# Cabinet bracing



## mhammer (Nov 30, 2007)

I made myself a simple 1 x 12" cabinet over the weekend. The baffle itself is 1/2" plywood, secured along two edges, rather than all 4, and the rest of the cab is solid pine. I'll call it "semi-closed" because there is an opening of about 5" in the back. The speaker is an old Jensen C12R, with a production date somewhere in the mid-to-late 60's. I was powering it with my Champion 110 last night, and found that, while the tone and efficiency was nice, and fuller than the stock 10" in the amp, the bass transients were a little annoying. They stuck out and made me want to insert a compressor or limiter. Switching to the amp's dirty channel tamed the bass somewhat, because the clipping diodes prevented big bass transients from occurring. But I don't see playing dirty all the time as any sort of workable solution.

So I was wondering whether bracing inside cabs helps to limit bass excursions without eliminating the bandwidth. Or maybe when the cab is light, the answer is a thicker, less compliant baffle, secured all around.

Alternatively, does a brace that connects baffle and rear panel help to reduce unwanted resonances? I'm curious about what exists "out there", either in terms of strategies use in commercial products, or simply ideas that people have tinkered with.


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## jbealsmusic (Feb 12, 2014)

Speaker excursion is the distance the speaker can move away from its resting position. You can't (or shouldn't) try manipulate that.

In your case, because of the thin baffle and comparatively loose attachment, the vibrations created by the speaker's movement may be causing the baffle to resonate enough that it is louder than the speaker itself. The purpose of mounting a speaker on a baffle is to absorb those vibrations. A thicker baffle that is attached on all 4 sides might be enough of a solution that you wouldn't need to brace it. Adding bracing would also help. The other potential problem is standing waves, which can be reduced by adding angled pieces in corners, adding internal baffles to break up the waves, and/or opening the rear baffle some more to allow more waves to escape out the back.

Something else to consider is that if your C12R is anything like the modern ones, it is a VERY bass heavy speaker. It would be tough to design a box that would tame that low end response. Short of putting an parametric EQ somewhere in your signal chain to dial out the most resonant frequency, if the above suggestions don't work I'm not sure it can be tamed.

Does anyone know or have access to the T/S parameters of a vintage C12R? That would certainly help.


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## Mooh (Mar 7, 2007)

You'll know better than me, but my ears tend to prefer either a fully open back or a front vented cab.


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## mhammer (Nov 30, 2007)

All good suggestions. Since it is the simplest of all, I think I'll start by adding bracing strips across the top and bottom front edges and secure the baffle on all four surfaces, instead of just 2. We'll see how that goes. I can also remove the back panel and see what difference that makes.

I have an Eminence Legend 12" (can't tell which model) that I might also try out in the same cab, to see if it's the cab design or the speaker.

Alternatively, after playing almost exclusively through 6" and 8" speakers the last half dozen years or more, it may be my sonic expectations and unfamiliarity that may need tuning, rather than the speaker. Eight-inchers generally deliver no thud when you hit the low E.


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## Granny Gremlin (Jun 3, 2016)

Excursion is not a bad thing. It is how far the cone/voice coil moves in and out (relative to resting position). It has to move that far if it's getting that much power at that frequency (whatever those are in your case). Do not try to tamper with that - it needs to do what it is doing.... the only way to effect that at all (safely) is to use a well tuned sealed (aka Acoustic Suspension) cabinet which will reign it in a bit (and increase power handling of the speaker a few percentage points above the manufacturer's published 'free air' spec).

Secondly, with open back cabs bracing and resonances are moot (aside from general build quality issues). But yours isn't exactly open back. The easiest thing to do is make it a proper open back - you may have accidentally made a poor bass reflex cab (which could explain the woofiness). You could convert to sealed but that might not work ideally, but if you stuff it it should be fine (a poorly tuned sealed will just not have as much bass response as it could have and won;t be as responsive to playing dynamics). 1/2" wood all around is fine without bracing (but stuffed) for 30 watts or so and down. If it's taking more power than that or you play a 7 string or a drop tuning (reg 6 string tuning does not go very low and bass resonaces are hardly problematic unless you made the dimensions perfect for a 80 to 100 -Hz standing wave, 82ish Hz being low E - that would be some amazingly rotten luck). 

If low E is too big and rude, try using the amp's tone controls or switching to not-neck pickup (it is to be expected that you would need to change your EQ settings when changing the speaker you are using - especially when changing size and not just model). Yes, using such small speakers may have skewed your expectations and palate somewhat. ... have you had the bass knob cranked (to some degree) because it was such a small speaker? Compression is not the answer, check your EQ and maybe even your playing style (you may have been digging in too hard all these years due to the smaller less bassy speakers you have been using).

Putting the Eminence into the same cab tells you little other than which speaker sounds better to you in that cab. The cab (if sealed or ported, even if by accident) must be tuned to the speaker (the Jensen and Eminence will be different); only with open backs does it not really matter.


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## mhammer (Nov 30, 2007)

"Excursion" may have been the wrong term, though my sense is that the cab itself seems to give less pushback at those lowest frequencies, such that the speaker responds more, um, "enthusiastically"; even when the amp's bass control is turned way down.

I'll try securing the entire perimeter of the baffle first, and see what that does, with and without the rear panel. Cutting another back panel that encloses less is no big deal.


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## greco (Jul 15, 2007)

This old thread might interest you....

about "high" end closed back speaker cabinets..

I still find my Garnet cab (which I think was originally designed as a bass cab) *with the port AND the batting* to be very bass pronounced/"woofy" when it is closed back. When I open the back totally, I like the sound but the bass seem very "loose" (logically). The speaker is an Eminence C-Rex (hemp cone) and it might be a bit bass heavy for starters.

I'll be following this thread with interest.


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## cboutilier (Jan 12, 2016)

My 1x15 closed back cab with the Jensen C15n is rediculously bassy. If I'm turned up loud, I have to keep my bass control at 3 or 4 max. And that's with my Tele and the tone knob rolled down nearly halfway.


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## mhammer (Nov 30, 2007)

I was using my Turser Tele. Even with the bridge pickup, it was too bass-heavy for my tastes too.


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## Granny Gremlin (Jun 3, 2016)

mhammer said:


> "Excursion" may have been the wrong term, though my sense is that the cab itself seems to give less pushback at those lowest frequencies, such that the speaker responds more, um, "enthusiastically"; even when the amp's bass control is turned way down.
> 
> I'll try securing the entire perimeter of the baffle first, and see what that does, with and without the rear panel. Cutting another back panel that encloses less is no big deal.


I have no idea what you mean by "pushback" - kinda sounds like you are describing the acoustic suspension effect but there is no such thing in an open back cab (they do nothing except delay the front and back waves hitting each other in order to get some bass response that would otherwise be cancelled out). ...though there would be a bit of that if your semi-open cab is behaving like a mistuned bass reflex (ported) as I suggested earlier - in that case what it sounds like you're saying makes total sense. The opening in the back is an accidental port tuned so low that it's a woof monster. Open up the back completely and see what happens.

Could also be that the speaker has a particularly soft suspension and just moves a lot - that's not what I recall Jensens being like though. It will certainly appear to move more than a 6 or 8 inch due to the size difference. ... wait [google, google] those Champions are 25 watts. That's around the power handling limit for a C12R (personally, with the size of that magnet, I'd say that's a bit optimistic). You could just be running the speaker at or close to it's limits. Converting to a proper sealed cab would take the edge off that (if that's the case - easier to try full open back first to see if the problem is accidental bass reflex).



cboutilier said:


> My 1x15 closed back cab with the Jensen C15n is rediculously bassy. If I'm turned up loud, I have to keep my bass control at 3 or 4 max. And that's with my Tele and the tone knob rolled down nearly halfway.


Tone knobs roll off treble - try opening that up and see what happens; might balance better. A closed back 15 will be bassy; that's just what they do. Jensen 12s are not that bassy in an open back or even sealed cabs.


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## epis (Feb 25, 2012)

Hi Mark, what are your cab dimensions ? How deep it is ?


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## mhammer (Nov 30, 2007)

epis said:


> Hi Mark, what are your cab dimensions ? How deep it is ?


Baffle is 20 x 16.5". Outside cab dimensions are 21.5W x 19H x 12D.


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## epis (Feb 25, 2012)

Your cab is too deep, that "amplifies" low end, you shouldn't go more than 8" for your sound preferences. Cheers, Damir


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## epis (Feb 25, 2012)

Width and height of your cab are good for oversized 1x12 cab, I like the sound of them, full and rich with big bottom, but never go more then 9 1/2 deep (standard 10" pine from Home depot). Even greenback in cab like this would sound big and rich.


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## greco (Jul 15, 2007)

epis said:


> Width and height of your cab are good for oversized 1x12 cab, I like the sound of them, full and rich with big bottom, but never go more then 9 1/2 deep (standard 10" pine from Home depot). Even greenback in cab like this would sound big and rich.


This maximum depth of 9 1/2 inches only applies to closed back cabs...Correct?

How do you determine the "best" depth for a cab with a back that is 1/4 to 1/3 open?

These concepts interest me very much.

Cheers

Dave


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## epis (Feb 25, 2012)

Well Greco, this is my own practical experience, same for open or closed cabs, usually I make 1/3 open cabs.
Theory isn't my strong side


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## greco (Jul 15, 2007)

epis said:


> Well Greco, this is my own practical experience, same for open or closed cabs, usually I make 1/3 open cabs.
> Theory isn't my strong side


I'm just curious as to how much a "reasonable" depth (i.e., 8 inches to 14 inches for example) of a cab would influence the sound when the back is TOTALLY open.


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## epis (Feb 25, 2012)

I don't know, let's wait for Mark to open cab fully and tell us his experience. I would say even more than 10" deep cab is unpractical.


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## JeremyP (Jan 10, 2012)

I ran into pretty much the same thing last year with a homemade cab I got from a friend. To me all the bass sounds like faint fart noises in the background of my amp , it drove me nuts. I finally just set it aside but I am super curious to hear how you make out with the changes and extra bracing. My baffle is attached on all sides just a piece of plywood. But mine is also deep with a small gap at top of back..


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## greco (Jul 15, 2007)

The plots thicken....

Sorry for the derail Mark, but I hope some of it is applicable.


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## mhammer (Nov 30, 2007)

Thanks for the recommendations, Damir I hope these will help. I'll have to think a bit about whether I can put the existing cab "on a diet", or simply have to cut more wood for another home for the baffle. I did play it last night, and even with everything the amp EQ could do, and using a SC bridge pickup, the low-end thump was more than I cared for.


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## Granny Gremlin (Jun 3, 2016)

Sorry, but I have to point out that you guys are thinking about this all wrong: Depth, in and of itself, has no influence on ANY speaker cab's bass response.

With an open back, depth is an extension of the baffle; a cab with a 2x2 foot baffle and 1 foot depth (sitting on the floor vs hung) is roughly equivalent to that same speaker mounted in a 3x3 flat pc of wood, i.e. a baffle only with no sides (the main difference in terms of bass response comes from directionality - I talk about this below and it applies to all cab types), but the hard cuttof point for bass response (where the front wave cancels out with the back wave) would remain the same. In a sealed cab (as well as ported), the operative factor is internal volume (of which depth is only a factor - L x W x D - hardware, so the volume of the speaker frame at the very least; bracing and recessed handles etc if you have them). This is similar to acoustic guitars, where the internal volume determines bass response. This is the reason that the dreadnought body type was invented - larger volume for more bass.

For any cab type, baffle size has a limited but important effect on bass response (google baffle step effect) in that the minimum dimension determines the point at which a low frequency wave can wrap around the cab and radiate backwards vs reflecting forward only - a transition from 2 pi hemispherical radiation to 4 pi spherical radiation. This creates a -6db shelf that bottoms out at the point where the smallest baffle dimension = 1/8 the wavelength; above that the response rises 6 db over an octave or two to the 'normal' response of that speaker (e.g. it's efficiency spec). The midpoint of this rise (3 of 6 db) is determined by the formula: F3 = speed of sound / baffle width (or height; whichever is smaller). You know how Hifi guys tell you about bass being nondirectional so you only need 1 sub and it can be nearly anywhere in the room? That point at which sound becomes non-directional is not universally fixed but determined by what I am talking about right here. This is why you get an increase in bass response when you put your cab directly up against a wall (or very close to it) - you are effectively making the baffle MUCH bigger. This is also the reason you have room mode switches on studio monitors.


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## mhammer (Nov 30, 2007)

That's intriguing stuff. The point about depth being largely irrelevant with an open-back design makes sense...up to a point. Clearly, if I had two cabs with the same "frontage" but one was 8" deep and the other was 24" deep, the semi-cavity or whatever one wishes to call it, would shape the frequency content, and provide certain "fuzzy resonances". That is not at all to contest your point, and the relevance of distance from the wall or other rear surface; just to underline that not all open-back scenarios are equivalent.

In my own case, the back is _mostly_ closed. Not really by design, but simply because of the width of the wood I had on hand. The opening available is of the dimensions one might expect if it was a combo amp and you wanted space for a chassis, or at least to access the controls.


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## Granny Gremlin (Jun 3, 2016)

Yes, it would allow the backwave to bounce around inside a bit more, but in a full open back they still bounce out vs staying in the cab (unless there's at least a partial back, as in your case or most combos as you say); there are no resonances to speak of (aside from the cab vibrating due to direct transfer from the vibration of the speaker frame mounted to it - gasketting can reduce/eliminate this). Now in the case of semi-closed back like yours (and admittedly most other 'open' guitar cabs), it depends on the details (as regards how much back there is, and where - is the opening in the middle or the bottom for example), but generally still no significant resonances due to cab dimensions and backwave bouncing around in there (still the direct transfer thing as mentioned). Once a cab has significant resonances it stops being (semi) open back and becomes bass reflex or aperodic (essentially a strategically leaky sealed box - see the venerable Dynaco A25 or A35 vintage hifi speakers). The main thing that I could see being maybe something to worry about with a semi open back is backwave distortion - when the back wave bounces off the inside of the cab and hits the rear of the cone - this causes additional vibrations not present in the source signal feeding the speaker at that moment. Still not as much of an issue in most semi open cabs as it would be in cabs with a full back (sealed or otherwise). This is a problem in all speaker cab types with the exception of full open back (nowhere near a wall), and transmission lines (my favorite cab type for any application including guitar - the clarity and detail of a full open back with the bass extension of a horn)


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## greco (Jul 15, 2007)

@Granny Gremlin ...Very interesting!! 

Baffle Step Compensation

@mhammer ...There is an interesting electronics "project" in the above link.
Unfortunately, it is not appropriate for speaker line placement.

Copied from the link:

_"To this end, I have determined a simple passive network that allows you to adjust the level with a pot, until the response is optimum. The frequency is fixed, and is easily determined based on the width of the baffle. The equaliser circuit should be placed between the preamp and power amp - it is not suitable for use in the speaker lines. Although circuits exist for use in the speaker lines, I would not recommend them in any situation, since power losses are very high, and they cannot be adjusted easily to suit your listening environment."_


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## Granny Gremlin (Jun 3, 2016)

Yes, but _where_ is your cab? In 99% of jam spaces, right up against a wall so baffle step is moot. At shows less likely to be close enough to a wall (sometimes but not at most venues), but if it's close-mic'd that's just as good - more bassy even due to the pronounced proximity effect of the dynamic cardioid mics typically used. We're typically not missing any bass with our 12s and 15s (this thread being a case in point).


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## mhammer (Nov 30, 2007)

I picked up an empty cab for a single 12 today via Kijiji. Very well made finger-jointed construction and open back design. It actually has the samedepth as the one I built and overall greater internal volume. I was intending to install a higher wattage Eminence 12 in it, but this will provide a good opportunity to be able to separate what is the speaker and what is the cab.


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