Bracing 828 Cabinets

[John Stover]

I just reread Jim Dickinson's article on the small VOT's and think I'm about ready to take the plunge on adding additional bracing to my early 828 cabinets (3/4 plywood, braces on the outside back panel). But I'm confused by the directions contained in the article. I apologize in advance for being dense.

First, I assume I close off the horn flares by fitting plywood pieces on either side of the woofer mounting board and then filling the cavities with blown-in insulating material. Should those plywood pieces be the same thickness as the woofer mounting board (which I think is thinner than the plywood used for most of the cabinet). Or is it better to completely replace the woofer mounting board with a single (thicker?) piece that will cover both flares?

Second, the article says: "Screw and glue 2x4's on edge diagonally on the inside of the enclosure on all large flat areas of the reflex chamber. Cut other areas in half with 2x2's." Is the reflex chamber the entire area inside the speaker cabinet, or just the lower half? Does anyone have a diagram or better description for how the bracing boards should be placed in the cabinet? Does the additional bracing further reduce the reflex volume (I'd think it would) and is that a problem?

If JimD is Jim Dickinson, I would really appreciate some clarification, + I would like to hear from others who have tried his modifications.

Thanks in advance.

John

[Steve Burger]

Jim D is indeed Jim Dickinson.

[marcuswilson]

John,

The altec cabinets can sure use some bracing.

After designing and building scores of cabinets, this is my recommended method.

Firt, understand the purpose of bracing. It is to stiffen the cabinet walls, so they do not vibrate and radiate sound themselves as well as absorbing energy.

You are correct in assuming 2X4s will take up space and they are not ideal bracing material. The best material is 1/2" or 3/4" plywood cut into 2" or 3" strips. These are glued to the cabinet walls edge on. If you are really keen, glue them in a 6" to 12" criss-cross pattern over as much of the inside panel walls as you can. Make sure they are accurately cut, so the edges mate up squarely and can be glued where they join.

The 2X4s provide no greater stiffness to the panel than the ply, but weigh much more. This weight changes the resonant frequency of the structure a lot and negates part of the stiffening effort. They also take up too much space, as you have noted. I have found that 2X4s attached on the flat perform very poorly.
The 1/2" ply is as good as the 3/4" when glued properly, but if the cutting is not very accurate or the original panel is rough, the glue bond on the 3/4" ply may be superior. The strength of the glue bond is very important. All joints in a speaker cabinet must be very well glued. Merely re-glueing an old cabinet can make a huge difference to the sound.

The bracing is trying to stop the panel flexing in and out, the ply strips on edge are very stiff in this direction and will make the panel extremely stiff with minimal weight increase or volume loss.

[Charles Chance]

Rather than diagonal bracing on the side surfaces, I glued 2?x2?x4? wood blocks on either side about half way down. Then I tied the blocks to each other with a 2"x1" stringer. Since the vibrations of the 2 sides are in opposite directions, this technique will stop that. All joints are glued and screwed.

As to the woofer mounting board, if you try to remove it, you may very well damage the horn component. If all is glued and screwed tight, removal will be difficult. I used Space Invader insulating foam by GE. It comes in an aerosol can and is available at my Wal-Mart. It takes several cans but I f_i_l_l_e_d (editing program changed the real word to *****d) up the voids behind the LF horns. Use one can at a time and allow it to expand and harden before using another. This not only fills the voids but also deadens the horn itself and makes it much more solid.

[marcuswilson]

Charles,

Your technique works well for the basic movement of the panel, but if you have only fitted one brace, you have basically split the panel into two. Now there are two panel radiators on each side with their fundamental frequency 2 to 4 times the frequency of the whole panel.

I advise to keep bracing the panel until the resonant frequency of the remaining unsupported panel sections rises an octave above the passband of the cabinet.

This is easy for subwoofers crossing over at 100 Hz or so, but much harder for bass/mid cabinets.

Once the panel sections get small enough to resonate over 250Hz or so, elastic damping material becomes effective, eg. rubber or tar mats. This can be applied to the flat surfaces.

You can check the frequencies radiating from the back and sides of a cabinet by putting it's face through a doorway and blocking off the rest of the door area with something solid. If you are in the room with the body of the speaker, the rear and side radiation is much clearer. It should really be inaudible.

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In "Loudspeaker Enclosure Walls" (JEAS, October 1961) Peter Tappan shows, through direct measurement of a test panel, that the most effective bracing of a rectangular panel is fixed at the center line of the panel, parallel to the long dimension. His paper claims that, based on measured response, a perpendicular center brace is least effective (in fact, virtually ineffective) and a diagonal brace only half as effective as a longitudinal brace.

[Fitts]

My low cost, low tech solution:

Brace the large rear panel of your A7's using pillows made of the firmest foam you can find. Place the pillows between the back panel and the wall behind the cabinet and push the cabinets up against the wall firmly.

Of course this presumes that your walls are solid and that you have your cabinets against a wall. I have done this for years and find it works great.

[lvd1569]

hello john, i also would like clarification on this matter with both jim d. and bill eckle also thunder bob, the plans are not completely helpful. maybe they have pasted into vott's heaven. may they rip for they did try.

[marcuswilson]

The bracing tests used in Peter Tappan's paper used the rise in resonant frequency of a panel as the measure of the effectiveness of bracing. His tests proved that a brace along the cenre line of a panel parallel to the longest side gave the greatest rise in resonant frequency.
This work is correct within it's parameters, but does not prove that this is the only bracing required. You need to be aware that by raising the resonant frequency of a panel you can make the vibration more audible.

By using multiple braces of the type I described earlier, the resonant frequency of the panel sections is raised right into the midrange. At these frequencies damping materials are very effective.