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Pyramid Powered Isobaric Subwoofer

Part III

April, 2006

Kevin Lichterman

 

Constructing the Box

Now for the fun part making it happen. There are a number of ways to walk you through the construction process. To keep things simple, I'll break down the project into a number of goals and enumerate the steps I took to accomplish the goal. Following the plan as outlined should get you a completed speaker without too much drama, but feel free to modify them to your own tastes. Along the way, I'll also call out a number of tips that apply to the various steps. Consider them a list of my own lessons learned on this project and the 'why' I did things a certain way. Also, I'll be referring to the parts I used in the project, and this list is located at the end of the document along with costs and possible sources.

Creating the Basic Speaker Components

The first goal is a big one, to create each of the thirteen panels required to construct the speaker. Refer to the diagram below for the dimensions of each of the panels. Notice that each panel has a label that may be referred to below.

 



Tip: Refer to the Top and Bottom panels to see final box dimensions.

1. Cut all panels to size. Mark each panel as noted in the cutout diagram (Top, Bottom, etc). Note that because we are building a speaker with sloping sides, not all of our cuts will be made with the saw blade set at 90 degrees. There is roughly a 5 degree angle at the top and bottom of each panel to account for the slope of the box. Because of this angle, the Front, Back, Left, and Right panels will have the shape of a rhombus if examined on edge. For the Top and Bottom panel, this 5 degree angle is canted in toward the center of the box think severely squashed mini-pyramids. The interior panels for the Isobaric Chamber can each be 90 degree cuts.

Tip: There are a number of ways to make these cuts. First you could use a table saw. This can be very accurate, but the largest panels can be awkward to handle on the saw. What worked well for me was to use a saw guide (available at many hardware stores) and a circular saw. The guide can be clamped to the panel being cut to provide a straight edge to guide the circular saw cut. Both methods use a 5 degree tilt in the saw blade (from 90 degrees) to achieve the correct angle.

Tip: Remember to account for the size of the saw blade when making the cuts

2. Find and mark the center lines (long and short orientation of each, and therefore the exact center of each panel) of the Right, Left, Top, and Isobaric Top panels.

3. Using the center line you just marked on the long access, find and mark the 1/3 and 2/3 points along this centerline to divide the panel in thirds.

4. Mark each panel to indicate the intended orientation of each panel. Mainly what will be the outside and inside of each individual panel. This is important since the panels have a correct orientation due to the angles required in the pyramid.

5. Cut the holes for the 12" drivers in the Isobaric Top Panel and the Front panel. In the Isobaric Top, the center of this hole is the 'x' you marked earlier in the center of the panel. The center of the hole in the Front panel is somewhat flexible. I tried to go as low as possible in the panel to maintain a low center of gravity (best to keep this beast on its feet!) while allowing clearance for the Isobaric Chamber. This turned out to be roughly 10" from the bottom or slightly under the panel third marked above.

Tip: Verify the dimensions of the hole before making any cuts. Use a tape measure or ideally the cutout template provided by manufacturer.

Tip: To cut all these holes in the box, I used a Jasper circle jig and a router. The circle jig is similar to the school compass (you know - you always had to get one for school but rarely used it for anything but a paper punch) that is designed to attach to the bottom plate of many routers to cut nice clean holes. You could also get the holes made using a hole saw, jig saw or something similar.

Tip: When using the Jasper circle jig, make sure that there is clearance under the router bit or you could potentially put a nice smile in your work bench not that it EVER happened to me (I deny that I have photos)!

6. Cut the holes in the Top panel for the two 2" ports. The centers of the cutouts for these ports are the Xs that were made at the 1/3 and 2/3 points along the centerline of the Top panel.

7. Cut the 1" holes in the center of the Left and Right panels marked out earlier for dowel cross bracing (these are holes through the panel).

Tip: Instead of a hole saw or circle jig, I used a 1-1/8" Irwin bit to make the holes the slight oversize helps account for the slope of the box - just remember to use plenty of glue when assembling to seal things up.

Click Here to Go to Part IV.

Copyright 2006 Secrets of Home Theater & High Fidelity

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