June 12, 2012. ©
Damping of ringing from Altec horns 811 and 511 (800 Hz and 500 Hz lowest bottom x-over), PART 1 from October 2011 News, is continued here in PART 2 with text and pics. This is a professional approach to solving a sensible ringing problem from both horns, and a simple cure with text description and photos of the working process. The Altec aluminium casting is too thin related to the areas. I have been using vibration test equipment with extremely light sensors, to find positions and frequency levels of the resonances. Part 2 starts here.
I will continue with the 811 back bows. I have made models of the necessary side panels made from 3-4mm plywood. Press double sided reinforcemented carpet tape to one side of the panels (make 2pcs.) of the size shown on pic. 1 for the 811 model and completely cover the wood area, but only on 1 side. The other side of the wood will be used for the bottom bow. You'll need to saw out 4 pieces for a pair of horns, using a fretsaw and woodblade. Only use one panel 2 times. See the measurements on pic1. When the side panels have been sawed out, tape the 2 panels fully, but only on one side. Let the tape go +2,5 cm out in both ends. See pic2, the top panel for the 811 horn, is taped for top left side, the 90 degree corner is right side down. The high wood end near the frontplate. The tape must start at the front plate, the bottom of the panel 3 mm in parallel over the welded line on the flat area. Press the panel hard and carefully all places to the flat horn side. This is now top left bow seen from the driver side.
Take another 811 panel, and press tape on in the same way, but on the other wood side +2,5 cm in both ends. Press it on the other top side of the horn, in the same way as the first panel. You are now ready to mix, but on the back bows we do not use resin/hardener, but a blend of beach sand and rapid cement, which much more economical. Mixing ratio here is 1 : 4 volume, 1 part cement, 4 parts beach sand.
If you don´t live near a beach, try to get the beach sand (typically white or white/grey) in a pet shop, you don´t need much, 5-6 kilo. Cement can be bought in stores with timber and building materials, down to 5 kilo bags. They also sell beach mortar min. 15L /25 kg, also cheap. This is also fine. If you have a friend who also want to dampen his horns, you might share such a bag.
Mixing: Use e.g. a 500 ccm disposabel clear plastic cup. Start with 500 ccm cement, and put it in a small bucket. Then measure 4 times 500 ccm sand or mortar, and add it to the bucket. Blend this carefully, and then add a little water, and blend. Better to be a little on the dry side, instead of too wet. Keep adding small portions of water, and blend, but avoid to get floating water on the top of the mix. The mid size tool shown on pic3, is fine for the mixing.
Before applying the mix to the 811 horn, place a big piece of cardboard under the horn, to take up aby water running down from the mix. Apply a kilo or so near the panel edges using the tools, spread the blend uniformly along the panel and pressing the mix a little so the mix is pressed down to the aluminium here. Fill up approximately to the screw line on the front panel, and finish with approx. 1 mm height at the bottom of the panel.
Place pieces of wood and cardboard below the horn where the driver is bolted to the horn, and adjust with a small spirit level to horizontal surface of the mix when the total area between the side panels is filled up and adjusted to flat level of the whole surface with the tools. Use eventually a piece of clear plastic foil on the top of the blend, it is easier to get a fine surface this way. If uneven use a small spoon to fill up, and use the biggest tool to get it even.
If the surface is too wet, use kitchen roll paper to suck up surplus water. Also balance front plate to horizontal level.
If there's a tendency to topple over to the front, just add weight to the driver plate, using a string with weight tied to a bolt hole.
Do not remove plywood side panels until after 12 hours of hardening.
Good luck with the work, pics added HERE. Remember to paint your work. Per Hasling.