Here's a simple electret mic capsule / circuit I use for frequency response measurements, . .basically just a panasonic $2 WM-61A (chosen for its linear response) capsule mounted on one end of a hollowed-out ball-point pen and a 9V battery and a couple of other miscelaneous components hanging off the other. I taped this contraption onto a spare tripod and voila !! . This is fed to the LINE-IN (not MIC) input on my audigy 2 sound card, which has a flat enough response for frequency response measaurements. I use ARTA for actual measurements.
The only downside to this simple circuit is that it can't drive long lengths of cable (mine is a couple of meters long and works just fine).
A sample measaurement