GM's MLTL with 416-8B

[westend9]

Thanks, GM, for iterating what must seem to you, like school yard stuff. I've never built an enclosure of this size so construction techniques need to be learned. Thank God, I have enough of my faculties still in place to absorb most of it. And, thankfully, there are still a few souls left on the Planet that are helpful.

I'm beginning to understand that anything that will not diminish the pipe action, WRT braces should be part of my focus. If I think of this as an aerodynamic exercise, removing as many surfaces that would cause an eddy or disturbance to the "airstream" would be good. Do I have a handle on this?

I do have adequate dust collection systems and cutting or machining lumber (and thereby creating sawdust) is also part of my everyday life. I tell customers, when creating sawdust, that it is not dirt and happens to be the main constituent of floor cleaning products. This is not always received as good information.

I will do some calculations WRT as-built volume with braces added. I did size the first panels with a doubled top and bottom panel in mind but could easily forgo the doubled bottom. Removing the inset bottom panel will add about a .25 ft.^3 to the volume so it does have merit in that regard.

Thanks, also, for sharing your experiences at the advent of TL speaker design, priceless information, IMO. I have a pair of single driver BLH's and the designer reached the conclusion that much of horn's reinforcement was due to TL action and not from the horn, itself.

FWIW, I could not have picked a better time to launch on this project since we are caught up in a late winter storm that is making travel a struggle. It has been raining and snowing since last night and the roads are miserable. Staying in the shop and keeping the wood stove fired is a good priority, today.

Picture of woofer cutout marked---44" center from bottom of baffle.

Well, a thick bead of PL400 is a pretty decent gusset all by its lonesome. I lay it in with a caulking squeegee..............

Correct, you want stiffeners running along the longest axis, same as a horn.

I use 1x3 boards on edge or similar size scrap plywood.

There's no real advantage to vertically staggering them and if you get them too close to one end you create a 'hot spot'.

Well, you're creating offset 'X' frame braces with the lumber , but if you prefer to make all those extra cutouts that's fine too. Me, I was always in a time crunch and never had any effective dust control system, so tried to minimize cutting as much as practical.

If used, minimizing how much they protrude in the airstream along the walls is to *maximize* pipe action.

Unless the bottom is raised up off the floor, doubling up the bottom is a waste of time/materials and even then, bracing is sufficient since all the pipe's pressure is concentrated at the closed end (top) and why the panel bracing is along its axis.

Except for the full length vertical brace to tie the top, bottom and all four sides together there's no need for any more tie bracing, i.e. the object is to not have any bracing near any corners as these areas are already self braced by having glued joints as well as unbraced areas too small to resonate with any consequence.

There is a mathematically 'perfect' position along a pipe's length to place the woofer for smoothest response, though while a good point, I disagree with MJK's axial length 'L' x 0.349; instead using a different formula that includes the pipe's cross sectional area (CSA), but never having compared the two, I don't know if the differences seen in sims using his software are audible except possibly with the most minimal of internal damping.

Regardless, since your HF horn is on top, unless you sit really far away, like > 25 ft, then placing the woofer right up at the top of the cab is best overall and it also maximizes pipe action, ergo max vent damping.

Yeah, believe it or not, I get excited every time someone builds cabs like I learned to do so long ago. By age 22 when I did my first pipe horn designs, I was already pretty jaded WRT to high performance/SQ sound, but these took them to another level of BLH-like without all the extra bulk/woodworking, yet with the smoothness of low Qt sealed with none of 'boom'/'flabbiness' of the BR cabs of the day.

Of more interest to me though was that such alignments caused much greater improvements with 'el cheapo' drivers and I'm glad that now with MJK's software, the various TLs and their mass loaded (vented) variants are the de facto alignments for so-called 'full-range' drivers and their woofer + super-tweeter variants.

For this we have RCA's Harry Olson's circa 1949 MLTL design for his LC-1 'full-range' driver to thank, though he just called it a reflex, and the college student working at Altec that mentioned it to me when I toured the plant that year.

GM

[Altec Best]

Cool! I sent you a PM the other day, but hadn't heard back. I still haven't installed the HF phragms yet.

Darn I didn't see it.I'm sorry GM, I had a look at it.You replied to an email I sent February 3, No wonder I had trouble remembering what it was about that was 7 weeks ago ! . Just Kidding you explained the last month were more health oriented. How are you feeling now ? I Hope Well ! It is always nice to see you on the boards.When I don't see you for a while I know your not feeling well.And that is Beat ! I'm glad to see up and around ! :lollipop:

I hope you get a chance to check those Frams out soon ! As I'm really curious as to how they sound.You don't hear much about those models too much.

[westend9]

Took some time, today, between errands and moving snow, to finish the cuts of all the panels except the interior braces. Looking at this stack of goods and imagining the final aspect of the boxes, if this speaker thing doesn't work out, I might drill some holes and ship myself South.

I will do some light trimming on the table saw and then proceed to vent and driver cutout. The original description of this MLTL design was to use a vent the same size as the original Model 19 and that would be positioned close to the bottom of the cab on the front baffle and centered, I would presume. I have the woofer cutout marked at 3" below the top edge. That will allow me to get a bolt into the frame of the woofer underneath the doubled top panels without too much trouble. I have established the diameter as 14 1/8" as per Altec's spec sheet.

One thing that is nagging at me is what I'm going to do for access to the interior and to provide for future vent modification. The latter isn't that critical as I am thinking of more than a few workarounds should that be necessary. I know GM advocated cutting the front baffle and that would certainly allow provision for access and vent mods but I am very reluctant to do that, mostly for cosmetic reasons. Would it be possible to use the bottom as an access port if I went to somewhat extraordinary measures to insure there is a tight seal at that point? I'm thinking of a neoprene or butyl high tech gasket material and through bolt the bottom panel into cleats or window pane of one the doubled bottom panels to accept the exterior bottom panel. Would I be better off, functionally, by using a purpose built access port in the back panel?

[GM]

The original description of this MLTL design was to use a vent the same size as the original Model 19 and that would be positioned close to the bottom of the cab on the front baffle and centered, I would presume.

One thing that is nagging at me is what I'm going to do for access to the interior and to provide for future vent modification.

Would it be possible to use the bottom as an access port if I went to somewhat extraordinary measures to insure there is a tight seal at that point?

I presume also since I don't remember which of the many m19 variations I've done is the one you're referring to.

Yeah, a removable bottom is probably the best alternate since the sheer weight of the cab bearing down on the gasketing ensures a great seal. FYI, my gasket of choice is either NEMA 5/12 (dust/rain tight) or 3R (rain proof) Neoprene since one can drill through it and will self seal around bolt/screw threads. Not particularly cheap, but I've rotated the 515's individual baffles many times over the decades and still no air leaks as best I can tell, and this is with just large wood screws that are obviously not as hard to remove/replace as they once were.

GM

[westend9]

Whoops, your design for this iteration is discussed here.
In post #13 the sizes of the cabinet are discussed and the simulated responses of the stock 19 and two sizes of variants are in post #15. I am building the MLTL 19.long, the 16 cu. ft. model (we should, at some point, maybe give this a name ).

I will follow your experienced advice on the gasket material, I have access to just about everything. There will be some minor engineering and machining on the seal area. I plan to break out the router and table to cut dados and ease edges and so forth so cutting a dado to accept a gasket will only mean a bit and fence change.

Right now, I am squaring up the panels on the table saw. I still have the interior panels to cut, including the vertical window pane brace. For the support for the woofer's magnet, I was planning to cut the brace to fit around the magnet, recessed from the magnet > 1/8" to accept a strip of felt and to allow for shimming. Is this the usual way? I had planned to make two more window paned vertical braces to tie to the side panels. I think I can get away with this if I substantially reduce their area, i.e. mostly cut out.

An alternative would be to add hardwood cross braces from the center brace to the side panels. These cross braces could tie into other vertical bracing on the side panels. The vertical braces on the side panels could be dado'd to accept these cross braces and that would make for a good, secure joint. It is also a lot of time to do it this way. Another thing I've seen with panel bracing, mostly in older designs, is to position the braces on an angle across the panels.

Back to the saw!

[GM]

OK found the sim and since no one has built one yet I've changed its name to 'Westend'.

Yes, though I use scrap Neoprene.

Yes triangulating panel areas really stiffen them up, just try to bend a triangle, but not a good plan for stiffening up TLs as we don't want the turbulence they would create.

GM

[westend9]

Lol, I like the name! I feel like I'm in the ranks of H. Ford, A. Bose or the much more revered T Crapper.

I am about to cut braces and two panels for the top and bottom. That will pretty well chew through the last of five sheets of plywood for those that are keeping count. At 65lbs/sheet, the weight of each cabinet, just the plywood, is 162.5 lbs.. I have an appliance dolly.

BTW, GM, no traingles will be forthcoming, I want this to be like the leading edge of the Darpa.

[westend9]

OK, we now have all of the panels ready for routing a dado to accept braces. Rather than chronicle in this Forum, the techniques used, I've posted a parallel thread at Audiokarma that shows some detailed processes of the cabinet build. I'm not going to leave the experienced Altec guys in this forum without build specifics but thought the Audiokarma crowd would benefit more from the tool usage and such.

Last night, after sweeping up, I decided to investigate further the Golden Ratio and how I could use it for the bracing. I actually went through the proof of the theorem to better understand how Phi is calculated. Since digesting the mathematics, I've come up with some numbers for brace lengths and placement.

My initial thoughts on bracing were to build "X'd" window pane braces, one to run vertically that would tie the front and back panel together and also support the driver motor. The other window pane brace would be halved to join the first brace and also run vertically, to join the side panels to the center brace. This would, in effect, tie all of the six sides of the cabinet together.

After reading GM's advice on keeping the disruption of sound waves to a minimum and the fact that I have burned through most of the BB plywood at hand, I've decided to keep the first driver supporting brace and to use a smaller vertical brace inset into each side panel. I will use three cross braces that will tie the vertical side panel braces into the central window pane brace from each side. This will also bring all six sides together and allow for the least turbulence inside the cabinet. All of the bracing will be inset into dados because it is easy to do and allows for the most effective joint.

The attached picture shows some detail of the dado and the insetting of a brace piece.

[Altec Best]

or the much more revered T Crapper.

My Favorite plumber ! And his most famous invention the "Crapper Valveless Waste Preventer"

[Vint_age]

OK, we now have all of the panels ready for routing a dado to accept braces.

If you cut a dado in the middle of the panel, you have just reduced the stiffness of that panel. A better way to attach the brace is to use pocket screws and glue. Also, much easier than cutting a dado.