[aktiondan]

It was years ago that I built this little impedance analyzer, and the method for measuring impedance was much more time consuming. It involved using a signal generator and a DMM and you had to go through and manually test and measure each frequency. REW just replaces the signal generator and DMM, and it does it quite nicely too. Since all it's really doing is comparing an output voltage to an input voltage.

I've got a little 1/8" mono plug that runs to an RCA jack with a couple of wires soldered to it, and that hooks up right across the woofer/speaker. The 8 ohm resistor can be used to calibrate the measurement, so you have a reference to 8 ohms on the graph in REW. Though it shows up as dB, you can convert back to ohms with the formula Z=CALz*10^(A/10) where CALz is your reference load (8 ohms in this case) and A is the delta dB from the 8 ohm reference. For example, if you set it up so that 80dB is your 8 ohm reference, then if the plot rises 3 dB to 83 dB, that would mean the impedance actually doubles from 8 ohms to 16 ohms. A 6dB rise would be 32 ohms. The trick would getting the plot into Excel so you can manipulate the data.

I hadn't thought about using it to evaluate the ideal amount of stuffing, but it certainly can help you "see" what you're doing by adding/removing stuffing. For measuring the tuning frequency of a newly built sub this method is ideal. A more accurate way to measure the Fb of a ported box is to plot the impedance and measure the frequency at which each of the two lower peaks occur and label them Fl and Fh. Then seal the ports, and measure the frequency at which the single peak occurs and label it Fc. Run the numbers through the formula Fb = SQRT(FL^2 + FH^2 - FC^2) and you get Fb right on the nose.