second thoughts on the bracing of VOTT cabinets?

[fallot]

Hello, this is my first post, and please pardone me for my poor English. I am in the process of DIY an A5 system. I have all the units I need and now I need to build the cabinets. Most of my friends who are using VOTT speakers braced the cabinets. They swears by it. I have search this forum and did find some inputs on this subject. However, I would like to know if there is a before and after comparison of the cabinets. And any regrets for bracing their cabinets? Pros and Cons? YOur input will be much appreciated.
Thanks

[bowtie427ss]

Let me first preface my post with the disclaimer that i am neither an electronics or acoustics technician and have no formal training in either field.

My basic understanding from a physics point of view is that any cabinet movement or resonance is undesireable. Physical movement of the box is the result of electrical energy that your driver/speaker has effectively converted to acoustic energy, and has now been lost or given up as mechanical motion within the speaker box instead of audible sound waves as intended.

I've seen/read many accounts where it's suggested that certain speaker designs rely on the above mentioned box motion or resonance. I do not subscribe to this school of thought, and in my novice opinion it's simply an excuse for poor engineering/construction.

Before i built my A7's i spent many months gathering as much data and opinion as i could. My decision to build my own boxes was at first based on economics, but as i read and learned more about the design and it's many revisions over the years, as well as some of the other opinions expressed by users i realized i could probably build something of an improvement.

I basically followed the print for the 828H A7 box with the following changes/additions:

The horn flares are constructed using 3/8 fir plywood kerfed every 1/2 inch to allow smooth bending.

I used 3/4 inch hardwood for braces behind the horn flares, the print shows just 2, i used 4 vertically spaced about 3-1/2 inches apart and used generous amounts of glue to ensure very solid construction.

There seems to be a consensus regarding too much internal volume, i addressed this by closing off and isolating the area behind the horn flares by using a mounting baffle that spans the width of the interior of the box.

I also reduced the overall port area by about 15 percent.

My feeling was that a 1 piece back panel is stronger than a 2 piece, the back is reinforced with 2 large 2x4 braces running the verticle length of the back panel.

Also note the hardwood bracing i've added in the lower portion of the box where there are otherwise very large areas of un-reinforced panel. I chose to angle the braces parallel to the bottom horn flare to keep a symmetric look, my sloppy painting has negated that benefit.

Note that a 1/4 inch spacer had to be added to allow the use of non-altec woofers.

[Old Guy]

I pretty much have to back Bowtie's play- any flexing of the cabinet is energy lost, not to mention adding unwanted things.

His idea of sealing off the horn portion has merit. We did the full width baffle board trick, then drilled a hole on each side and filled the area with mix and pour urethane foam. I do this to any void areas when building cabs.

[Charles Chance]

Old Guy,
Just curious - If you fill the voids with foam, is there a purpose in sealing it off first? Won't the foam seal it off?

[Old Guy]

You are certainly correct. However, be aware the full width baffle board add signifigant structural integrity also.

The ones we toured with were fiberglassed all over the outside, functionally making them a one piece cabinet.

[Kinnick]

Hello, I have been lurking in the background for some time.
I also built A7's (515-8G / 902-8B / 511B / 8500 x-over). I built the 828 enclosurer out of 3/4" MDF then surrounded it with 3/4" oak plywood. The horn flares are almost completely solid by using cnc cut 3/4" plywood stacked up from bottom to top (except for the angled area)then covered with 3 layers of 1/8" masonite glued to the curves so i could fit the angle. Where did i screw up--- the back is only 3/4" mdf with huge cut outs to access the drivers. It is extremely weak and the bass sound terrible and i have not had time to fix it.
So---Brace, Brace, Brace whereever you can. Also brace the angle that defines the horn, mine needs it bad.
If i listen to a bass track at low volume, it sound great, but as soon as the music get complex and loud the lows fall apart
If you search "Big Woofers" you can find some photos of my creations.

[gearfreak]

Howdy, First post/newbie.
A7-500 cabs (416-8A/802-8A/511)

So for filling the horn flares, does foam accomplish the same as sand-filling?

Is (e.g.) the self-expanding-foam-in-a-can good for this, or should a specific type be sought?

I would think the mass of the sand would be preferable for the flares and rest of the box - but perhaps it is overkill?

thanks!

[Cal Weldon]

Having listened to both braced and un-braced I found the difference may be different from what you expect. A braced cabinet is the way to go for sure as you really do wish to control cabinet resonances but but an un-braced seems "bassier" (bass, bassier, bassiest) and "bigger sounding". I sure as heck might be wrong but it may be that the Altec engineers, who no doubt understood resonance, left them unbraced to get that extra "bigness". They used underpowered (by today's standards) amps to fill an auditorium so...

Anyway the braced cabinets give you what your ears expect, a tighter, much less muddled bass. More Hi-Fi if you will.

I think the high cabinet resonance can be likened to a large passive radiator.

<font color="#FFFFFF" size="1">[ September 30, 2008 08:36 PM: Message edited by: Cal Weldon ]</font>

[GM]

Originally posted by gearfreak:

So for filling the horn flares, does foam accomplish the same as sand-filling?

<font size="2" face="Verdana, Helvetica, sans-serif">Yes and no. It will keep the voids from resonating, but it won't mass load the cab as sand, Portland cement, etc. will, so I prefer kitty litter as a compromise.

Any of the expanding foams will work, just be careful as too much can bow and maybe even blow, the flares off.

GM

[GM]

Originally posted by Cal Weldon:

I sure as heck might be wrong but it may be that the Altec engineers, who no doubt understood resonance, left them unbraced to get that extra "bigness".

<font size="2" face="Verdana, Helvetica, sans-serif">I have no doubt to cover the near-field where the horn is acting like a direct radiator. This was the problem I had with the 210s. I couldn't sit far enough away to be in their far-field enough to experience anywhere near their full performance capability and having braced them to a fare-thee-well they actually sounded a bit 'thin'/'p i n c h e d', though I didn't fully realize it until they were gone and I just had the same dual 515s in big low tuned cabs. I'll never have compression horns again in a HIFI app, only waveguides (WGs) to control directivity if horn loaded.

GM