NHT-1295 Subwoofer

This Section Last Updated: 11-5-2005

This article was originally published at "Secrets of Home Theater and High Fidelity"
http://www.hometheaterhifi.com/volume_5_1/diy70398.html

The Driver

The subwoofer is based on a design that is all over the Internet. The driver is a really nice piece of work. It is used in NHT's highly acclaimed NHT 3.3 system. The 50mm diameter, 43mm long, and two layer voice coil allows long excursion, high thermal capabilities, and tremendous output. It also has a very heavy cast frame that minimizes vibrations to the box. The nominal impedance of this speaker is approximately 4 Ohms with a sensitivity of 83 dB/w/m.

The Enclosure

The box measures 3 cubic feet internally. Made of two layers of 3/4" MDF, it weighs in at 120 pounds with the driver mounted. Make sure you know where the finished beast is going to reside beforehand, as it takes two people to lug it around. Is two layers of 3/4" MDF a little extreme? Yes, but I only had 3/4" MDF readily available to me. So, rather than special order 1" MDF (nobody in my area carries it), I decided to overdesign the cabinet and use two layers of 3/4". IMO, the minimum wall thickness for this sub should be 1". I also used one shelf brace, and 4 dowel rods to further stiffen the box. The driver was also flush-mounted on the front baffle. The subwoofer is a sealed-box design, which makes for a very smooth response curve. The diagrams above map out the enclosure dimensions for the sub.

The Crossover Network

The sub output on the receiver that my brother selected (Marantz SR-870) has a built-in active crossover with a low pass cutoff of 60Hz, so no internal crossover was needed in his particular setup.

The Assembly

Here are the cut dimensions and quantity needed to make one subwoofer:

To make the inside box of the subwoofer you need:

2 (19.5" x 18") These are the top, and bottom.
3 (18" x 18") These are the sides and the shelf brace.
2 (19.5" x 19.5") These are the front and back.

To make the outside box of the subwoofer you need:

2 (21.5" x 19.5") These are the top, and bottom.
2 (19.5" x 19.5") These are the sides.
2 (21.5" x 21.5") These are the front and back.

The sub is basically made in the same fashion as the fronts and center speakers. The only difference is that you are working with a box inside of a box. I assembled the inside box to the point where one side was not attached. I then sealed the inside seams of the box with silicone sealant and clamped the outside box in place. After finding the center of the front face where the driver was to be mounted, I drilled a hole through both layers of the MDF at the center location. I then routed the flush-mount ledge on the front baffle where the speaker would sit. Then I routed the inside face using the center drill hole I made. You obviously dont need the flush mounting ledge on the inside of the baffle. All you need to do is rout the inner hole out. Once the speaker hole was routed, and everything aligned up, I added the bracing. I cut the holes for the shelf brace with a jigsaw so all sides of the brace were 2", and I cut the dowels with a miter saw. The dowel rods have to be the perfect length. If they are too big, they will bow the box walls, and if they are too small, the dowel will not do its job of bracing. I put two drywall screws in each end of the dowel rods to secure them and keep them from rotating/vibrating.

To attach the outside box to the inside box, I used Liquid Nails For Projects. It is an adhesive made by DAP that stays flexible after curing. A layer of soft material in between the two boxes helps dampen the box. I used the serrated edge of a trough to spread a thin layer of Liquid Nails on each surface of the inside box, and started attaching each face of the outside box. Once all the faces were attached, I used more dry wall screws to attach the outside box to the inside box. I let the Liquid Nails dry overnight, and then made the hole for the terminal cup with my jig saw. I applied the veneer, stain and topcoat, just as in the MTM description. Since the walls are 1.5" thick, you need to make sure the machine screws are long enough to go all the way through the wall and into the T-Nut when installing the driver. There was no crossover for this subwoofer, so I ran the speaker wires from the terminal cup directly to the driver, and mounted the driver with the machine screws. I powered the subwoofer wuth a Marantz MA-500 (125 watt into 8 ohms / 180 watts into 4 ohms) monoblock, and crash, bam, wham, away my brother went.