Bracing101 2021

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camp10
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Bracing101 2021

#1 Post by camp10 » Mon Apr 12, 2021 5:02 pm

HI all....I have a sub I am trying to tie up my loose ends with....it is a sealed box of the PushPushSlotLoad variety....I have 2 18s facing each other in the slot and threaded rod going from one side of the box to the other mounting the drivers....Potentially overkill in that area...but....oh well....

I have come to understand that the majority of vibrations come from direct contact of the drivers to the baffle....the force cancellations of the opposing woofers should take care of most of this one would with....the way the bracing is setup so far with the threaded rod...the baffle directly behind each woofer should be about taken care or close to it.....but there is a back baffle that I need to address...the top baffle are skinny and likely are fine without additional bracing....

I can have come to believe that matrix bracing and what I call TL style bracing is the best. M atrix bracing, I think everyone here is familiar with and TL style bracing is just inspired by the bracing cause by folding a line, like you guys do with your horns.

In regards to matrix bracing....does anyone know what the baseline spacing is? A brace beam every 4" maybe?

the attachments show my bracing start so far, and examples of matrix bracing and what I call TL inspired bracing or atleast one iteration of....for a sealed cad one could build a 1/2WL closed transmission line, folded, for the same results.

Thanks in advance for all the help and wisdom =)
Attachments
EEEPEE.jpg
Bracing.jpg
all-dowels-650x554.jpg

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Seth
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Re: Bracing101 2021

#2 Post by Seth » Mon Apr 12, 2021 5:40 pm

Hey Camp, welcome to the party. I'm Seth. Good looking project you have there :thumbsup:
camp10 wrote:
Mon Apr 12, 2021 5:02 pm
I have come to understand that the majority of vibrations come from direct contact of the drivers to the baffle.
This is a common misconception, that I had as well. If you were to hold the driver flange in your hands in free air while playing a low note, you'll realize that very little vibration comes from the mass of the cone moving back and fourth. Or, breaking them in on a work bench, they don't dance around and vibrate out of control.

The vibrations come from the pressure inside the cab, trying to force the sides of the cab in and out. You're on the right track, because that's really where the importance of bracing comes in, keeping the sides from flexing. To be really honest, the true answer you seek is beyond me. But, I will say that your bracing needs depend on the pressures you expect to have and the ability of the enclosure material to resist it.

Since your cab design is sealed, I'm pretty certain your pressure swing will be less than if it were a vented enclosure, or horn enclosure. To give you at least a place to start, it seems most of the plans here have bracing no more than 8" and the designs use ½" plywood. So, any portion of any of the panels within the enclosure is 8" or less from a support, whether it be an adjoining panel or a structural brace.

Hope this helps.
Build in process - 2 WH6, one Alpha 6a loaded, one PRV Audio 6MB250-NDY loaded

Two 2x6 shorty SLA Pro's
One T39, 16", 3012LF loaded
Tall AutoTuba, 20" wide, 2x 8" MCM 55-2421
TruckTuba, 8½" wide, 2x 8" MCM 55-2421

camp10
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Re: Bracing101 2021

#3 Post by camp10 » Mon Apr 12, 2021 8:53 pm

Interesting!!! Thank you for replying. Regarding pressure swing of vented vs sealed, that is very interesting...I was almost certain that the pressure swing in vented design was less than a sealed considered the pressure is relieved by the vent....that is interesting, I'm sure i can find some who disagree but it is kinda not here nor there...in the sense that the wisdom I seek has to do with optimal bracing spacing and regardless of the truth of those aspects...an optimum of some sort would remain unchanged...

Every 8 inches sounds like a good starting point. If this is the trend that you see in Bill's designs then I'd take that as a good indicator...I've been blessed to receive council from some big names in audio, via the internet, but I hold Mr. Fitzmaurice as the master of bracing. Hopefully he will weigh in on the topic :fingers: ...

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Tom Smit
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Re: Bracing101 2021

#4 Post by Tom Smit » Mon Apr 12, 2021 10:33 pm

The middle picture has way too much wood for braces...I wouldn't want to carry it. If you look at shelves or inside cabinets, you will notice that a brace along an edge is between 1 and 2 inches (wider is better). That same idea could be incorporated inside the cab.
The lower pictures shows dowels near a side panel, but, that would be a case of overkill. Only the top/bottom dowels would be sufficient for the baffle.
I disagree with Seth (dui bu qi) because I think that the vent would relieve the pressure.
TomS

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Seth
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Re: Bracing101 2021

#5 Post by Seth » Mon Apr 12, 2021 10:54 pm

Tom Smit wrote:
Mon Apr 12, 2021 10:33 pm
I disagree with Seth (dui bu qi) because I think that the vent would relieve the pressure.
Tom (yaba daba do) :D ...

(Moved content to other thread)

Yes, shelving bracketry... good analogy for bracing :thumbsup:
Build in process - 2 WH6, one Alpha 6a loaded, one PRV Audio 6MB250-NDY loaded

Two 2x6 shorty SLA Pro's
One T39, 16", 3012LF loaded
Tall AutoTuba, 20" wide, 2x 8" MCM 55-2421
TruckTuba, 8½" wide, 2x 8" MCM 55-2421

camp10
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Re: Bracing101 2021

#6 Post by camp10 » Mon Apr 12, 2021 11:58 pm

I can maintain my own thread... I don't mind the discussion either.
(content doesn't need moving)
Seth wrote:
Mon Apr 12, 2021 11:01 pm
From another thread, in regards to rear chamber pressure of a ported cabinet vs. sealed cabinet...
Tom Smit wrote:
Mon Apr 12, 2021 10:33 pm
I disagree with Seth (dui bu qi) because I think that the vent would relieve the pressure.
Tom (yaba daba do) :D ...


I'm fairly certain the port velocity crams more air into the box creating higher pressure and sucks more air out of the box creating lower pressures than that of a sealed enclosure, especially at the tuning frequency. Below the tuning frequency of a ported enclosure, yes the positive and negative pressure peaks are lower than a sealed enclosure. Which I believe is why drivers exhibit lower excursion at the tuning frequency in a vented enclosure and do a ton more travel below the tuning frequency.

I'm sure a quick googling would clear it up, but let's carry on with a little banter... if you don't mind? Bruce's thread's got me all riled up. LOL

:mrgreen:

camp10
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Re: Bracing101 2021

#7 Post by camp10 » Tue Apr 13, 2021 12:31 am

Discussion on this topic is interesting but a quick trip to HornResp tells most of the story...it seems pressure peaks around tuning is highest and drops off above and below tuning....which makes sense...the lower the tuning note the higher the pressure.....that also makes sense...
Tom Smit wrote:
Mon Apr 12, 2021 10:33 pm
The middle picture has way too much wood for braces...I wouldn't want to carry it. If you look at shelves or inside cabinets, you will notice that a brace along an edge is between 1 and 2 inches (wider is better). That same idea could be incorporated inside the cab.
OK wait I think you are talking about the full width boards...You are probably used to seeing holes and such cut into the bracing or dowels etc...pros and cons... in the above example each full plane of bracing breaks up resonate modes into higher less energetic notes...if its worth it? I don't know...

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Re: Bracing101 2021

#8 Post by Grant Bunter » Tue Apr 13, 2021 3:19 am

Perhaps the most obvious thing to point out with the second and third pics is that their bracing reaches all panels, not just the sides, but front and rear as well, and the brace planes are not points, and the second pic also extends to contact significant lengths of each panel.

Yours, and the third pic, contact at points on the panels only, and I would think that may be insufficient.
The idea of bracing, is to prevent whichever panel it touches from flexing (regardless of what causes that flex) over significant area of that panel, as flex robs output by up to 3dB.
It's also irrelevant which way the panel flexes, either out or in, as that is still flex.

To that end, you would need to add lock nuts internally on the threaded rod to achieve that, even then, the rod itself will flex...
Built:
DR 250: x 2 melded array, 2x CD horn, March 2012 plans.
T39's: 4 x 20" BP102 , 2 x 28" 3012lf.
WH8: x 3 with melded array.
Bunter's Audio and Lighting "like"s would be most appreciated...

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Bill Fitzmaurice
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Re: Bracing101 2021

#9 Post by Bill Fitzmaurice » Tue Apr 13, 2021 7:14 am

camp10 wrote:
Mon Apr 12, 2021 8:53 pm
I was almost certain that the pressure swing in vented design was less than a sealed considered the pressure is relieved by the vent..
That's a common misconception. The vent doesn't relieve pressure. It increases it, because it's not a simple hole in the box, it's a Helmholtz resonator. Many is the bass player that uses a sealed cab because they think it controls excursion better than vented. The opposite is true. This is related to how a vent works. The usual assumption is that the rear wave from the driver escapes the enclosure via the vent. It doesn't. The air mass within the vent vibrates back and forth, just like a driver cone, creating its own wavefront that emanates from where that air mass meets the outside air.

camp10
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Re: Bracing101 2021

#10 Post by camp10 » Tue Apr 13, 2021 7:25 am

Each rod goes through a threaded insert at every baffle...locknuts hold the woofers, there is a good amount of tension on each rod, but I get your point. I've got to figureout how to keep the rods from ringing at this point...I'm hoping tape will do.

Thanks to the bracing provided by the threaded and my new info regarding brace spacing (8"-12" separation) The side panels are near finished...The top/bottom baffles are 12" wide so, bracing is not needed there. I think a few cross members and some ribs jutting out several inches from the back wall running top to bottom will take care of the back baffle which is 32Wx29H...the front baffls are 12" wide and so no bracing needed there either.

camp10
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Re: Bracing101 2021

#11 Post by camp10 » Tue Apr 13, 2021 7:39 am

Bill Fitzmaurice wrote:
Tue Apr 13, 2021 7:14 am
camp10 wrote:
Mon Apr 12, 2021 8:53 pm
I was almost certain that the pressure swing in vented design was less than a sealed considered the pressure is relieved by the vent..
That's a common misconception. The vent doesn't relieve pressure. It increases it, because it's not a simple hole in the box, it's a Helmholtz resonator. Many is the bass player that uses a sealed cab because they think it controls excursion better than vented. The opposite is true. This is related to how a vent works. The usual assumption is that the rear wave from the driver escapes the enclosure via the vent. It doesn't. The air mass within the vent vibrates back and forth, just like a driver cone, creating its own wavefront that emanates from where that air mass meets the outside air.
Hi Bill! I don't know if you remember me but I have crossed your path many times in the past at PE forums...actually you and Wolf are the only ones I have found memories of from that place, and I wish your presence was at diya forums, I think you are Genius and ty for contributing to advancement of DIY! :clap:

I opened up Hornresp and ran a few simulations, it shows what has been said here as truth regarding pressure of sealed vs vented. There are areas where the pressure of the vented enclosure are lower than sealed of the same volume but it is well away from the tuning note for sure (its hard to say 100% through hornresp..with sealed alignments it shows pressure at diaphragm but not exactly box pressure...for vented design I can see pressure at diaphragm, horn throat, and horn mouth..maybe thats all that needs to be seen) the pressure peak rises as the tuning note lowers as well. I feel like I once knew this and forgot? I've ran so many simulations over the year or two...I think one thing that just clicked for me is the connection between pressure and exaggerated decay. Where I see the pressure climb in the sealed and vented design is also the areas where decay extends

You did not comment on the bracing spacing concept...that suggest to me that the discussion is on the right track :D

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Bill Fitzmaurice
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Re: Bracing101 2021

#12 Post by Bill Fitzmaurice » Tue Apr 13, 2021 7:57 am

On the bracing the last picture is the best scheme. Six inch spacing is adequate.

camp10
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Re: Bracing101 2021

#13 Post by camp10 » Tue Apr 13, 2021 11:21 am

Ty kindly for the quick spoon feeding

I wonder how you came to this conclusion for spacing...has it any relation to sound modes?

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Bill Fitzmaurice
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Re: Bracing101 2021

#14 Post by Bill Fitzmaurice » Tue Apr 13, 2021 11:49 am

Experience.

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Seth
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Re: Bracing101 2021

#15 Post by Seth » Tue Apr 13, 2021 11:55 am

camp10 wrote:
Tue Apr 13, 2021 11:21 am
Ty kindly for the quick spoon feeding

I wonder how you came to this conclusion for spacing...has it any relation to sound modes?
You seem the type to really dig in and learn about stuff. You may find some insight in researching the term Flexural Modulus
Build in process - 2 WH6, one Alpha 6a loaded, one PRV Audio 6MB250-NDY loaded

Two 2x6 shorty SLA Pro's
One T39, 16", 3012LF loaded
Tall AutoTuba, 20" wide, 2x 8" MCM 55-2421
TruckTuba, 8½" wide, 2x 8" MCM 55-2421

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