After testing the subwoofer carefully, the next step is to prototype an enclosure for the subwoofer. For the prototyping the program will look at Q_TS and V_AS more than any other driver parameters.

To begin, create a new enclosure resource and then bring up the enclosure properties and put the driver into chamber 1. In detail, do:

1) Resource / New / Enclosure. Name it appropriately (mine is NHT1259 Sealed Box).

2) Bring up Edit / Properties (Ctrl+E) or right mouse button or menu.

3) Go to the Chamber 1 tab and click the Select... button to pick your driver.

4) Go to the general tab and set the frequency range to 10Hz to 400Hz (or so), which is appropriate for a subwoofer like this.

5) Click OK to close the properties dialog and begin the prototyping.

Before deciding on sealed versus vented I tried prototyping both kinds of boxes. The sealed box Q can range from 0.5 to 1.3 for reasonable results. As the Q approaches .7 the response gets flatter and flatter. Over 0.7 the response starts getting a small hump at the resonance (which keeps moving up as the Q increases).

Use the Sealed Box dialog (Calculate / Sealed...) and vary the Q and look at box size and the various curves. I tend to prefer a Q of 0.7 but the box size got pretty big. Also, for a subwoofer a higher Q can be nice because the response will be "flatter" near the resonance. Finally, the subwoofer amp has a small hump at around 24Hz which fits perfectly with a Q of closer to 0.8.

For comparison I looked at a BB4 vented alignment. Here is a screen capture of the curves for both sealed and vented alignments:

The red curves are the sealed box curves and the black curves are the vented. As you can see the vented curve has a much lower frequency response (vented is -4dB at about 16 Hz while sealed is -4dB at 25 Hz). Conversely the vented box has a hump of about 2-3dB at 27 Hz and excursion issues as the frequency drops below 18Hz (see the excursion start climbing dramatically) and the group delay and transient response for the vented are just not as good as sealed. I once built a system with great response and terrible group delay and it sounded just awful - high values of group delay are clearly audible.

In defense of vented note that the cone excursion is actually lower around 20Hz and as a result vented boxes can play louder with less distortion near their resonance. Actually, using a BB4 alignment is pretty tempting, but not a great match with the amplifier. Also, there is not much content below 20Hz sonically (none, in fact) and so the vented improvements are not all that great here.

I like the QB3 alignment sonically but it required a box that was 83 cu ft!

Finally, the sealed box (with a Q of 0.9) was 5 cu ft and the vented box was 12.5 cu ft. A sealed box is usually smaller than spec (filling increases the apparent box size) while vented boxes are generally built to spec (filling doesn't change the alignment much). For my purposes 12 cu ft is a pretty big box - too big.

On the next page I go into more detail about the sealed alignment selection process.