Frequency response Round 3 ->
Frequency response was measured crossover at its max and min settings and also bypassed if available. 10 second sine sweep from 200 Hz to 10 Hz was used. Level was matched at 90 dB at 50 Hz at 2 m distance. Mic was on the ground and the distance was measured from the acoustic center of the subwoofer. For sealed subs acoustic center lies right in front of the voice coil (front baffle was used for consistency). For vented and subs with passive radiator(s) acoustic center lies somewhere in between the driver(s) and the port(s)/
PR(s). But because most of the output (vent/
PR affects only near tuning frequency) comes from the driver, I measured the 2 m distance from the (active) driver also with vented/
PR subs (using a directional line from the mic). If the sub had a port or
PR on the back, the sub was rotated so both were equidistant to the mic. In some cases I also measured all combinations and choosed the one which gave better FR. Being consistant and fair to all subs regarding this matter is extremely difficult because of the various driver/port/
PR combinations subwoofers have. If all subs were single driver, sealed, front-firing subs, there wouldn’t be any problems. The key thing was to measure the 2 m distance from the point where the
most of the output comes from, but also keep the other sources (if there was one or more) as close as possible to the 2 m distance. Subs were kept in their
normal operating positions, for example base plates weren’t removed or subs turned on their sides etc. Same orientation/position was used during all measurements. All graphs are using a full 1/24 octave resolution. +/- 3 dB points were calculated using maximum or bypass (if available) crossover setting.
Round 1 and Round 2
Subwoofer’s frequency response was measured with TrueRTA’s Quick Sweep signal. The signal is a short-duration, digitally synthesized logarithmic sine sweep from 0 Hz to 24 kHz (when using a 48 kHz sampling frequency). It takes only a few hundred ms’s to sweep the normal subwoofer operating range. Unfortunately this method isn’t as accurate as the normal sine sweep (which was the reason why I switched to it) under relatively loud background noise. Therefore small variations can be expected between these methods. +/- 3 dB points were calculated using maximum or bypass (if available) crossover setting. 
+/- 3 dB points: 18.5 Hz - >200 Hz
The best possible anechoic FR depends of the room/space where the subwoofer will be put in. Usually gradually sloping (6-12 dB/oct.) response gives the flattest in-room response due to room gain. The bigger the room the lower the response can be flat without having problems with elevated low end response. Below can be seen two frequency responses. The first one is more suited for small/mid-sized rooms while the second one is more suited for larger spaces with less low frequency room gain. If the second sub will be put in a small (rigid walled) room, frequency response will be strongly elevated below 30-40 Hz, which will sound too "bottom heavy”.
