OT: 12" or a 15" for a 6732 cubic inch bass reflex ?

I found a pair of 6732 cubic inch bass reflex boxes at the side of the road (in the garbage) the other day. They don't have any brand name on them but they have 1267A with 95DZ43 under that first number on the back panel. They are factory made. The grill cloths (brown/yellow grill cloth) were torn open and the speakers were removed.

They are made of 3/4 in plywood with a 1/16 inch vaneer on the outside with bass reflex baffle holes below the speaker holes. They have 12" baffle adapters in 15" precut holes. Above the main speaker holes they also have a rectangular hole covered with wood inside the baffle. One of them was sanded down a little and one is unsanded. Looks like the previous owner was planning to fix them up but didn't have time or whatever.

Nothing much wrong with the boxes except some small scraps and scratches, nice old style boxes, with legs, made of real wood and plywood. I'm planning to put some drivers into these boxes, according to the interior cubic inches, what size speaker would sound the best in these 6732 cubic inch bass reflex boxes? A 12" or a 15"?

6732 cu in or 3.9 cu ft.

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Originally Posted by VolvoHeretic

6732 cu in or 3.9 cu ft.

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that's what i came up with. but that doesn't sound right.
how about some measurements?

What type of system are you trying to build? Do you want to use these cabinets for bass only, and what frequency range are you trying to cover? Got a picture you can post?

Need a little more information to go on.

Every speaker is different and require different sized cabinets. You're working backwards trying to shoe horn a new speaker into an existing box.

If you get WinISD Pro, for each driver under consideration you can play with different box sizes to see what can be made to work. There's usually some latitude and tradeoff between box size, port tuning, rolloff etc.

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Originally Posted by Phil-G

that's what i came up with. but that doesn't sound right.
how about some measurements?

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Here are basic dimensions of the outside Attachment 2085.

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Originally Posted by miker1

What type of system are you trying to build? Do you want to use these cabinets for bass only, and what frequency range are you trying to cover? Got a picture you can post?

Need a little more information to go on.

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Mainly for bass, highs would be separate enclosures and bi-amped.
Attachment 2086Attachment 2087Attachment 2088Attachment 2089Attachment 2090

Oh, those are pretty nice looking cabinets!

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Originally Posted by johnobub

They are made of 3/4 in plywood with a 1/16 inch vaneer on the outside with bass reflex baffle holes below the speaker holes.

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OK, what is the exact area of the vent?

Net Vb wise, it's between a 612 and 614, so there's some 12" and 15" Altec options, though the vent may need re-tuning.

GM

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Originally Posted by GM

OK, what is the exact area of the vent?

Net Vb wise, it's between a 612 and 614, so there's some 12" and 15" Altec options, though the vent may need re-tuning.

GM

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The port measures 13 1/4" X 2 3/4, not sure what the radius is at the two outside ends of the port. Attachment 2091

Looks like a perfect half circle in the picture, so ~34.815"^2 = ~60 hz tuning [Fb], so a little bit higher once a driver is installed and maybe a little bracing. Pretty high, so will need to block some of it off to get it down to around the 612's, 614's ~42 Hz.

What's the size of the HF horn cutout?

GM

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Originally Posted by GM

Looks like a perfect half circle in the picture, so ~34.815"^2 = ~60 hz tuning [Fb], so a little bit higher once a driver is installed and maybe a little bracing. Pretty high, so will need to block some of it off to get it down to around the 612's, 614's ~42 Hz.

What's the size of the HF horn cutout?

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The size of the HF horn cutout is 8" X 2 1/2" Attachment 2093

Only takes it down into the 50s, but adding a 2.75" matching duct behind it and blocking off the existing vent flush or from the front will work.

All that dealt with, I've been assuming an Altec 414, 416 or similar driver, so....... what's it going to mate to as it could render all this moot?

GM

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Originally Posted by GM

Only takes it down into the 50s, but adding a 2.75" matching duct behind it and blocking off the existing vent flush or from the front will work.

All that dealt with, I've been assuming an Altec 414, 416 or similar driver, so....... what's it going to mate to as it could render all this moot?

GM

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Well, I have two Altec 416 8A's that I could use for now. What do you mean by "adding a 2.75" matching duct behind it" ? Do you mean adding a duct on the back panel?

Right, it already has a ducted port 0.75" long and a 3.5" long duct will tune the cab to around 42 Hz once the driver's, etc. volume displacement is accounted for, so a 2.75" long extension required and make sure there's no air leak.

GM

I'm not sure what you mean by,

"it already has a ducted port 0.75" long and a 3.5" long duct will tune the cab to around 42 Hz once the driver's, etc. volume displacement is accounted for, so a 2.75" long extension required and make sure there's no air leak."

could you show this change on a quick diagram? Thanks!

Sorry, not set up to do drawings. All you're adding is a 2.5" x 8" i.d. rectangular tube made out of scrap 1/2 - 3/4" thick wood to extend the baffle cutout into a 3.5" long vent.

GM

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Originally Posted by johnobub

I'm not sure what you mean by,

"it already has a ducted port 0.75" long and a 3.5" long duct will tune the cab to around 42 Hz once the driver's, etc. volume displacement is accounted for, so a 2.75" long extension required and make sure there's no air leak."

could you show this change on a quick diagram? Thanks!

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He's working with your existing port. He's saying to make the existing port 3.5" deep....... it's already .75" deep, the thickness of the plywood. If you add another 2.75 it will make it 3.5" deep/long and the box will be tuned to around 42hz. All this depends on how much volume the driver takes up, and I suppose the extra material needed to extend the port. You could add it onto the front of the panel instead of the back of the panel, I suppose.

I wonder if the front baffle is removable? If so you could change the port without having to work with the existing one. The shape of the existing port is kind cool/unique, Though.

Actually, I'm working with the blocked off tweeter horn cutout rather than dink around with the oversize vent with its half round ends, which of course will need to be sealed off. FWIW, I knocked off a 1/2 ft^3 in my calculations to allow for the driver, etc.. Anyway, what's a +/- ~1 Hz tuning variance among friends? ;)

GM

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Originally Posted by GM

Actually, I'm working with the blocked off tweeter horn cutout rather than dink around with the oversize vent with its half round ends, which of course will need to be sealed off. FWIW, I knocked off a 1/2 ft^3 in my calculations to allow for the driver, etc.. Anyway, what's a +/- ~1 Hz tuning variance among friends? ;)

GM

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Oh, I see.

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Originally Posted by GM

Anyway, what's a +/- ~1 Hz tuning variance among friends? ;)

GM

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Right on Brother ! :biggrin2:

Let me see, so, in knocking off 1/2 ft^3 for the driver, duct, and bracing the inside area becomes 3.3 ft^3.

A tuning frequency of 42hz is the same as a 612 or 614 but for deeper bass response a lower tuning frequency should be built into the duct. A 42 hz tuning frequency would produce bass response similar to what a 612 or 614 would but it maybe possible to produce a deeper bass response by increasing the port length longer than 5 1/2 inches. Tuning the boxes to 39hz would produce deeper bass but not as loud as at 42hz. Tuning to 39hz would require a port length of 7 inches.

The tuning frequency would be 47.5hz using a 3 1/2" duct. For a 42hz tuning frequency
a duct 2 1/2 inchs high and 8 inches wide with a 5 1/2" vent length should be made.

Port Length Calculator

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Originally Posted by johnobub

Let me see, so, in knocking off 1/2 ft^3 for the driver, duct, and bracing the inside area becomes 3.3 ft^3.

A tuning frequency of 42hz is the same as a 612 or 614 but for deeper bass response a lower tuning frequency should be built into the duct. A 42 hz tuning frequency would produce bass response similar to what a 612 or 614 would but it maybe possible to produce a deeper bass response by increasing the port length longer than 5 1/2 inches. Tuning the boxes to 39hz would produce deeper bass but not as loud as at 42hz. Tuning to 39hz would require a port length of 7 inches.

The tuning frequency would be 47.5hz using a 3 1/2" duct. For a 42hz tuning frequency
a duct 2 1/2 inchs high and 8 inches wide with a 5 1/2" vent length should be made.

Port Length Calculator

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can I get any feedback?

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Originally Posted by johnobub

Let me see, so, in knocking off 1/2 ft^3 for the driver, duct, and bracing the inside area becomes 3.3 ft^3.

A tuning frequency of 42hz is the same as a 612 or 614 but for deeper bass response a lower tuning frequency should be built into the duct. A 42 hz tuning frequency would produce bass response similar to what a 612 or 614 would but it maybe possible to produce a deeper bass response by increasing the port length longer than 5 1/2 inches. Tuning the boxes to 39hz would produce deeper bass but not as loud as at 42hz. Tuning to 39hz would require a port length of 7 inches.

The tuning frequency would be 47.5hz using a 3 1/2" duct. For a 42hz tuning frequency
a duct 2 1/2 inchs high and 8 inches wide with a 5 1/2" vent length should be made.

Port Length Calculator

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Correct..By lengthening the vent/port tube you lower the tuning.Or raise it by shortening vent/port tube.

Thanks, and I'll be updating my progress on the cabinets, they are an amazing find. I was just thinking to make some new boxes and now I have these to use for experimenting. What an amazing day it was and I'm looking forward to the final results of my garbage find. Anyway I believe I will build some A9's in the long run after I get up some extra cash.

Hmm, closer to 3.396 ft^3 based on your i.d. dims minus a 1/2 ft^3.........

Most vent calculators assume a perfect Helmholtz resonator, so will tune a cab lower then predicted, which may, may not be acceptable. A more real world calculation for an un-damped cab puts the 3.5" long duct at ~44.5 Hz and the fact that there's some cab damping with a big pad of it right next to the vent will lower it some more, hence my more or less [~] 42 Hz somewhat educated guess. At 5.5", ~37 Hz.

Regardless, human hearing isn't very acute down this low plus the room's modes will dominate, so in room, most folks can't tell the difference between this small fractional [~0.18] octave spread tunings even when the vents are driven to audible distortion. Actually, you may find that moving this damping pad to the bottom of the cab will sound subjectively better even though it will raise tuning a little.

Bottom line, nobody's figured out how to shrink the LF BW, so if you want deep bass at high SPL [< this cab's ~60 Hz], think big cabs tuned at least at/below the lowest frequency you want to reproduce and/or be in a smaller, better sealed room to increase 'cabin gain' and why some vehicles can do true infrasonic bass at high SPL with a small sealed cab loaded with a high power handling driver.

GM