Dayton Audio BR-1 6-1 - Manual (Page 4)

About Zobels
This simple circuit consists of a single resistor and
capacitor. It is commonly referred to as a Zobel
network, named for the Bell Laboratories engineer
whom invented it. An excellent starting point can be
calculated by using this formula.
R = 1.25 x Re
C = Le / R squared
Re is measured voice coil DC resistance in ohms.
Le is measured voice coil inductance in henries
R is in ohms
C is in farads
I measured the individual 295-305 woofer parameters
using the LMS measurement system, and the Le was
found to be 1.87 millihenries. Using an accurate
ohmmeter, the Re was 6.5 ohms. Using the above
formula:
R = 1.25 x 6.5
R = 8.125 ohms
so
C = .00187 / 66
C = .0000283 farads or 28.3 uF
Although these values will give a good result, actual in box impedance measurements indicated an even better
compensation can be had with a 47uF capacitor and a 8 ohm resistor.
As can been seen on the graph above, the impedance is now very flat through the crossover region. From 250 Hz to
10 KHz, the impedance is 7 ohms +/- 0.5 ohm.
Page 4
6.5" Impedance w/ Zobel Network