Now that the subwoofer is finished (see the photos) it is time to evaluate the sub and make sure it is working correctly and built right. 

First, the finished sub is 83 lbs and so carting it around is a hassle. I try to do this just once.

Begin by putting the sub near your computer so that you can do the tests. The first test I did after construction was to check the frequency response. I did this with a nearfield test - the microphone is placed about 1/2 inch from the center of the dustcap of the subwoofer driver (the closer the better). Then I did a signal record (using a sweep) and checked the amplitude, ensuring the amplitude was as close to 30K as I could get without clipping. Since the woofer has its own internal amplifier it was easy to set the level.

Then I selected the NHT driver and did a Nearfield test (there is a button for it on the button bar labeled f_N). Here's the nearfield chart

This chart is almost unusable, so doubleclick the dataset and turn off the phase display (which just confuses things) then change the Frequency range to something more reasonable (here I used approximately 10 to 200Hz) to get the following chart.

This chart is very reasonable, but to make it even easier to read I did a 1/16 octave smoothing (Transform / Smooth / select 1/16 octave using absolute), producing the following very easy to read chart:

From looking at this chart, the subwoofer is down about 10dB at 20Hz, with usable output down to 15 Hz (20dB down), it peaks around 35-40Hz with perhaps a 2dB peak and then it slowly drops off. The dropoff at the top end could easily be due to the nearfield test process - a nearfield test is most accurate at low frequencies - where it excels at removing room interactions.

For comparison I looked at the Sealed Box analysis using a volume of 4 cu ft (about the size of my box) and saw the following:

As you can see, the correlation is excellent. The next step is to fill the box with dacron or some other good quality fill to increase the apparent volume to 5 cu ft and improve the top end substantially. But, for now, things look good.