Building a Speaker Cabinet

[zendragon63]

Hello all. I am wanting to build a custom combo amp cabinet for a Emenance 16 ohm Red Fang and need to get a couple of things worked out--to try to get the most mid to lower frequency response out of what looks like a great speaker. Having read the post by blandman74 and the comments by PRR's and others, I am thinking that, with a little help, I could pull this off. The spec on the speaker:

http://www.eminence-speaker.com/pdf/redfang-16.pdf

Resonance given is 89 Hz and the recommended enclosure volume is not specified and beyond that, I am pretty lost on the nuances of the other small parameters.

If I understand PRR correctly, the general rule for volume is (1.1 x speaker diameter)cubed; for a 12" driver, about 2300 cubic inches; non centered and uneven box dimensions.

Couple of questions. Do I need to increase the calculated volume to off-set any volume displaced by the speaker voice coil/magnet and the amp chassis? And then how much baffling to apply the open end (back side) of the cabinet.

Many thanks in advance for any insights. Regards

dennis

[jjasilli]

Not clear - are you building an open back, or closed back speaker cabinet?  (You say it's a combo, but it's possible to build a combo with aclosed back speaker section, and an open back amp section above that.  Traynor & others have done that). 

If open back, then the dimensions of the cab don't matter much.  If closed back, then Theile parameters come into play.  Internal volume counts, and so do the size of all 3 dimensions.  You can copy a good known cab; use a design program; or follow PRR's suggestions in past threads.

[zendragon63]

Thanks jjasilli. Sorry I wasn't as specific as I could have been. It is to be a open back combo cabinet and while I have read where the volume/dimensions of the box are not as critical as those of a sealed cab--within reason of course--I was thinking that there might be a few simple variables that I may or may not have overlooked to get a the best frequency performance in any given situation. For example, having different size back panel(s)/baffles on open back cabs seem to make a difference in some instances. I dunno--so much to try to know.

Yeah, I can copy an existing combo cab--and probably will--but still appreciate the input. Back to work...Regards

dennis

[tubesornothing]

I would love to supply a decent answer other than: copy something that works.  I have a bunch of scrap open back combo cabs sitting in the basement.  Some sound great, others sound like crap (with the same speaker and amp). I have given up trying to "design" my own - I now just copy from others what sounds good to me.

[jjasilli]

You have lot's of design leeway with an open back cab.  I design so it's easy to carry the cab on my narrow staircase -not too tall or wide, and fit in & out of my car - not too deep.  

[Shrapnel]

IIRC, most speaker building books (usually for hi-fi, and thus, Thiele and all the other formulas can come into play for all the various Q's and V's and F's... most we don't have on guitar speakers) you add about 10% to box volume for speaker allowances. Tuned port enclosures included... As pointed out too, if 3/4 open, 1/2 open, or any other degree of open backed cab is used, you can throw all that out the window and use the size and dimensions  that fit best for your build.

[zendragon63]

Maybe too much leeway. I have a Matchless Spitfire 2x10 that has great clean mid to bass response for the size of cab that it is. My general observations so far are that--in addition to a wide range of dimensions--the back 'baffle' surface area varies enough between combo manufactures so there is an area to experiment with on the couple of open backs that I already have built.

Hey, I needed to take a break from solder fumes anyway. Regards

dennis


  

[jjasilli]

Rear baffle  & tone experiences:  My Silvertone 1482 is in a stock combo cab; 12" speaker with a rear baffle.  The cab & rear baffle are all 1/2" thick masonite finished in the finest contact paper  ; exept I installed a 1/4" plywood front baffle board to replace the stock 2-piece!!! unjoined!!! masonite!!! front baffle board.  (The baffle board is so small, that the 1/4" seems to be stiff enough; and the cab is so shallow that speaker clearance at the rear of the cab is an issue, for a thicker front baffle board.)  The cab is so small overall that Thiele parameters are a joke here.   

Anyway:  the rear baffle is almost full, except it's cut short (stock) to allow access to the tubes.  I guess you could say it 's a full rear baffle with a wide rear port.  The amp sounds quite different with the rear baffle mounted, or with it off - I think it sounds good both ways.  Platefire is into these amps, and I think prefers his open-backed.  I keep mine closed-backed for variety, because it's the only closed-backed cab I have.

My two Traynor Bassmates are combo's each with a 15" speaker in an enclosed bottom section of the cab.  The enclosure is just large enough to house the speaker.  So, again, Thiele parameters are grossly violted by the manufacturer.  I agree with the general consensus that these amps sound much better with the rear panel off (for regular guitar).

[PRR]

> (1.1 x speaker diameter)cubed

For non-optimized closed boxes.

Although, an open box tends to have to be no smaller than a closed box, and generally bigger.

In raw theory, the main variable in an open baffle is the distance from front of cone to back of cone. When this is a half-wavelength, output cancels. So for best bass, make the baffle very-very-very big. All else is some compromise.

In practice, you need to "fold the edges" to reduce height and width, make it self-standing, and to hide/protect the magnet. Again there is no good simple theory, except that in practice for the -same- lowest frequency, an open bent-edge baffle tends to occupy a larger volume than a closed box. (The trade-off is that open-back doubles the midbass power efficiency into the room.)


> the back 'baffle' surface area varies...

Yes, and this gets into a grey-zone between open baffle and tuned port. AFAICT, there is little joy and less (simple) theory in this area. A large opening resonates up in the midbass and should cancel deep bass. A small opening (smaller than the cone area) can whiffle at high output. Put rails on the sides, cut a set of 8", 4", 2", 1" cross-boards, and try full-open, half-open, etc to closed. If you find that an 8, a 2, and a 1 give you "the sound", cut an 11" board for final use.

[zendragon63]

OK, I had to do a little research on the baffle and the "folded edges". Thanks PRR.

I took the bass-deficit home built combo cab and rebuilt the baffle; went with a 1/2" board, tee nuts for the speaker, screwed and glued the baffle mounts solidly to the sides of the cab mading it essentially air tight when I tightened up the baffle to the front end of the cab and--voilà--I now have the bass reinforcement that I should have had. Apparently mediocre building techniques deliver mediocre results. 

Not a big back panel either as it got boomy. Thanks to all. Regards

dennis