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Cab design help

The boa

The boa

Nov 12, 2016
200
256
Maryland
#1
ok so I have 2 celestion bl12 200x speakers laying around, and I have decided to build a cabinet for them. I am trying to figure out the right size to build it. I have been doing online calculators and reading over websites all day and can't seem to find a simple walk through on how to size an enclosure for a particular speaker set.

Any one out there feel like giving me a hand figuring out the volume I need the box to be? Building it will be no problem as I am a carpenter by trade.
 
#2
Cab design is a juggling of tradeoffs; there is no single "right size" (and, if we're talking about ported boxes, no single "right tuning frequency") - it depends on the specifics. That's one of the reasons why there is no simple how-to walk-through. What are your goals, where can you compromise if needed, and what are your constraints?

Assuming your goals include "decent low-B and decent fartout resistance, with the box no bigger than it needs to be", you might try 3.5 cubic feet internal volume, tuned to about 50 Hz by two 4" diameter ports, each 4" long. This internal volume takes into account the space occupied by the woofers and ports; bracing usually doesn't make enough difference to worry about. This box size and port tuning isn't any sort of definitive answer - the result is just one set of points within a multi-dimensional continuum.

If possible, I suggest aligning the woofers vertically, and locating one of the ports below the lower woofer's magnet, and the other port above the upper woofer's magnet. This will give you a bit of "chimney effect" cooling, which can help minimize thermal compression (which can happen near the end of a loud set as the magnet heats up and temporarily loses strength).
 
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#3
Well, now that I think about it, my goal is to make a great sounding cabinet and fully utilize these two drivers, and to have fun doing it. I don't play any lower than drop-D, so low b is of no concern. I also am not concerned about size or weight.

I have been looking at offset and side by side boxes. Maybe an offset 2x12 with the ports in the upper and lower opposite corners. That would always have the ports one above and one below the magnets. Maybe 28" tall, 18" wide, 12" deep, that's 3.5 cubic feet, and would have attractive proportions.
 
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#4
Thanks for the additional information!

I couldn't help but notice that you chose internal dimensions that are very close to a golden ratio. Did you do that just off the cuff?

Okay, given your lowest tuning (drop-D) and size constraints (none), I'd suggest 4.5 cubic feet internal volume. For the ports, assuming your cabinet walls are 3/4" thick, just cut two 4" diameter holes. That will tune the cab to about 52 Hz, and your low end should be essentially dead flat (my modeling program says +.3 dB) at the first overtone of drop-D (73 Hz).

One advantage of going with a golden ratio for the internal dimensions is, you can get away with less internal damping material to tame internal reflections in the midrange region. Too much damping material can suck the life out of the sound. I like to use as little as I can get away with, and that involves trial and error. So stick with your golden ratio approach... I was going to mention the golden ratio in my earlier post, but decided that was getting too geeky, and then you went there on your own. Nice job!
 
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#5
Actually I picked those dimensions by what looked good scaled down. They do say the golden ratio is the root of natural beauty.

Thanks a ton for the info. Looks like I should just scale up the dimensions to get to 4.5cu'. That would keep me very close to the golden ratio, which apparently makes sonic beauty also.

Maybe 30x20x13.
 
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#7
For the most part 12" speakers are usually looking for 2 cubic feet per driver for decent ripple response. some dont do much more lowend depending on the driver design below 1.5 or 1.8 so making the box bigger just chews up power handling. other drivers are looking for a bigger box so the ripple response isnt to boomy and 2 cubic to 2.3 would be more benificial and losing 10 to 30 watts power handling is no big deal at all.

Good place to start to see what a driver is looking for is to model it in a closed box and see what it wants at a ideal .7 ripple response. if that size is to ridiculous large. then you see if you get something decent at .8 ripple response. after you find the ideal volume for good ripple response then you simply port it as close as you can to 40hz if the fs of the speaker lets you do that. some 12" speakers have high fs around 48 to 52 hz so yes you would want to tune a little on the high side around 50hz. some 12" speakers have very decent Fs thats low around 39 to 44hz so you would defintaly tune low to 40hz.

in a vented enclousure the cone movement is very limited at Fb or the tune frequency of the cabinet. that is the point where the speaker is no longer accurately producing notes. cone movement is limited at Fb and its when the speaker has little movement and the port takes over. which you will see in the cone movement screen. its all poof of port. even though the spl chart shows great lowend performance with a vented cabinet. a static graph isnt telling you that there is poor transient response around Fb of a vented/reflex enclosure. all people see is lowend response on the spl chart and think its amazing in reality its all poofy of porty. so basically you wanna tune as low as you can go and try to get as much accurate cone movement as possible to reproduce the fundamental of whatever signal your designing the system for. in this case electric bass with a fundamental around 40hz for standard tuning.

Anyways the celestion likes a bigger box and gets a .7 ripple response at exactly 2 cubic feet. which is perfect likewise for a reflex/vented application. likewise it also has a decent fs at 44hz. so it would be a good candidate to just tune at 40hz. high tuning the 220x at 50hz only saves about 15 watts of power handling and the lower tuning would be much more benificial to produce a accurate reflex system.

since your using a 2x12 pair obviously your looking at 4 cubic feet for the pair. and since the volume of the ports, volume of the speaker and volume of the bracing will take up space. you would want a total of about 4.5 cubic feet and the volume acquired by the other elements would give you the ideal 4 cubic feet at 40hz.

likewise if using a standard rack or not a 19.5 rack with half inch of case on either side will be about 20.5 wide so a 21" width is a good place to start. and yes golden ratio works out rather well so the ratio to that 21" for your height with a average of 14 to 16 inches of depth will probably put you pretty smack daddy to 4.5. remember depending upon your grill of choice your baffle setback will be 1 inch or 1.5inch for the grill so. you need to subtract the baffle setback and of course the wood depth for your actual volume. likewise deeper cabinets will help cabinet Q and make a more stable stack able form factor
 
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#8
Wow, thanks for the help guys. Cabinet design is definitely something I want to learn more about.

Now when you guys are talking about the ports, are you referring to just a hole or a sleeved hole?

Also what is some good beginner reading material to get an understanding for what all the different values and parameters for speakers represent?

I'm hoping to start building this cabinet this week. It's probably wise to build a prototype out of cheap MDF to test first so that's what I'll do.
 
#9
The boa said:
Now when you guys are talking about the ports, are you referring to just a hole or a sleeved hole?
Click to expand...

In my first post above, I was talking about a plastic tube with a 4 inch internal diameter, and a length of 4 inches. In my second post, I was talking about a 4-inch diameter hole cut in 3/4" thick plywood. A "port" (or "vent") can also be a long or short rectangular or square or triangular slot, among other things.

The boa said:
Also what is some good beginner reading material to get an understanding for what all the different values and parameters for speakers represent?
Click to expand...

I recommend Vance Dickason's "The Loudspeaker Design Cookbook" as a text that is both accessible and thorough, and a box modeling program so that you can trial-and-error designs without having to actually build them. Also take a look at Francis Deck's (TalkBass member "fdeck") page on bass cab design: Invalid Link Removed
 

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