Sub woofers in 825/8 cabinets

[jnorv]

There was a thread here that proposed putting 18? drivers into the 825/828 cabinets. I just acquired a pair of 2245?s with the purpose of doing this. I have 828?s with the horn flares sealed off from the rest of the cabinets. I think the resulting volume is 10 cubic feet. I have 515B?s as base drivers now. Has anybody done this mod and, if so, can you offer any suggestions?

Thanks,

Jim Norvell
San Jose

[GM]

Yeah, I started it off when I mentioned installing a 411 in my old A7s to fill in the bottom end and Roland latched onto the idea like a bear trap. He used a JBL 2242 IIRC, but last I heard, he hadn't finished them due to a hip injury.

Speaking of which, I haven't seen him post on any of the forums for quite awhile, I hope he hasn't had any bad complications......

You'll need to seal off the 515 area, reducing the net Vb to ~6.5ft^3 after factoring in the driver/additional bracing. With a 2245, tuning it to ~27Hz with a 6" vent looks good. If it's too boomy, just make the vent longer.

If the 515 rolls off too much, reduce the rear volume with s***** of wood, and if it's peaky, create an aperiodic leak around the rear access cover as required. Bi-amp with plenty of power to prevent clipping/steep active XO, EQ to achieve flattest response in-room. If you have the space, I recommend using the original 'wings' to improve the match.

HTH,

GM

[jnorv]

Thanks. I will let you know how they turn out.

Jim N

[Roland N R]

Hello Jim and GM:

Passing by and noticed the discussion. I am still physically limited in activity but I will add some details.

Basically before converting the cab to built in subwoof you need to reinforce the cab. This is useful in all 825-828 cabs even w/o the subwoof mods. I will describe what I did although I am sure there are other, just as good, ways of doing this.

Bottom ...was reinforced with 2x6" as follows two pieces of 24" and two pieces of 20" were cut and 45 degree mitered glued and screwed with galvanized screws from the inside. Roller bearing wheels were bolted thru at the corners of this. This limits resonances keeps the bottom away from water and allows easier transport since the cab does get very heavy by the end of all the mods.

Rear...a 2x4" brace was added across the back at the point where the separation between the subwoof and 515 chamber will be made. The 2x4" was placed on edge the bottom edge at 17 1/4" measured from the top inside where the back cover fits. A couple of notches are made to allow it to be at the same level as the molding that holds the back cover in place. This not only strenghtens the rear cover but also allows a split back to be used if benefits of separate backs for the sub and and 515 chamber are desired.

To separate the 515 and subwoof space MDF 3/4"
8' by 28 1/2" is then screwed on to the 2x4" and the edge of the 3/4" plywood used to cover the horn flair. Additional 1x2" pieces and caulking ensure an airtight fit. Also foam strips are placed all around where the back cover sits and also in the new added mid brace.

Horn Flairs need to be *****d with sand, kitty litter or two part expanding foam your choice.

Front baffle used is MDF 3/4" thick 28 1/2"x 20". After removing the board used to limit the size of the front vent the front baffle was temporarily screwed on. Then the border of the old baffle which is essenctially a 2 to 4" border was marked on the new baffle from behind from inside the cab. The JBL 2245 was then placed in midline position (5" from the both sides as I remember) and 1/4" holes were drilled thru the bolt holes of the speaker thru to the front baffle. Also the position of the speaker was also marked on the front baffle from behind.

Once the baffle is so marked then it can be cut with a jig saw (router). But first you must reduce the size of the hole by going in 1 1/4" from the actual speaker size marking. Please also note that there will be a flat area in the cut at 12 o'clock and six o/clock where the old baffle passes thru it was my intention not to cut the old cabinet since it is not necessary.

An additional piece of wood the thickness of the
old front baffle needs to be placed to fill the opening of the old baffle. this can be attached to the new baffle and then the speaker opening cut using the already cut new baffle as the template.

Additional bracing was done from the inside of the sub compartment by making a 3/4" MDF carved out baffle. This made contact with all four walls of the cab... that is the bottom and two sides and the angled bottom of the horn. This measured 23" by the 28 1/2". The top was cut at 27 degrees so it would fit flush with the angle of the bottom of the horn. Most of the material inside of this baffle was jigged sawed out and then it was glued and nailed from the inside outward to again show no signs of mods. This bracket then tied all four walls together at the middle of the sub enclosure.

The back cover of the cab can also be reinforced with 2x4" on flat as desired.

The sub enclosure once stuffed fully with fiberglass gives an apparent volume between 8-9 cubic feet which is the JBL recommended cab size for the JBL 2245. Therefore their recommendations regarding vents etc can be followed. I decided to use a low freq tuning of 22 Hz as I remember. This was mainly to protect the driver from being overdriven below cab tuning. To tune it to that point required two 3" vents 3" long or four 1 1/2" vents 1 1/2" long. Both of these will fit around the speaker w/o having to cut or alter the original cab front baffle. A single six inch port although theoretically less noisy will not.

Final tuning will really depent on your particular driver's T/S parameters and room acoustics and I leave that up to you. A rane XO with time delay or DBX digital rackdriver will be needed to time delay the sub relative to the 515. Additional tuning suggestions as per GM may also be useful.

I plan to do a complete detailed report (with pictures) on construction and measurements hopefully including free field but unable to move this beast downstairs for now.

Enjoy.

Roland

[jnorv]

I had planned on just adding volume to the cabs with a bottom box. My real question is how do I treat the 515B. Do I port the back side or do I treat it as a fully horn loaded driver with the back enclosed?

Jim N

[Roland N R]

Hi Jim:

Greater cab volume for deep bass is always useful but the idea was to limit yourself to the actual 825-828 cab. JBL recomends an 8 cubic foot enclosure which with a little deep bass Eq with Q slope of two gives low bass into the 20's Hz.

The 515 back chamber will support bass output below actual horn bass output so can use the 515 as fully horn loaded.

[Roland N R]

Hello Igor:

All mod needed details are included here. Hope this helps

[djnagle]

Roland, This helps very much. Will my 416 respond to the sealed MR horn as the 515 do? P.S. My wife said that they just can't look that funky (altec gray)in our family room so I've talked to a good friend of mine and his is going to vaneer the cab in cheery with oak in the horn flair. Against all love of Altec Green, I am painting the 511s dark Candy Apple Red with chrome standoffs. Even though I can't afford the subs at this point in time I want to make all the mods before the vaneer goes on. P.P.S. I am DJN from the HESF.

[Roland N R]

Igor:

416s should do fine. Try using finished plywood
instead of veneer this would also help stiffen the cab. Available in 1/8" to 3/4" as oak etc finish with not much difference in price between thin vs thick...I go with 3/4" oak finished plywood. Take pictures and share with us.

Candy apple red on the horns!!! COOL!!

Good luck

Roland

[djnagle]

I am using the cheery on all surfaces except the horn flare sides and top/bottom. For the horn flares I will use the oak vaneer. I like your idea about using cheery finished plywood for stiffness. Thanks.