Toccata Grande, Part 2

By Lars Mytting
20 Jan 1998

The dance around the jigsaw

Okay... woodwork hour has come. As mentioned, the best school of cabinetry is to make a box that is so heavy, so rigid and so stiff that the drivers have no chance to put the cabinet into vibrations -- something like bolting the drivers into the foot of the Taurus Mountains. This is possible, but if we want to do it with the sizes necessary for our transmission line, we would encounter dimensions recalling Forum Romanum. Instead, I had to use thinner materials and try to make the cabinet as stiff and rigid as possible -- at the cost of some vibrations that were to be dissipated into a sand layer. The drawing explains all the important features of the cabinet. You should note the following details:

• The bass drivers are supported to the internal wall with a brace. A layer of asphalt was placed on the support and heated before the driver were mounted to give full contact.
  
• There is a additional brace between the baffle and the bass driver support brace.
  
• There is another brace between the rear wall and internal wall.
  
• A brace located at the fold of the line couples front wall, internal wall and rear wall.
  
• The baffle is made of double plates.
  
• There are angle-cut boards at each corner to improve air flow (minor effect), and to improve the strength in corner joints (good effect!).
  
• The midrange baffle is fastened to the front wall with eight supports.
  
• The midrange drivers have magnet supports of asphalt.
  
• A 5 cm wooden moulding is profile-cut at 60· angle, and fastened to the lower edge of the speaker and painted black. This creates a a "base" for the speaker, for cosmetic reasons.
  
• The dividing wall that forms the t-line is also bracing the cabinet.
  
• Extremely good bracing could be obtained by extending braces 1 and 3 to the full length of the cabinet!
  

Building the cabinet is fairly straightforward Fig. 1. Toccata cabinet plan, front Fig. 2. Toccata cabinet plan, side. The quality of the saw used has a direct impact on solidity and thereby sound quality. Use polyurethane glue for all leakage-critical applications. This glue expands and gives a completely air-tight seal. Fasten all internal cables with hot glue to avoid rattle. Place the crossover in the port for easy access Fig. 3. Toccata at early lumber stage.

Building instructions
1. Place square wood lists along all edges of the cabinet. (Marked with red on the drawing). The thickness determines the thickness of the sand layer. I used 9 mm thickness. Fasten with glue. Ensure that all fit tightly, or you will have a sand leak in our room!

Set the lists approx. 2 mm outside each edge. When you have fitted the outer board, you can get sharp, perfectly flush edges if you remove the excess with a power sander or a wood planer.

2. With a saw, cut a 6-7 cm long, cone-shaped piece of the upper list. Cut more holes at the other sides if needed to ensure complete fill. This is where we will fill the sand Fig. 4. Preparements for sand-filling. Do not throw the bits away, they will be used to plug the holes.

3. Place short wood lists (same thickness as in step 1) evenly and asymmetric on the cabinet wall. These will form fastening points for the outer wall. Make sure the lists are positioned so that they do not block the path where the sand is to be filled.

To reduce mechanical coupling, use no more than the number needed for secure fastening of the outer board. There should be no more than needed to keep the outer wall from swelling. The amount of fastening points depends on the strength of the outer wall, and on the amount of sand. For a small cabinet you need just a few small points in the middle. The approximate number used in the Toccata is shown in blue.

4. Cut the board that is to form the outer wall. It should be a few mm larger on each side than the cabinet. When the excess is removed with a power sander you get perfect fit. As mentioned, thin walls are preferable. This is also a very simple method of getting good-looking wood finish on the speaker. By choosing a veneered board you get the wood type you prefer. The veneer on the Toccatas is teak, impregnated with a oil used for boats.

5. Place glue on all the fastening points, position the outer wall and place heavy weights on it to ensure that it gets fastened firmly. Apply as much pressure as possible!

6. With a wood planer or a power sander, remove the excess from the sides. It is better to do this before filling the sand, as the cabinets will get heavier and difficult to handle.

7. Fill sand through the hole on the top. Wiggle the speaker and knock on the side with a hammer (use a chipboard between to protect your precious veneer!!) to ensure that all areas are filled with sand. Add glue at the filling hole, position the piece of wood, and hit it down with a hammer. A layer of veneer on top will hide the holes.

As this was a prototype, I made it of lousy, 17 mm chipboard. I played a lot of music with it, and the cabinet showed massive vibrations, with heavy coloration of the sound in the 250-450 Hz region. Then I added the outer wall and sand, and supported the driver magnets. The result was astonishing. Even though the new layer added only 15 mm diameter to each wall, and I only damped two sides (not top and rear), it reduced the vibrations to great satisfaction.

From the first moment after applying sand damping everything in the sound stage became far cleaner and more precise. Bass became tighter and harder. Midrange clarity was greatly improved at high sound levels. You could now clearer distinguish the pedal kick on the skin of the bass drum. Heavy attacks on guitar strings were controlled and loud. Sounds echoed out on the stage instead of just becoming distorted fog.

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