Closed back cabs for guitar

[bcmatt]

I've decided to build my own 2x12 closed back cab (I think). I am struggling over dimensions and whether I want porting or perhaps only a 3/4 back.

This is actually a result of wanting a 2x12 cab for my old 100W Traynor Amp Head after having spare wood and Vinyl from building an open back combo amp cab for a DC-30 Kit I finished.

My question is: "what I should do with the back?".
So, what I have understood so far is that closed back cabs project the sound outward more to the crowd, while open-back fills the stage space with sound better. I've also heard that the closed backs have a tighter bass response...but then I have also heard that this is not necessarily true for the frequencies that a guitar in regular tuning can produce, and only helpful for lower frequencies that I would not be using. I do want strong bass and mids, but, I don't expect to really do detuning.

Any thoughts? I would like to keep small and portable if it means I'm not sacrificing the sound I want.

[Darkbluemurder]

If you are not playing loud and do not have a super monitoring system for your gigs then open back would be good.

Why not build a convertible back? Then you have both options.

Another option could be a detuned cab. For a 2x12" this would be as large as a 4x12". It would have the four speaker holes but only two speakers installed. The sound would come out from the speakers directly and through the holes. The concept is similar to a ported cab but in contrast to that it is not tuned to a specific frequency (hence the name "detuned cab"). I read first about this concept from the London Power website (www.londonpower.com).

[bcmatt]

hmm, ya, I am thinking about doing some sort of convertible back to figure out what I want. I kind of wish that porting was easy enough to add afterwards, but it sounds like a real science, and not just about cutting a hole in the back of the cab. The detuned cab sounds like a neat idea, that I have been hearing lately from people. A 4x12 cab completely ruins my whole point of going small and lighter though.

[dartanion]

Go with a detuned cab. I've done them all and keep going back to the detuned config.

[CaseyJones]

My question is: "what I should do with the back?".
So, what I have understood so far is that closed back cabs project the sound outward more to the crowd, while open-back fills the stage space with sound better. I've also heard that the closed backs have a tighter bass response...but then I have also heard that this is not necessarily true for the frequencies that a guitar in regular tuning can produce, and only helpful for lower frequencies that I would not be using. I do want strong bass and mids, but, I don't expect to really do detuning.

Closed back changes the cab tone substantially compared to open back. Depending on cab volume the enclosed volume of air damps speaker movement resulting in, for lack of a better term, a "tighter" response. Think Marshall which is generally a closed back tone compared to Fender which is generally an open back tone although that's a gross oversimplification.

Any thoughts? I would like to keep small and portable if it means I'm not sacrificing the sound I want.

Then "de-tuned" cabs are out, in general they're larger than "normal" versions of the same cab.

For 2 x 12" 26" wide by 19" tall by around 10 1/2" deep are about as compact as you can go, any smaller and the speakers won't fit. You can cut the height down from 19" down to around 16" but the cabinet volume becomes sub-optimum.

You can keep your options open. Glue cleats around the perimeter of the rear of the cabinet so it can be fully sealed for a closed back, OR you can use Fender combo style top and bottom cleats for open back tone, OR you can cut a hole through your full coverage back panel for semi-closed.

Having options means that you can experiment and let your ears decide what's best, OR you can open or close the back of the cab to suit your mood or your needs, OR you can change the cab configuration to work best if you change speakers.

[bcmatt]

Thank you Casey,
I appreciate you sharing your expertise. I think you have a good grasp of what I am trying to do.
So, is there an optimum interior volume(space) that I should be sure to stay above?

Using 3/4" birch Ply wood, I am thinking of matching the length of my combo amp, which is about 28 1/2". I am thinking of a height of about 16" and perhaps a depth of around 12". I'm willing to play around with the height and depth some, but I want to make sure that I reach the optimal minimum volume within the Cabinet. Do you know how to find that out?

I'm also curious about what sort of cleats you are suggesting for the perimeter of the rear. Are you referring to an external sort of cleat that fastens to the outside rear of the cabinet? Or do you think that the sort of bracing I used on my combo amp: http://yeomansinstruments.blogspot.com/
around the perimeter of the inside of the back of the cabinet that the back piece/pieces can butt up against?
Thanks for your suggestions.

[skyboltone]

Using 3/4" birch Ply wood,

I'm gonna stick my neck way out here and say......if you want light weight, and good tone, why not do the regular thing and use pine? I "think" I can hear the difference between a pine cab and a ply cab. The pine sounds SO much better in a guitar cabinet. YMMV
Dan

[bcmatt]

Using 3/4" birch Ply wood,

I'm gonna stick my neck way out here and say......if you want light weight, and good tone, why not do the regular thing and use pine? I "think" I can hear the difference between a pine cab and a ply cab. The pine sounds SO much better in a guitar cabinet. YMMV
Dan

I've heard good things about pine too, and perhaps that may have been a good idea for my combo cab.
However, for closed back cabs, I seem to remember hearing that thickness and density are important for that low end tightness, and it may be better to go to ply-wood. But, I'm no expert; have you heard of anyone using pine fore closed back cabs?

[skyboltone]

I'm not absolutely sure but I would guess that all the old closed back Fender "Piggy Back" cabs were pine. It's a guess though. I've carried a few around and they don't heft like plywood.

Dan

[CaseyJones]

Thank you Casey,
I appreciate you sharing your expertise. I think you have a good grasp of what I am trying to do.
So, is there an optimum interior volume(space) that I should be sure to stay above?

There's an optimum interior volume... and I generally ignore it.

JBL, Altec and Electro-Voice all published cabinet dimensions and plans years ago to promote sales of their loudspeakers. The "optimum interior volume" is only relevent to sound reproduction in hi-fi or a sound reinforcement applications, in a musical instrument application we can use response spikes and sympathetic cab resonance to our advantage.

Using 3/4" birch Ply wood, I am thinking of matching the length of my combo amp, which is about 28 1/2". I am thinking of a height of about 16" and perhaps a depth of around 12". I'm willing to play around with the height and depth some,

Then that's your cab.

but I want to make sure that I reach the optimal minimum volume within the Cabinet. Do you know how to find that out?.

Well, yeah. I could look it up... or I could go with experience gained from building hundreds of guitar cabinets. No one has ever told me my 2 x 12" sucks so I guess they don't suck!

I'm also curious about what sort of cleats you are suggesting for the perimeter of the rear. Are you referring to an external sort of cleat that fastens to the outside rear of the cabinet? Or do you think that the sort of bracing I used on my combo amp: http://yeomansinstruments.blogspot.com/
around the perimeter of the inside of the back of the cabinet that the back piece/pieces can butt up against?
Thanks for your suggestions.

Yeah, you've got the right idea. Some sort of mechanical joint at your corners like a finger joint or at least a dado will make your cab much stronger but what you have is good. Do your cleats like the cleats on your combo and you'll be o.k..

I'm not absolutely sure but I would guess that all the old closed back Fender "Piggy Back" cabs were pine. It's a guess though. I've carried a few around and they don't heft like plywood.

They're pine. Loaded with JBL D120s or D130s they heft more like stone!

[bcmatt]

Awesome! Thanks!
Do you ever play around with the complexities of porting? Or do you think that I would be ok if I make 2 back panels for the cab; one being 1/4 of the back and the other being 3/4 or the back and choosing whether to use one or both based on how it sounds?

[CaseyJones]

Awesome! Thanks!
Do you ever play around with the complexities of porting?

Yes. But your life will be complicated enough without worrying about porting.

Or do you think that I would be ok if I make 2 back panels for the cab; one being 1/4 of the back and the other being 3/4 or the back and choosing whether to use one or both based on how it sounds?

On your back panels... My first two choices would be a solid back panel followed by two narrower panels for a semi-open back. The solid back panel would of course be a closed back and you want it to mount solidly to the cabinet, use lots of fasteners. For high gain tones I prefer a closed back cabinet.

I prefer two panels for the rear of an open back cabinet with an upper and a lower panel. If you leave a narrow gap between the panels say an inch or so it will start to restrict the airflow from the rear of the cabinet.

The main thing to remember if you want to try a port is that the port is to prevent phase cancellation at bass frequencies. The acoustic wave from the rear of the cone is exactly out of phase with that from the front of the cone, if they are allowed to combine then you LOSE bass response. The trick is to delay the wave and combine it more in phase with the sound from the front of the speaker. One easy way to accomplish that is to use a 3" cardboard tube and extend the tube from the front baffle to within around an inch of the back of the cabinet, changing the length of the tube and the depth of the cabinet changes the phase relationship of the two waves.

[bcmatt]

Alright.
I really appreciate this.
I will make a one-piece solid back, knowing that if some sort of porting is needed in the future, I can do the hole in the front Baffle with a tube going back (realizing it may be a lot of experimenting to find the right length to cut the tube).

The great thing about doing this at the same length as my open backed combo amp is that we do have a partial back piece already cut that we didn't use. I could use that with another for the option of open back. But for this cabinet, I've been primarily thinking closed.

Thanks for all your help Casey. Do you make amps/cabs as a living or a hobby?

[CaseyJones]

Thanks for all your help Casey. Do you make amps/cabs as a living or a hobby?

Long hours divided by short money equals what?

[novosibir]

Long hours divided by short money equals what?

Not payed mortgage and a hungry family

Larry