Fourth order (24dB/octave) Two-Way Crossover 
* The phase shift on a fourth-order crossover is 360 degrees = 0 degrees (no phase shift).
Zobel Circuit (Impedance Stabilization) 
* Even though speakers are rated at a certain "resistance" (i.e. 4 Ohms), the actual impedance varies with frequency (speakers have inductance). To compensate for the non-linearity of speakers (on mainly subwoofers), Zobel circuits are used.
* Re is the DC resistance of the woofer (can be measured with an ohmmeter)
* Le (or Lces) is the lectrical inductive quivalent of the driver.
L-pad (Speaker Attenuation) 
* An L-pad circuit will attenuate a speaker.
* L-pads keep the load "seen" by the amplifier constant, affecting only the power delivered to the speaker. The power delivered by the amplifier remains constant.
* Since L-pads are made from resistors, it does not induce any phase shifts, or affect frequency response.
