# Rebuild cabs or add bracing



## drumdude (Aug 26, 2007)

As seen in a different post  here I have been playing with rebuilding\improving some existing speakers. I have gotten my waveguide where I am happy with the response and am ready to work on the boxes. The boxes are 1/2" particle board with some sort of a composite plasic front as seen below.










I can either brace the existing cabinet a little bit trying not to change the volume too much and overlay the waveguide into the inset as seen here. Without the tape of course 








or build a new baffle of 3/4" MDF
or rebuild the entire box of 3/4 MDF. 
Obviously rebuilding the whole box would be more work and the end result will be heavier (I am going to be mounting these on brackets held off the wall) but I would be willing to if the sound quality improvement is significant. 
The current box is 11"w x 21"h x 8"d with no bracing and I can feel some vibration when I crank it. I guess I am wondering if bracing will be good enough or is the difference between 1/2" particle board and 3/4" MDF significant. btw there are going to be crossed over at 80hz so not a ton of bass.


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## mgboy (Jan 17, 2007)

I'd use 1/2 MDF + bracing on the inside (and whatever sort of polyfill, etc) instead of a full on 3/4 in. MDF because you'll save on weight and since it'll be crossed over at 80 Hz and have bracing - resonance shouldn't be a problem. 

Cheap, effective - what more can you want? Oh and interesting results on the waveguide. Interesting = good. =)


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## avaserfi (Jul 5, 2007)

mgboy said:


> I'd use 1/2 MDF + bracing on the inside (and whatever sort of polyfill, etc) instead of a full on 3/4 in. MDF because you'll save on weight and since it'll be crossed over at 80 Hz and have bracing - resonance shouldn't be a problem.
> 
> Cheap, effective - what more can you want? Oh and interesting results on the waveguide. Interesting = good. =)


Cabinet resonances are actually exponentially more problematic in terms of required bracing/dampening ascending upwards in octaves rather than descending. The difference is at lower octaves the resonances are far more pronounced [physically] and thusly more noticed by the listeners. The subject of resonance audibility has been discussed thoroughly in the paper by Floyd Toole: _The Modification of Timbre by Resonances: Perception and Measurement JAES Volume 36 Issue 3 pp. 122-142; March 1988_.

To the OP ideally you would rebuild the cabinet from the ground up using 3/4" void free cabinet grade plywood (it is lighter than MDF while being more rigid). Along with this use of a constrain layer and oak bracing and you would have a lower resonance cabinet relative to most all commercial and DIY speakers. Sadly, if you are rebuilding the speakers you might as well just start from scratch and get some new drivers that would be more capable rather than simply salvaging an older speaker.

If you do want to minimize resonances in the current cabinet simply use a constrain layer along side steel angle bracing. This would minimize resonance greatly while effecting internal volume minimally.


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## drumdude (Aug 26, 2007)

avaserfi said:


> Cabinet resonances are actually exponentially more problematic in terms of required bracing/dampening ascending upwards in octaves rather than descending.


I wasn't aware of that, seems the opposite of what I would have thought. good to know.


avaserfi said:


> ideally you would rebuild the cabinet from the ground up using 3/4" void free cabinet grade plywood (it is lighter than MDF while being more rigid). Along with this use of a constrain layer and oak bracing and you would have a lower resonance cabinet relative to most all commercial and DIY speakers.


I wish I had thought of plywood before I dragged home that sheet of heavy MDF :foottap:



avaserfi said:


> Sadly, if you are rebuilding the speakers you might as well just start from scratch and get some new drivers that would be more capable rather than simply salvaging an older speaker.


 yes in a perfect world I would be building something like the  Usher701 or one of these  or simply buy some Mackies or Genelecs. I am rebuilding building these as a cheap learning experiment. This is my first DIY effort above 100 hz. They are going to temporarily be used as nearfield monitors until I can get something better. They will than become garage or bedroom speakers. They sound pretty neutral in the midrange as is, but the tweeter dies above 8k. 



avaserfi said:


> If you do want to minimize resonances in the current cabinet simply use a constrain layer along side steel angle bracing. This would minimize resonance greatly while effecting internal volume minimally.


Not sure I understand. Do you mean from inside out: original box, angle iron, wrap with another box?


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## avaserfi (Jul 5, 2007)

drumdude said:


> Not sure I understand. Do you mean from inside out: original box, angle iron, wrap with another box?


No, the constrain layer and angle iron would be placed inside the box along with high grade acoustic dampening material such as OC705 or 8lb mineral wool. If you do this properly, minimal amounts of internal volume would be used as the constrain layer would only need to be 3/4" thick and pieces of 1/8" x 1" angle iron would barely take up any volume.

What I am suggesting is a lot of work and is likely more work than should be done on a speaker such as the one you are looking to modify, but if you build a new cabinet from scratch you could design it such that it would work with more capable drivers in the future as well as working with the ones you have now. This will save you money in the long run and allow for an easy upgrade in the future.


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## Bill Fitzmaurice (Jun 14, 2008)

Brace it and that's all. Pound for pound cross-bracing is at least four times as effective as increasing panel thickness in reducing vibration. If the interior is accessable the easiest yet most effective method is to glue 1/2" dowels across the cab, spaced within a 6 inch or so radius.


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## drumdude (Aug 26, 2007)

I happen to have a pile of 1/2" dowels lying about. I will use them and save the MDF for another speaker project sometime in the future. Finding or building sturdy speaker mounts is difficult enough without adding more weight.


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## avaserfi (Jul 5, 2007)

Bill Fitzmaurice said:


> Brace it and that's all. Pound for pound cross-bracing is at least four times as effective as increasing panel thickness in reducing vibration.


While cross bracing is far more effective than increasing cabinet thickness it alone is insufficient if ones goal is an audibly resonant free cabinet. Without using specialized materials such as B&W does with their 802D midrange enclosure a viscoelastic constrain layer along with a dense XYZ bracing matrix is required.



> If the interior is accessable the easiest yet most effective method is to glue 1/2" dowels across the cab, spaced within a 6 inch or so radius.


This method will still result in a highly resonant cabinet, but would likely remove the most noticeable large vibrations in the lower octaves.


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## drumdude (Aug 26, 2007)

avaserfi said:


> While cross bracing is far more effective than increasing cabinet thickness it alone is insufficient if ones goal is an audibly resonant free cabinet. Without using specialized materials such as B&W does with their 802D midrange enclosure a viscoelastic constrain layer along with a dense XYZ bracing matrix is required.


as someone who is here to learn, I am filing this away for future reference. My current goal is to quickly improve some existing speakers that I have replaced the tweeters in. I think at this time such efforts would exceed the value of what I have to work with, but good to know.




> This method will still result in a highly resonant cabinet, but would likely remove the most noticeable large vibrations in the lower octaves.


Any improvement would work for me at the moment. I am currently delaying laying down some drum tracks in order to get this done. After I have something I can work with I will attempt something better in the future when I am not on a time crunch. 

To address Bills thoughts on Cabs. I am aware of the compromises made in SQ for raw SPL in the pro audio world. But as a semi-pro musician who has been his own roadie way too many times I would prefer to carry around lightweight well-braced plywood cabs. This has me wondering why consumer systems seem to take the opposite approach, using MDF, particle board or plastic with little or no bracing. 

I know subs need good bracing but I'm trying to get a feel for how important high density, ie MDF and bracing is for midrange & up.


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## Bill Fitzmaurice (Jun 14, 2008)

avaserfi said:


> While cross bracing is far more effective than increasing cabinet thickness it alone is insufficient if ones goal is an audibly resonant free cabinet.


This 24x24x48" pro-touring subwoofer is built entirely of 1/2" plywood, single layer. At output levels in excess of 125dB it does not vibrate. And it only weighs 90 pounds.










I don't use anything thicker than 1/2" plywood in any of my designs, and none of them vibrate. Using heavier materials and complicated constructions is one option, but it's not the only option.



> This has me wondering why consumer systems seem to take the opposite approach, using MDF, particle board or plastic with little or no bracing.


Using proper bracing is a labor intensive, ie., expensive method of building cabinets. It's far less expensive to use thicker heavier materials with little or no bracing. In the pro-sound world you save at the point of purchase, and pay back those savings and more in spades everytime you lift the cab, or fill up your tank.


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## avaserfi (Jul 5, 2007)

Bill Fitzmaurice said:


> This 24x24x48" pro-touring subwoofer is built entirely of 1/2" plywood, single layer. At output levels in excess of 125dB it does not vibrate. And it only weighs 90 pounds.


The typical passband of a subwoofer does not require large amounts of bracing to remove resonance. Cabinet flex due to pressure is another issue that is also dealt with easily as well. I was primarily referring to resonances in higher octaves which would alleviated by typical bracing methods. It should also be noted that using ones hand as a test for vibration is insufficient in terms of recognizing audible resonance. To get an accurate representation of resonance in the easiest manner one must use an accelerometer.



> Using proper bracing is a labor intensive, ie., expensive method of building cabinets. It's far less expensive to use thicker heavier materials with little or no bracing. In the pro-sound world you save at the point of purchase, and pay back those savings and more in spades everytime you lift the cab, or fill up your tank.


Pro-sound is rarely if ever concerned with maximum fidelity due to a variety of factors from room acoustics to SPL requirements etc... I do not see how these restrictions apply to home audio especially if one is building their own cabinet which seemed to be the point of interest in the original post.


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## Bill Fitzmaurice (Jun 14, 2008)

avaserfi said:


> Pro-sound is rarely if ever concerned with maximum fidelity due to a variety of factors from room acoustics to SPL requirements etc... I do not see how these restrictions apply to home audio especially if one is building their own cabinet which seemed to be the point of interest in the original post.


I posted that as an example of how high weight is not a required element of proper cabinet design. I don't use heavier than 1/2" plywood in my HT and hi-fi designs either, and they are inert.


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## avaserfi (Jul 5, 2007)

Bill Fitzmaurice said:


> I posted that as an example of how high weight is not a required element of proper cabinet design. I don't use heavier than 1/2" plywood in my HT and hi-fi designs either, and they are inert.


From your limited description of your "hi-fi designs" I highly doubt there is no audible resonance exhibited by said cabinets. Would you possibly be able to provide an accurate accelerometer reading of your cabinets? If not at least providing pictures of the internal structure of your designs would be beneficial.

To further expand on this notion: Take for example an example of an audibly inert cabinet the B&W 802D. This speaker uses a specially developed resign to achieve an inert cabinet. At the same time we have the Dali Ikon 6 a speaker that uses typical (both commercially and DIY) bracing methods which does have audible resonance exhibited*.

B&W 802D:










Dali Ikon 6:










Both plots are taken from Stereophile reviews. Do note that the Dali's sensitivity is 1.5dB higher than that of the 802D's. This factor is not accounted for in the accelerometer testing done by Stereophile and thus the Ikon's graph would be approximately 1.5dB lower than it is for an even comparison.

*In referencing audible resonance the thresholds determined in the paper _The Modification of Timbre by Resonances: Perception and Measurement JAES Volume 36 Issue 3 pp. 122-142; March 1988_ are being used.


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## Bill Fitzmaurice (Jun 14, 2008)

avaserfi said:


> at least providing pictures of the internal structure of your designs would be beneficial.


You may find those in back issues of Speaker Builder, Performer's Audio and AudioXPress magazines, where they have been published over the last 15 years.


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