Configurable bracing?

[thoff]

My first post!

I have been scanning various posts concerning bracing strategies, not exclusive to Klipsch, and one aspect of bracing that I'm not entirely comfortable with is the relative permanence of glued wood braces. Also, it's my impression, which may be wrong, is that it's not necessarily known what the exact change in resonance will be when adding bracing. Sure, if done correctly, it will most likely raise the resonant frequency, but what sonic impact it has does not seem absolute This is just my impression from various readings, so please correct me if I'm way off here.

So, it got me to thinking that maybe using a non-permanent, configurable bracing system would be useful for testing the sonic impact. For example, it occurred to me that using two threaded rods with a turnbuckle in the middle and small wood blocks against the panels would make for a simple bracing mechanism. This type of bracing scheme would also have less impact on the internal volume of the cabinet.

I have not seem mention nor tried the adjustable rod method myself, so I thought I would ask the more experienced community for their insight and opinions.

Thanks in advance...

[moray james]

Hey thoff: welcome to the forum. this is a great place with decades of accumulated knowledge about all things Klipsch. To address your post. Braces are fully intended to be 100% permanent and that is exactly what you want. Yourr understanding is not correct, rest assured that the mechanics of cabinet brace work was fully developed a very long time ago before you were born I would imagine. If you look into this precise science you can find wonderful and lucid text from the 30's 40's and 50's which go int great detail abut brace work and how to apply it and what it does. For example if you divide a panel with a vertical brace of reasonable dimenasions you will double the panels resonant frequency from say 100 Hz to 200Hz. Sub devide the panel again for a total of three braces and the panel resonance is nor 400 Hz. The use of stringers can further push up the panels resonant mode. This is very predictable and as I said very well understood. Ideally you want to push any cabinet resonance up beyond the passband of the driver in the cabinet. Higher frequency resonant modes are far easier to damp than are lower ones. That's why you hear the bass coming from a house party half a block away and the treble comes blasting out as sooon as the door is opened. Low frequencies at high energy levels carry a long way and simply move through most things in their way or they make those things resonate at the same or similar frequencies.

Using threaded rods is a partial solution but it is single ended as you are only wedging the cabinet in one direction and not the other and they only stiffen the panel at one point they are not neaas effective and a full brace and stringers. Panels require braces to stiffen them and then the panels need to be tied together with stringers to insure that they cannot bulge in any direction. You simply do not under any circumstances want or tollerate panel motion, your goal is zero motion but you can only try you won't achieve. Quite effective brace work can be achieved with correct material selection and application with well under 5% total cabinet volume use which is not significant. I have attached a link for you to see a retro brace job on a set of new Heresy 3 cabinets done with 3/4" solid oak through out. This cabinet is now very rigid and structurally quite sound. The stock cabinet which is a leaky hollow box is simply a joke in comparrison. Larger cabinets affford greater volume and the opportunity to brace with larger more substantial materials. The optimum ratio for a brace made from stif non flexible material is thickness times three however in smaller cabinets you can achieve very similar stiffness with a ratio of thickness times 2.5. I hope that this helps. Box talk is your enemy as it obscures vital low level detail. I have seen very poorly designed and built cabinets in which the box actually generated more acoustical output than the driver in the box just think about that for a moment. I hope that this has been of interest and assistance. Best regards Moray James.

http://www.audiokarma.org/forums/showthread.php?t=438234

http://community.klipsch.com/forums/t/172130.aspx

[thoff]

Moray,

Thank you for the welcome and great food for thought. It was reading many of your posts that contributed to my impetus to post this question.

When you mention: "The optimum ratio for a brace made from stif non flexible material is thickness times three", I interpret this to mean the depth of the brace should be 3x the thickness of the wall, correct? Please clarify. This would seem to imply that the mass of the brace as applied to the wall is more important than the pressure the brace exerts on the end points of the cabinet (i.e. the adjoining two walls). Is that right? So, it's not so much about pressure exerted by the brace between two opposing walls as it is the additional mass and stiffness applied to the wall itself.

The reason I'm asking all of this is I would like to try bracing my Fortes, but I would prefer to minimize the amount of material added to the structure. Not having a concrete bracing plan to go by makes me a bit nervous as to how the additional material is going to impact the operation of the bass driver and bass reflex.

Thanks again for your thoughtful response.

[moray james]

thoff: take a look at the link below in particular at the Quarter cabinet which is virtually identical to a Forte cabinet but 3 inches shorter. Thei is an excellent brace job using minimum materials. the results were excellent. Brace work is not about madd you want to use the lightest and stiffest materials availabel adding mass is counter productive. for small to medium size cabinets (2 - 3.5 cu.ft. volume) braces made from py or hardwood like hard maple birch or oak 3/4"x2.5" is fine for a retro job. If I were building from scratch a cabinet of theis size I would use 1"x3.5" braces and adjust the cabinet size to accomodate the additional volume consumed but the brace material. In large cabinet (5-10 cu.ft.) I would use solid hardwood or built up ply braces in a 2x4 size format which works out to 1.5"x3.5" dressed size. As far as panel resonances go it won't make a lot of difference if a brace extends to meet up with the adjacent panels at each end. That said I don't see that as a bad thing just a lot of workandnot practiaal for a retro job. See the link below for what I conssider to be a realistic and most effective brace job on a set of cabinets almost identical to yours.

Braces are not about excerting pressure they are about not permitting motion you do not want to wedge braces into place that will result in a damaged cabinet or broken ones if the job is done in a dry climate and shipped to a damp one later. Stiffness is the key. The more that you can interconnect your brace structure with stringers and the more that you can interconnect the stringers the more effectively you can distribute energy within the cabinet. The place where you want to concentrate your efforts in as far as stiffness goes is the main baffle especially around the woofer and passive you simply cannot make these areas stiff enough so do your best there to make them as rigid as possible all hard wood blocks where ever you can fit them in.

Here is the game plan I use. Each side and top gets one length wise central brace 3/4"x2.5" leave the bottom alone. The back gets two verticals above the passive and two short ones below. The front gets short verticals (2) below the woofer then a horizontal below and above the woofer and above the mid horn. Use 3/4"x1.5" stringers to connect everything together in as many places as possible (see my photo) and glue in as many small small blocks and 45 degree wedges around the woofer and as many as you can around the passive. You can see in my pictures that I used vertical braces under the passive. This is the device which makes the lowest bass in this system so make the back baffle rigid also.

The volume of the brace work is not significant in terms of overall cabinet volume you can easily off set this with some high density fiberglass damping material and you can extend your bass response further by retuning the passive with some added dead weight 2.5 ounces worth of steel flat washers works very well adding 3-4 Hz of extra bass extension to the cabinet. The reason thet AI suggest you leave the bottom panel alone is that you may want to install a seriously upgraded crossover at some point down the road at that will be the only place you will have left open to mount it. it is a more than fair compromise. To not build serious xovers for speakers with tismuch potential would be a real shame. I hope this helps. Yopu can see from Brians CF3 post that this is the format that was used. If you were building from the ground up I would do a more substantial job but this make for an impressive improvement as it stands . Perhaps you might like to chat with Brian ind hear his comment about before and after. Brians jobe went very well for a first time attempt and only a hand full of days effort. No reason you can''t do exactly the same.Best regards Moray James.

http://www.audiokarma.org/forums/showthread.php?t=372783&highlight=klf20

[wuzzzer]

Lots of good info here! In a speaker like my Chorus (or any speaker in general) what exactly will be the benefits in sound by adding bracing? Tighter bass? Deeper bass? Both?

[dtel]

Nothing wrong with bracing just don't overdo it, if overdone your changing the internal dimensions, less cubic inches from the design. Most good speakers are there size for a reason, you don't want to get to far from that.

This could also be part of the difference in sound, different tuning point with different cabinet volume ?

[thoff]

Moray,

Thanks again for the in-depth reply. There is a lot of helpful information here, obviously gained from experience.

Another question that comes to mind, with respect to the resonant frequency issue: how do you know where your panel resonant frequencies are so that you know how many subdivisions and stringer additions to make? Do you have a method you utilize for measuring the resonant frequencies pre and post modification?

I have to admit it's this aspect of a bracing project that I find somewhat intimidating (I'm fine with woodworking). I don't have a good feel, or rather audio sense, for what "specific" sonic problem I'm solving with bracing. Sure, I understand the intent, but just have no way of quantifying the specifics of what needs to be addressed.

I have already upgraded the tweeters and crossovers and the bass sounds fantastic...how can I identify what will improve with a bracing project?

Many thanks again for the insight.

[moray james]

Well you will find that the bass will be leaner not fat an wooly like it is with box panels flapping. Baass will be clean tight contrled and articulate but not deep thoug you might motice it as deeper bcause it is so much cleaner. You can add som dead weight to the dentre of the inside of your passive to re tune them a little lower 2.5 3 ounces on the Chorus ll passive will buy you 3-4 hertz more bass extension but don't use more than that or you risk the woofer getting unloaded and you don't want that.the woofer can only do what it can do you just cant make it go any lower than that but I can assure you that an extra 3 Hz will be very noticable and very nice to have you will not be disapointed. Hope this helps best regards Moray James.

[moray james]

most reflex designs can stand 10% variations the brace work I am suggesting would not hit 5% so if you add a little additional damping in the form of high density fiberglass ther are no worries along those lines and iven if you did brace so much that you gave up say 2 Hz of bass it would still be worth have tight controlled articulate bass. I simply refuse to listen to a hollow box I am sorry that is just not acceptable and there are no excuses for it in an after market product. A few days time and some material and the job is done. Fo me this is a must do modification and is of of a number which fall int that catagory.Best regards Moray James.

[moray james]

thoff: with retro jobe like I try to do as much as I reasonably dan and not go nuts I make an extreme effort not to eat up cabinet volume and in a retro job you simply do not have the room to do much more than I have recommended. I follow the basid game play mentioned for retro jobe and it has always worked very well. The only time I was really concerned was with the Heresy 3 and there as you saw we used all 3/4" solid oak I would have with hind sight added one more horizontal brace on the front vaffle above the mid horn as well. That said even with the material used and an almost solid damping packing of about 85% total volume with high density fiberglass you and othere simply would not believe the bass weight my H3 will make while up on 23" high four post stands. I can play Yello Touch and make the air in the room shake like jello. So don't worry a single vertical on the side and top and a pair on the back and horizontals below and above the woofer and above the mid horn too and you will be good with all the other small bits mentioned. Do leave your bottom panelopen as it will be the only addessable place you will have to mount your crossover if you upgrade later. In some cases it might even be a good idea to make the bottom od the cabinet removable ad cross braces and stringers may block access. think long and hard because once it is in theree it is not coming out. Hope this helps. Best regards Moray James.

PS. Bracing a cabinet will minimize or eliminate box talk which smears and obscures low level details from being heard. Shut the box up and you get to hear the speaker. it is just that simple. Do this once and I cannot ever see you going back to listening to hollow boxes.

[dtel]

From the way it sounds, you are doing some kind of test to be able to know what changes are going on. So I ask a question, since internal cabinet volume also can change the tuning point would it help to get an idea how much wood you would like to use and just place it in the bottom of the cabinet. Test and added a little more or less to get closer to what you want before actually installing it permanently.

Kind of like tuning it with volume before you make it stiffer, I ask because a little extra bracing would not hurt as far as strength goes, you may be able to use more than planned ? Or would the wood just sitting on the bottom rattle or vibrate to much ?

Just a thought

[moray james]

I have in past jobs calculated the volume of all the brace material and I don't much concern myself with that any more as I keep my basic game plan much the same . You can according to theory achieve some percentage over 22% or so of additional precieved volume with damping. I expect that would be a hard rabbit to pull out of the hat maybe on a good day using perlite or activated charcoal as the damping material . Point is that the amount of space my recipe takes up is easily offset with the use of good old fiberglass as damping material. So I just dont worry about it. If you are concerned then don't do it but your suggestion is a good one stuff it all in there and have a listen. Empty pop bottles (with sealed caps) give you a very easy liter at a time method or two liter at a time method of using up cabinet volume you dont have to cut a lick of wood just figure it out with a calculator. Me I did that now I just dont bother any more. So do what you need to do to reach you comfort level.Sometimes you just have to trust. I have beenthere myself and I have not been given bad advice so far (of what I have taken s maybe my BS metre is working just fine). Kills me to see the back side of a Cornwall open with all the nothing in there just kills me. thanks for thegood suggestion I know it is hard when you have not done this kind of thing and you worry about screwing things up. I guess I more or less stoed worring a long time ago. I have concerns but I work them out and I do what I do knowing some times I may have to sprig for ne parts or what eveer and it is what it is I don't often screw up dont remember havein to buy replacment bits in a long long time. Thats how I learn best I guess and I understand it is not for everyone. Best regards Moray James.

[kapsnb01]

I just finished bracing my CF3s using the same methods that Moray described. Pictures certainly make it seem as though the brace work is taking up a good deal more space than it truly is. The factory damping material that was wedged into the box actually took up more space than the bracing that I have in there now. I'm going to do some damping work here this weekend, so that will take up some additional space, but I agree with Moray, I seriously doubt it's close to a 10% change in cabinet volume.

Mark - you're always welcome to drop by and take a look in the boxes and have a listen if you want. I think you'll like the results if you go ahead with some brace work on the Chorus'.

[dtel]

I was just wondering, I thought I read somewhere that someone was playing with heresy parts in a bigger cabinet to try to get a lower output from the 12 driver since it really should go lower than what a stock heresy does. It may have been for a center I don't remember.

I know the Cornwall lll had a 2x4 going side to side and the shelf for the bottom port. I was just wondering if you could brace and also slightly change the tune with a little more wood, but I guess damping would do the same thing but also stop some of the sound bouncing around in there, good or bad I guess you would have to try it out to know.

I was asking because I have some old beat up Hersey's only missing a couple of parts, I would love to build some nice cabinets with solid wood like Mahogany or Oak and put the klipsch parts in them.

[moray james]

If you put a Heresy woofer in a larger volume cabinet yes you will get lower bass response but you will also get lower efficiency you cannot have both. The Heresy is an intentional design by a very sharp minded designer. The Heresy woofer does very well considering it has less than 1.8 cu.ft. of internal volume to work with and it is a low Qt driver. I have found that with a lot of damping which has to placed critically you can make the woofer think it is in a larger volume than it is. That combined with the fact that a sealed box rolls off at 12 db per octive andy you can get some prettry respectable bass out of a pair of Heresy. I am sure that most here would be shocked to hear what mine will do. To say that I am pleased with my results is a real understatment.

I have not spent any time looking at the Cornwall as I have never considered them an option for me as I generally do not prefer fifteen inch woofers I just happen to like tens in pairs or quads. Given the physical size of a Cornwalll I would think at least two vertical braced on the sides and three on the back and two on the top. I would make those braces 3/4"x2.5" and I would use oak hard maple or baltic birch only for added stiffness. I would make larger horizontal cross braces for the front baffle above the woofer and above the mid horn. I would then tie all those braces together with 1x2 stringers (which are actually 3/4"x1.5") three stringers per brace. That would be my minimun amount of brace work for a cabinet of this size and I think that should do a good job of stiffening the cabinet. If I wanted to build a Cornwall from the ground up I would double the size of all the braces and stringers and then make account for that in the new cabinets overall internal volume and I would move the vent from the bottom of the cabinrt to the sides to get it up off the ground plain. I would contrive to have the centre of the mid horn at my seated ear level and I would make the design a two way. Make sure that you keep any additional damping material to the top of the cabinet up away from the vent(s). But that's just me.

Speaking of Mahogany Heresy there is a pair for sale right now which are in mint condition and which look stunning I think the asking price is $600.00. Best regards Moray James.

[dtel]

Speaking of Mahogany Heresy there is a pair for sale right now which are in mint condition and which look stunning I think the asking price is $600.00. Best regards Moray James

I had thought about it because I had a lot of 1" thick mahogany, a plainer and a jointer. I gave most of the wood to a friend/neighbor who wanted to build something special for his daughter. I also built a cradle for our first grandchild, so I have little or none left, I do have some nice cypress 14" wide but that would not be good for a cabinet.