Let's build some scoops! - by - Paul C.
My friend Jay allowed me to lead him through building a pair of 15" scoop cabs for his livingroom, which you can see from the photos, is presently undergoing renovation. He decided to build the cabs right there before installing the carpet.
Suitable woofers are ones with high EBP (Efficiency Bandwidth Product), at least EBP = 90, and better 100 or more. Eminence publishes EBP, but you can also calculate EBP from EBP = fs/Qes. Also, for scoops you want fs (woofer resonant frequency) to be below the horn cutoff. For the 15" scoop this means you want fs below 40 hz.
Jay used the Eminence Gamma 15, but there are many others that will meet the simple criteria above.
In my opinion, only birch ply, 11 or 13 ply, is suitable for large speaker cabinets. MDF is too heavy, chips too easily, does not hold screws well. MDF is only suitable for small speakers, or large ones that will never be moved. For cabs that will be used for PA use, and will likely be moved often, birch ply is the only choice.
Jay started by cutting the major panels to size and with careful measurement, sketching placement of the internal panels on one side.
Note that instead of the very difficult to fabricate curved section in the horn mouth, with my advice, Jay substituted a flat panel at a 45 degree angle. Believe me, no one will be able to tell the difference.
Jay used carpenter's glue and a pneumatic nailer to tack it together until the glue dries.
This is OK as he will be leaving these speakers in his living room. If you will be moving them for PA use, use screws through the sides and into the internal panels.
Later Jay will veneer and oil these cabinets, but now it is time to test. For PA use let me suggest you use truck bed liner paint. This can be applied at home using a texture roller for a nice finish. You can also take your cabs to a commercial shop for an application of Rhino, Gator, or other brand name bed liner. This material is easy to repair later. Gouges can be filled with Bondo (autobody repair putty), sanded, and repainted.
Now Jay is testing with a simple crossover and Altec Lansing 511b horn with 902-8 horn driver. He later assembled a better crossover which I had sent him, which includes 2nd order (12 db/oct) with zobel components for the woofer section, and 3rd order (18 db/oct) with resistive padding and compensation to lift the high frequencies where they droop at the top. For these components he chose a 500 hz crossover point. He could also have used an Altec 811b or Eminence H290 horn with Eminence PSD2002 driver crossed at 1600 hz.
Here he has made a nice grill using cloth from Parts Express. This cloth looks like the old Fender grill cloth.
Not only is this speaker a rear loaded horn, it is also a transmission line. You may experiment with damping material, dacron poly fiberfill, as shown in this sketch. Don't pack it tightly, just lay it in. Experiment with the amount of fiberfill for best sound. Low bass frequencies will pass right through. Midbass and midrange (depending on amount of fiberfill) will be damped. Drill some holes in the bottom and back by the 45 degree panel in the mouth and spray in expanding urethane foam. Do not overdo this. Spray a little and let it expand, spray a little more. Excess will run out of the holes. This will stop that bottom panel from vibrating. You may also want to add a 1x4 vertically on the inside of the back panel to help prevent the back from vibrating.
With the Altec horn/driver combo and 500 hz crossover this speaker sounds amazingly real. And "It... it... it goes so low!"
Paul is the US Distributor of Saxrax products, a UK company www.saxrax.com
HERE'S THE PLANS!! - Paul kindly sent over the plans people have requested so we can host them! - This gives plans and dimensions for 12", 15" and 18" scoop!
[plans will print out to an A4 page]
(Useful tips if you are new to woodworking!)
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