Loudspeaker bracing: Shelves vs. Ribs - diyAudio
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Old 20th May 2008, 03:04 PM   #1
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Default Loudspeaker bracing: Shelves vs. Ribs

Hello,

Having built loudspeakers off and on since 1970, I can't help but notice how much shelf bracing has become so popular in modern loudspeaker design.

Shelf bracing appears to be easy to encorporate into tower style boxes.

I wonder if boxes which follow the classic profile where length to width to depth proportions are like the Golden Rectangle, would these boxes be more easily built with old style ribs glued directly to the interior panel surfaces.

It seems to me that where there are long spans between all the opposite surfaces, then shelf bracing ends up introducing a lot of extra weight into an already heavy box.

My case: I am building right now 4 full-range boxes which are identical to the 8 sided Tannoy System 1200, being approximately 25" x 16" x 15" (635mm. x 406mm. x 380mm.). Sidewalls and back are all 18mm MDF, and the front is 38 mm. MDF.

It would be much easier for me to attach about a dozen short ribs to panel walls inside each box, rather than create an elaborate matrix of 3 or 4 shelves which criss-cross inside each box.

Can anyone tell me what is so special about the use of shelf bracing that makes their choice over ribs so reasonable in spite of all the extra effort required to cutout the large holes in each shelf and the precision required to make sure shelves are perfectly sized to engage completely into each groove they mate with?

Thanks
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Old 20th May 2008, 04:54 PM   #2
Ron E is offline Ron E  United States
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Default Re: Loudspeaker bracing: Shelves vs. Ribs

Quote:
Originally posted by montreal
Shelf bracing appears to be easy to encorporate into tower style boxes.
It would be much easier for me to attach about a dozen short ribs to panel walls inside each box, rather than create an elaborate matrix of 3 or 4 shelves which criss-cross inside each box.

Can anyone tell me what is so special about the use of shelf bracing that makes their choice over ribs so reasonable in spite of all the extra effort required to cutout the large holes in each shelf and the precision required to make sure shelves are perfectly sized to engage completely into each groove they mate with?
I'd say you pretty much nailed it.
Shelves are great in narrow tall enclosures.
Ribs are fine, but it _is_ nice to interconnect the panels in a place or two - the vibration of a box is complicated.
Criss-crossing shelf braces seems like a potential recipe for buzzes, IMO.
DIYers like to overbuild - and overbuilding a box actually does provide decent returns if thought out well, but there is such a thing as overkill.

If ribs have worked for you, there is no reason to change.
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Old 20th May 2008, 06:00 PM   #3
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Quote:
Originally posted by Ron E

.......Ribs are fine, but it is nice to interconnect the panels in a place or two - the vibration of a box is complicated..
I agree and would never rely solely on ribs without adding cross braces to join opposite large panels.

In the end, we are trying to stiffen up any exterior panel which would otherwise vibrate at its own natural resonance frequency.

Having a stiff rib layed out along a diagonal line helps.

When I look at a shelf which has a number of large holes in it, I appreciate the idea that the shelf provides at the same time a rib that stiffens up the panel and a bridge between a panel and its opposite panel.

Unfortunately, the shelf often ends up being parallel to the panel edge and not along a diagonal.

Like with a Roman Arch, the holes in the shelf have to be circular or oval to add sufficient stiffness to the exterior panel.

The problem with making sure that each hole in a shelf has a generous curve to it is that the amount of material left in the shelf following the creation of the holes adds much more weight to the speaker box than the traditional rib and cross-brace approach which uses much less material.

We seem to have evolved in two directions in attempting to reduce panel resonance. Either we super-stiffen the panel to force the resonant frequency to be way above the range of the woofer, or we soften the panel by creating a sand filled sandwich to absorb energy, or by adding all kinds of elastic compounds like tar or flexible ceramic tile cement as an interior surface coating.

Thanks Ron E for taking the time to answer.
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Old 20th May 2008, 07:33 PM   #4
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ribs are usually enough.
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Old 20th May 2008, 10:01 PM   #5
Ron E is offline Ron E  United States
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Quote:
Originally posted by montreal
I agree and would never rely solely on ribs without adding cross braces to join opposite large panels. In the end, we are trying to stiffen up any exterior panel which would otherwise vibrate at its own natural resonance frequency. Having a stiff rib layed out along a diagonal line helps.
I saw a study which took a typical panel clamped at its edges in a very solid box and attempts were made to stiffen it in several ways. A single dowel brace in the center, a horizontal rib, a diagonal rib and unbraced. They measured resonant frequency and if I recall correctly, the horizontal rib beat the diagonal rib, which was about the same as the single point crossbrace, which was just better than unbraced. I think that it is best to break up a panel into smaller panels with ribs, and the distance between the ribs should be slightly random. The occasional shelf helps with the vibrations that involve more than one panel.

Quote:
Originally posted by montreal
When I look at a shelf which has a number of large holes in it, I appreciate the idea that the shelf provides at the same time a rib that stiffens up the panel and a bridge between a panel and its opposite panel. Unfortunately, the shelf often ends up being parallel to the panel edge and not along a diagonal. Like with a Roman Arch, the holes in the shelf have to be circular or oval to add sufficient stiffness to the exterior panel.
I think this is overstated a bit, a shelf brace that is more than a couple wall thicknesses proud of the panel, even with sharp corners, is going to be so much stiffer than the panel that a fillet won't make much difference. IT doesn't hurt, though...
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Old 20th May 2008, 10:20 PM   #6
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Quote:
Originally posted by Ron E

.....a shelf brace that is more than a couple wall thicknesses proud of the panel, even with sharp corners, is going to be so much stiffer than the panel that a fillet won't make much difference. ...
Your point is well taken and with sharp corners present in the shelf, that is going to eliminate a lot of extra weight.

Thanks for the great followup post.

Tomorrow I start my internal bracing using your recommendations.
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Old 20th May 2008, 10:35 PM   #7
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Quote:
Originally posted by myhrrhleine
ribs are usually enough.
Do you mean ribs or stick bracing? I interpret a rib to be a piece fastened to a single panel. This will do less to prevent the cabinet from flexing when compared with a stick brace.

I use stick bracing as I feel the amount of wood used (and time) to it's benefit ratio is better than with shelf bracing.
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Old 20th May 2008, 10:38 PM   #8
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Default Re: Re: Loudspeaker bracing: Shelves vs. Ribs

Quote:
Originally posted by Ron E
Ribs are fine, but it _is_ nice to interconnect the panels in a place or two
Agreed, maybe more than a place or two.

Quote:
[i]Criss-crossing shelf braces seems like a potential recipe for buzzes, IMO.
[/B]
I like to criss cross stick bracing and have not had a problem with anything coming loose.
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Old 21st May 2008, 02:39 AM   #9
Ron E is offline Ron E  United States
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Quote:
Originally posted by montreal
Tomorrow I start my internal bracing using your recommendations.
Draw a quick picture of your planned bracing scheme and share it, others might have input - and the devil is in the details.... In the study I spoke of , the panels were squareish, so vertical and horizontal were essentially the same. I believe ribs along the longest dimension of the panels are best.
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Old 21st May 2008, 01:10 PM   #10
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Quote:
Originally posted by Ron E


Draw a quick picture of your planned bracing scheme and share it, others might have input - and the devil is in the details.... In the study I spoke of , the panels were squareish, so vertical and horizontal were essentially the same. I believe ribs along the longest dimension of the panels are best.

Ron, thanks for the invitation to share a diagram.

First have a look at the attached jpeg publicity photo of this Tannoy System 1200 studio monitor which first was introduced about 10 years ago and has since been replaced by the System 12 DMT version which is four sided and smaller in volume but still essentially with the same 12" driver.

For those of you wondering why I'm posting a full range speaker project in the sub-woofer forum, my thinking is that System 1200 is more an old school speaker where it was large enough to provide all the volume down to 40 hz. New school speakers, no matter how full range they are or how tall they are, still tend to rely on a sub-woofer for the bottom end. And I believe for the studio use that this System 1200 was intended for, there was no sub-woofer hidden under the mixer board to assist it.

So my project to build four System 1200 format boxes faces the same challenges as building a sub-woofer box designed to handle full electrical energy down to 20 hz. without panel resonances.

I might add that while I have chosen the System 1200 box design, the actual 12" drivers that will be installed are all much older Tannoy Coaxials that I picked up in a second hand department of a major professional audio equipment rental company. They consist of three 12" Tannoy Monitor HPD315 and one 12" Monitor Gold.

Normally these four classic drivers would have been installed in boxes greater than 6 cu. ft. in volume. Thus my 2.3 cu.ft. volume for the System 1200 style box is quite a squeeze for them, and there may be unpleasant surprises at the low end that await me.


I don't have a sketch ready yet but let me tell you about the dimensions. From the photo, you see that the front panel aspect ratio is 1.65 to 1.

The internal depth is 13".

We are talking about 3/4" MDF side panels that are as follows:

Side Panel number 1 is 20" x 13" (2 per box)
Side Panel number 2 is 10" x 13" (2 per box)
Side Panel number 3 is 3" x 13" (4 per box)

Additionally:

The back panel is roughly 24" x 14", also in 3/4" MDF.
The front panel is the same dimension as the back, but in 1.5" MDF.

I may not bother reinforcing Side Panel number 3. These are already quite narrow and meet adjacent panels at 45 degrees.

The front panel is already thick enough not to vibrate.

The rear panel and Side Panels number 1 and 2 all require ribs and cross bracing.

For each of these selected panels for which I am choosing to add ribs, I will surface glue two ribs in an "X" pattern, creating a cross that follows the diagonal lines that join opposite corners of each panel.

Where a pair of ribs on a typical panel intersect, I will attach a cross brace to reach over to the identical rib intersection on the panel located on the opposite side of the box.

Of my 8 octagonal sides, where I provide ribs only to the 4 largest sides, I end up with two cross braces, one vertical and the other horizontal. I may even tie these two cross braces together where they cross each other in the dead center of the box.

A third cross brace from off center of the rear panel to off center of the front panel may also be desirable.

I have not decided if I will be using hardwood or MDF for all the ribs and cross braces, but I expect the rib cross section to be at least 2" x 3/4".

If anyone here needs a sketch to better visualize this arrangement, then I will try to draft one up and post it.

Thanks for your continuing interest.
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