4 THT's

[Old Mike]

Managed to get 4 hours in the shop today. Pilot drilled for the drive unit mounting screws 9/64 bit. Test mounted drivers to all the panel 4's using socket head screws from PE. Managed to glue up panels 1,2 and 3 for all 4 units. Some photos below. The Baltic birch ply is nice to work with, almost zero warp, I bought the genuine stuff, made in Russia.

First time using PL Premium, I like it, I found it easier to work with than regular PVA glue, it grabs. I dry fit the parts, pilot drill and install fasteners on the panels before glue up then I'm not fighting the job and getting glue all over the place.

Tried the Home Depot tube plug, I'll see if it has held tomorrow.

[Charles Jenkinson]

Nice work Mike!

That Russian birch may be straight, but a beverage always helps to hold it down, just to be sure.

[Old Mike]

Can't beat a beer or two after an afternoon in the shop!! One of life's greatest pleasures.

Managed to grab another 4-5 hours today. Installed panels 4 through 8 and all inter panel braces on one cabinet. I've decided to complete the units one at a time going forward, these 70 year old shoulders don't take kindly to heaving these things around. I destroyed my right bicep tendon and tore my rotator cuff muscle beyond full repair possibility when hefting a new steering box into an old Mercedes that I was rebuilding. The surgeon did his best but my strength in that arm is limited so I have to work around it, getting old is a bitch but at least I'm still looking at the green side of the grass.

My taper jig came in handy today, with the exception of the braces between 4 and 6 I was able to set the angle in place so minimizing the number of test cuts and also getting a perfectly matched taper so no distortion. The braces slid in perfectly and I held them in place with some 1 inch brads through the respective outside panel.

The home depot caulk tube sealing pin worked fine overnight, I just backed off the plunger, hopefully it will continue to work. Been wearing disposable gloves when anywhere near the PL, so far no glue on hands.

The plans are a dream and make this construction a pleasure, thanks Bill, the called out angles are perfect. My table saw is a 5 horse General Cabinet 10 inch and well tuned, it makes life easy. A tip for other builders, I always use the perfectly square factory edges as a reference to get perfectly square panels, if the edge is damaged just cut the part a little oversize and rip to size after getting a perfectly parallel cut using the factory edge as a reference.

Now to have something a little stronger than a beer for my aching shoulders!!!

If everything goes to plan I should have the first cabinet ready for final closure by the end of tomorrow. I need to finish installing the rest of the panels plus braces and cut the access hole. I'm building 2 right handed and 2 left handed units with respect to the access panels, that way the panel and wiring terminals will be hidden. They will be placed in the 4 corners of my room. I spent a lot of time experimenting with placement and the 4 corners gave me the best result regards minimum difference across seating positions for my existing subs.

[88h88]

I've decided to complete the units one at a time going forward, these 70 year old shoulders don't take kindly to heaving these things around. I destroyed my right bicep tendon and tore my rotator cuff muscle beyond full repair possibility when hefting a new steering box into an old Mercedes that I was rebuilding. The surgeon did his best but my strength in that arm is limited so I have to work around it, getting old is a bitch but at least I'm still looking at the green side of the grass.

If you were anywhere nearby I'd come help you haul crap about if only to get in on the product testing at the end of the build.

[Old Mike]

Hey 88,

How about a trip to Canada, beers not as good as the UK but not bad. I'm a Brit but left there in 1971 to travel the world, ended up in Canada in 76 after four years in Africa working in the Zambian copper belt and kind of got "stuck" here raising a family. Frequent trips back to Blighty but find it very expensive, twice the cost of living compared to Canada for just about everything. I used to miss the pubs but even they have changed from what I remember from my hay days.

Managed anther 5 hours today and took the first unit to physical internal completion. I'm going to put this one to then side and get the other 3 to the same state, then break out the paint to pre-finish the visible parts of the throat before closing with the final side. I'm going to a trick I learnt building the aircraft, use locating pins, otherwise known as panel pins with the heads cut off so there is no smearing around of glue trying to get the final panel into position.

I've attached a photo of the state of play at the end of the day.

Have to say SWMBO is very good at bringing me a beer or two about 30 minutes before I finish, walked in today and said "OMG those are big and weird, how many will there be!!!" "Just 4 darling, they're only slightly bigger than the existing subs". Look of disbelieve. Women really don't have any sense of "space and size", at least that's my experience, they'll argue that a tape measure is wrong!!!!.

Probably a pause in posts as I take the other 3 units to the same stage of completion.

[Old Mike]

Almost there on the basic construction, one cabinet to take to a state ready for closure. Pictures to come in a few days.

One thing I will be doing that may sound odd, especially given the emphasis on achieving an airtight closure for the rear chamber, is to drill a very small pressure equalization hole in the cover plate, something way less than a 1/16th on an inch.

This seems counter-intuitive when we go to great lengths to obtain an air-tight seal for the rear chamber however a very small equalization hole in the cover plate will not affect the performance, (it will behave as a restriction orifice under the dynamic pressure change conditions when the driver is operating), It will just allow the chamber to equalize to the atmospheric pressure.

The volume of the rear chamber space is a nominal 3 cu. ft. in my 24 inch wide cabinets. Consider a 10 degree rise in the chamber air temperature, if the speaker diaphragm were just a plunger, it would displace the cone about 0.8 inches. The cone has mechanical resistance so it would not move that much,it would however be moved from it's natural at rest position.

Ball park math. The size of space is a nominal 3 cu ft. Assume starting ambient temp is 20 C or about 293 K. Increase temp to 30 C or 303 K, if pressure remains constant then new volume due to temp change is 3 *303/293, (i.e. P1*V1/T1 = P2*V2/T2) or 3.1 cu ft. Nominal area of speaker cone is 1 sq ft therefore cone displaces 0.1 ft.

I have wonder how many "blown drivers" may have been in part due to pressure build up in the chamber when operated at high power/high temperatures? The venting on the voice coil assembly turns it into quite a good air heater for the air space behind the driver.

[Bill Fitzmaurice]

I have wonder how many "blown drivers" may have been in part due to pressure build up in the chamber when operated at high power/high temperatures?

Very few, if any. No matter how tight you make the box there will be some leakage somewhere.

[Stratispho]

I have wonder how many "blown drivers" may have been in part due to pressure build up in the chamber when operated at high power/high temperatures? The venting on the voice coil assembly turns it into quite a good air heater for the air space behind the driver.

Are we building with materials that are completely air proof? The subwoofer and all it's components, the gaskets, every single joint? I would think that much like a rubber balloon, over time, whatever change in pressure happens due to atmospheric conditions that the air would equalize simply because the materials we use allow air to pass through over time.

[Old Mike]

Time is the factor in air tight as you state. But yes we are building with "air tight" materials, without them the design would not work. There will be diffusion through the rubber surrounds and gaskets over long time periods but they are much thicker than the few mills of a balloon. The temperature in the rear chamber can and will increase quite quickly if the unit is driven at high power. An equalization hole will not affect the performance but eliminates the risk, after 40 plus years in the oil industry managing risk became second nature for me, especially when it costs nothing and does not have a detrimental affect.

Made some progress, photo of the first unit almost ready for paint, going to put it on one side for the bondo to fully cure before a last fill. Despite the claims it will shrink a small amount, as I found out refinishing several vehicles over my lifetime. Nothing spectacular planned for the paint job, just a latex paint the same colour as the walls in the home theater. Trying to make the units "disappear". I could use David Copperfirlds help in that. It's one thing to imagine an 18 cu ft box it's another to stare it in the "throat". Kudos to all of those on this site who hump speakers like this around for mobile DJ work. I'm already stocking up on beer for the buddies that have promised to help me load these into the room, they'll earn it.

[howiez]

It'll be one hell of a feat to hide 4 of those anywhere! Great looking build, keep up the good work. I can't wait to hear some impressions of the sound.

[Old Mike]

Been slower than I hoped. Life got in the way of the build. Started to get unit number 2 ready for closure and paint and as my wife was away for a week grand baby sitting I had to find a different way to heft these things around, the solution was already in my shop. The picture is unit 2 being lifted onto the operating table. This has made moving these beasts around much easier, not many speakers that need an engine lift!!!!!

[Tom Smit]

Haha! I think that is the first time that I have seen that on this forum.

[Bill Fitzmaurice]

It's quite similar to Nimrod's DR280 flying rig.

[Old Mike]

I'd promised to show some shots of the existing subs. Here's a photograph of my 3 mains, Linkwitz LX 521.4 units, before installation behind the screen and in the background is one of my Linkwitz Thor subs, there are now 8 of them in the room, two stacked in each corner. The room is nominally 24 feet by 23 feet, the screen is a 12 foot wide 2.38 ratio sound transparent unit.

The other photograph is out of focus, likely too many snifters of scotch whilst listening to some Eric Clapton. it shows the seating and the projector is just visible on the rear wall, no prizes for guessing which seat I use. The color rendering is not good, the carpet is very dark much darker than it appears, I frigged with it to get a better image of the speakers themselves and make the sub visible. The carpet was actually the darkest I could find that was not actually black.

Second unit close up today, tomorrow I should have it ready for paint and hopefully have unit number 3 closed up.

[88h88]