Sub Construction Summary
Saved from Audio Karma ::
Well, it’s Minnesota, it’s been snowing most of the day already. So, time for a speaker post.
Project One, the HPM-60 ‘restoration.’ Actually, these were in fantastic condition when I got them from a fellow AK’er … thanks again! So, there wasn’t much to do at all. But calling it a restoration sounds cool. Anyways, I ordered the re-foaming kit from PE, but then decided to just go with Midwest Speaker Repair, which I got a fine deal on (and they’re near my campus). After that, we reveneered the sides with walnut. Now, if I remember correctly, these speakers are exactly two feet high on one side. When we bought the veneer, we stupidly assumed that when they said “24 inches” they meant a full 24” … So, there is anywhere from a sixteenth to an eighth of an inch gap between the bottom of the veneer and the bottom of the speaker. With a little plastic wood and sanding, it’s pretty hard to see. Shame it had to go that way, but I am happy with the results. The finish is less shiny than the ‘stock’ HPM’s, but that’s how I wanted it! Finally, gave the front a good cleaning around the drivers to give it some life again and now they’re almost done! (Did I mention how perfect the grills are?) Any advice on how to ‘pull’ the midrange dustcaps would be appreciated. I could always replace them, but considering a dented dustcap doesn’t affect the sound quality, I am content to leave well enough alone.
Project Two, the DIY sub project. I got some excellent help here with this project about 2.5 months ago when it was in the planning stages, so here’s a quick recap of the project. From the start, I decided to base the project around the Shiva Mark IV 12 inch high excursion kevlar impregnated dual driver yada yada subwoofer from Adire Audio. Thus, I began using WinISD to model different cabinets after I entered in the Thiele-Small parameters. Naturally, I came up with all sorts of crazy designs, one being 30 inches high and about a foot and a half in its other dimensions (I just figured I could put it on its side if it got tippy). Eventually, I realized it would be better to go with a tried and true design which turned out to be the SBB4 alignment provided by Adire. I decided, instead of buying a port, to just make one out of PVC. I used WinISD to double check that 17 inches was the correct lenghth (general rule, add 5 inches to a flared pipe length if you are replacing it with a straight pipe) and things were good to go. Something you might be asking yourself: what about poly fill / acoustic foam for the inside? Well, I opted against this, being told that its not a good idea to use fill unless you have measuring equipment. True? I’m not sure. Adire Audio in an email said:
It actually increases the volume the driver sees. It makes the box appear larger that it really is, by about 8% [per pound]”
Thus, using this rule you could probably make things work ok. Either way, I figured I would simplify and leave the acoustic fill out. So, I began ordering things - I got a great christmas deal, no shipping (110 delivered!) on the Mark IV from www.acoustic-visions.com - I would highly recommend this site; great customer service, blazing fast support, etc. I then ordered a 250W subwoofer amp from Parts Express - this one has the rumble filter (which the others might have, but I didn’t see that specifically stated) which prevents the cone from unnecessary driver excursion when you are significantly below the tuning frequency of the cabinet. Seeing as the internal volume had to be exact, I did wonder if the displacement of the components of the amp would significantly detract from my internal volume (I had taken all that time to account for the bracing and the driver, why have that ruined by the amp?). So, I fired off an email to Parts Express - to which they replied:
Actually, woofers do not require “precise” volumes to work as they should. Few woofers are highly sensitive to cabinet volume, and none are sensitive in the amount the transformer and reservoir caps of the amp will occupy (.020 to .030 c-ft). Stated Thiele/Small parameters are usually beyond + or - 10% tolerance in even the best and most expensive speakers. Though we do try to get things close to optimal, 30-thousandths of a cubic foot is far too small to make even a measureable difference in performance, not to mention an audible difference.
This makes sense. Let’s see, some other details of construction: we used almost one full sheet of MDF for the cabinet and internal bracing - the extra MDF was useful to rout out circles on, etc. Note for others: never, ever use a circle bit with a battery powered drill. Our old corded drill died a while ago … so I went through about eight battery charges just to get the four 4 inch diameter holes drilled in the bracing. Construction was pretty straightforward, we routed out some slots in the bottom for those legs that you see - that’s cocobolo wood - a rainforest wood which is unbelievably hard. I was told it is “aromatic” when cut. Talk about aromatic … I guess they didn’t have the problem of it burning! Our circular saw didn’t get the blade spinning fast enough, so we got some burn marks and the garage was filled with this greenish brown smoke. Weird. Either way, the stuff makes fantastic legs because its harder than heck and only needs a good sanding to look fantastic (it’s a little oily by nature). Furthermore, schedule 40 PVC pipe is very slightly under 4 inches in internal diameter, so we cut 17.1 inches for the port tube (to get the correct volume), and just rounded out the sharp edges with a rasp. Assembly: the cabinet was glued and screwed together to make it solid. All of the internal seams were sealed with silicon caulk. You could probably toss this thing down the stairs and not worry about it breaking. Anyways, the port tube was glued in with epoxy, it sticks out a bit from the bottom of the cabinet so I could get 3 inches of clearance from the top of the box (3 inches is the recommeneded minimum space to have around the tube). The driver itself was secured using t-nuts and then a couple of lengths of rope caulk to seal it. One of the trickiest parts of the project was stapling the internal wiring (the driver is wired in parallel to the amp to present a 4 ohm load) to the internal bracing so as to avoid slap from the wires moving around during ‘spirited’ listening levels. Finally: the outside was finished using gloss finish Formica Laminate (the kind on your counter tops). It was attached using contact cement and machined very well using a carbide tipped flush routing bit. I guess it could double as a TV dinner tray if needed.
How does it sound? Well, I can say don’t use the plate amp in the pre/power loop - it introduces copious amounts of noise. So, I use the tape out loop to attach the sub to a receiver if I want to (I know it’s not good form) add some deep bass to the music. I have noted when listening to classical piano, the sub brings out the pedaling that the mic picks up - very interesting. The cabinet is absolutely dead - no rattles or anything. I am still figuring out how to best position the sub in the room, so I can’t give any real definite details about how it sounds. I can say there is a slightly noticeable room gain bump, and I don’t have the equipment to isolate and fix that. Nevertheless, the output can be very subtle as supplementary bass all the way up to ridiculous SPL’s. I feel like I got a 2000 dollar sub for way way less (but we all know price means nothing). Any questions on building I would be happy to try and answer.