing. As the speaker impedance decreases, its efficiency decreases. All that matters is that the acoustic output remain constant.
Another info link...
http://www.swee****er.com/forum/arch...hp/t-2193.html
More info under the impedance section...
Primer - Speakers 
