How to do the horn modifications on a SEOS15 for SmallSyns
since there was a question about drilling holes (ports) for the midrange --
(from a writeup I did a while ago but didn't blog yet
Mods to the horn involves first moving the two tweeter mounting holes to get them away from the midrange that would otherwise block them; then cutting port holes for the midrange; then fabricating and attaching a mounting plate for the midrange and filling in some resulting volume. None of it is too hard, and I avoided making changes that couldn’t be specified and duplicated accurately enough on a shop bench. But it will take a little care.
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First, cut new tweeter mounting holes. Place the SEOS15 mouth down onto a table, and take your metal scale and mark the center line across the horn’s mounting plate (you can see the position by looking at the mold seams on the side). Then, use the scale to mark the positions for the new holes as shown below.
Drill small pilot holes using your pin vise drill (for precision of placement) and then drill out the 3/8”D holes with a wood bit. You’ll discover something about drilling in the plastic the SEOS15 is made from -- that it is made from SERIOUSLY thick plastic, but it cuts easily. The drilling will start slow, but then when the drill gets about halfway through, it will suddenly dig in and pull with some violence through the CD mount! So don’t push too hard. This won’t cause any real problem here, but would become one in the next step when you drill the midrange ports, as the drill will want to gouge into the visible opposite wall of the horn when it jumps through. So these holes are good practice for controlling the drill.
To prepare the midrange mounting area, you need to cut away the support strut that is intended to strengthen the CD mounting plate, on the side that doesn’t have the SEOS logo. The plate is very thick and strong, and the tweeters won’t be all that heavy, so the strut is both un-needed and in the way. I used an old “un-cherished large wire cutters” to trim away the plastic strut a quarter inch or so at a time, you can do that or use a saw or any other means you have available to cut the ridge down approximately flush with the outside of the horn.
Lay the horn on its wide side on a table. Now, take your flexible scale and on the side you removed the strut/ridge from, measure and mark (or scratch) a line 1.25 inches out from the back of the mounting plate (see the sketch below). Measure along the body of the horn, with the scale pushed to the outside surface of the horn. Then locate the center of the horn again (where you removed that ridge from) and measure and mark 0.90 inches on either side of the center, at the previously marked line. At those points, use the pin vice drill to start small holes into the plastic, making the hole perpendicular to the TABLE (not to the horn wall).
Now, here is where it gets (a little) more complicated. So far, out of the 8 ports I’ve drilled like this into plastic waveguide walls, only two have happened without gouging into the opposite wall when the drill yanks its way through! I don’t have a lot of horns to keep doing this with, but the method that has worked successfully (well, except once!) was to have a piece of wood stuffed into the horn throat from the other side so the drill bashes into it rather than into the plastic wall. A 1” dowel worked once, but another time, the drill still pushed it aside and went past anyway. The best approach seems to be to use a piece of MDF sheet, cut to have a corner that is a little less than 90 degrees and pushed into the throat, to catch the drill. I’ll leave coming up with a way to hold the MDF in there during drilling to you (I held it by hand, but that was a bit clumsy). If the drill does tear through, it isn’t a disaster, only cosmetic – sand down the gouge and smear in a little wood putty and it will be fine. But it’s a shame to deface the pretty surface if you can avoid it.
Drilling also requires care because we want it near the desired angle with the horn walls. To get the port down near the tweeter’s throat, the hole is to be drilled at an angle toward the tweeter (rather than straight down perpendicularly through the horn wall). Set the SEOS on a flat bench, which will result in the CD mounting plate being at an angle to the bench. Start with the 7/16 brad-point drill bit perpendicular to the horn wall, so that it begins boring in at the right location where the pin vice hole was made. Then, when the drill begins to cut its outer diameter into the horn wall, tip the drill upwards with the drill running to be perpendicular to the TABLE (not the horn wall anymore). See photos below. This is easier than it sounds, just align the drill bit (via eyeball) with some vertical edge in the room, when checking it from several directions while you drill. The more difficult part is when the drill starts to come through the wall and gets over-enthusiastic about pulling in!
