| # Speaker Project # Evaluating the Subwoofer

> > > 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 fN). 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.

> > > On to the next page.

> > > last updated September 03, 2000