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bob_m10 02-02-2016 07:41 PM

Bracing again
 
1 Attachment(s)
Hi,

I know the subject of bracing has been talked about many times, but have a few questions in order to understand the basic concepts.

1) I have learned that bracing a sub-woofer box as the effect of raising the cabinet’s resonant frequency. My understanding is this will move unwanted sound due to vibration above the sub’s passband. In addition this will also make it difficult for the sub to vibrate the box because the resonant frequency will be above what the sub’s driver can produce. If true, won’t this simply add unwanted color higher up the range and interfere with the main speakers’ passband?
2) When I wrap on a speaker box, is the sound that I am hearing the panel vibrating at its resonate frequency?
3) Lastly, the attached picture is similar to the type of bracing I would like to use in a small sub cabinet I am building. Is there anything particular wrong with it?

Thanks Bob

Bill Fitzmaurice 02-02-2016 07:55 PM

Quote:

Originally Posted by bob_m10 (Post 41210833)
1) I have learned that bracing a sub-woofer box as the effect of raising the cabinet’s resonant frequency.

That does happen, but it's a by-product of what bracing really does, which is to make the cab stiffer.
Quote:

2) When I wrap on a speaker box, is the sound that I am hearing the panel vibrating at its resonate frequency?
Pretty much, although you'll also hear any resonances inside the box unless it's well damped.
Quote:

3) Lastly, the attached picture is similar to the type of bracing I would like to use in a small sub cabinet I am building. Is there anything particular wrong with it?
You're using at least three times the necessary material. This scheme gives as good a result as you can get but with the minimum of material:
http://www.billfitzmaurice.info/imag...es/bracing.jpg

It shows round dowels, but square or rectangular are OK as well.

wvu80 02-02-2016 08:25 PM

Quote:

Originally Posted by Bill Fitzmaurice (Post 41211153)
You're using at least three times the necessary material. This scheme gives as good a result as you can get by with the minimum of material:

It shows round dowels, but square or rectangular are OK as well.

Bill, am I correct that hard wood is preferred for the dowels/bracing? My understanding is that long strips of MDF is not desirable, I presume due to the fact that it isn't strong that way.

bitmap42 02-02-2016 08:31 PM

The style with the braces from front to back like Bill showed also helps with the issue of what to screw the woofer into. The 4 front to back braces can be positioned so that they line up with four of the woofer screw holes. This gives you a really solid way to attach the woofer without having to use threaded inserts or additional small blocks on the back of the baffle.

If you use round you can use a small forstner bit on the back of the sub to recess the end about 1/8 of an inch and it will sit well for gluing.

bob_m10 02-02-2016 10:04 PM

Quote:

Originally Posted by Bill Fitzmaurice (Post 41211153)
That does happen, but it's a by-product of what bracing really does, which is to make the cab stiffer.
Pretty much, although you'll also hear any resonances inside the box unless it's well damped.

So why is it important to get the resident frequency outside of the subs passband? If we do, can the driver get the panels to vibrate? How are the drivers different from me wrapping my knuckle on the side of the cabinet?

if ones does use shelf type braces, do you have to worry about getting to close to the driver? thanks -Bob

Bill Fitzmaurice 02-03-2016 06:27 AM

Quote:

Originally Posted by bob_m10 (Post 41214089)
So why is it important to get the resident frequency outside of the subs passband?

It isn't. You could have the panel frequency right at Fb with no ill effect provided that the combination of mass and stiffness was sufficient that it would not vibrate. But that combination is far less practical than using methods which also happen to raise the panel frequency.
Quote:

Bill, am I correct that hard wood is preferred for the dowels/bracing? My understanding is that long strips of MDF is not desirable, I presume due to the fact that it isn't strong that way.
MDF is fine. All of the forces are on the brace axis, so it only has to resist being either compressed or stretched, not flexed. I don't use dowels myself, I use strips of whatever material the box is made of. The advantage to dowels is when you're retro-fitting a cab that wasn't properly braced. Cut them a little short of the required length, then glue them in, using a small wooden wedge on one end to get a tight fit.

Quote:

if ones does use shelf type braces, do you have to worry about getting to close to the driver?
They can't touch it. If you use shelf braces there's a right way and a wrong way. In the pictures below the left image is the wrong way, as it uses more material but is less effective than the right image. A complication of window braces is the added difficulty of having them connect the top and bottom as well as the front, back and sides.
http://www.billfitzmaurice.info/imag...indowbrace.jpg

chadamir 02-03-2016 08:28 AM

Quote:

Originally Posted by Bill Fitzmaurice (Post 41211153)

It shows round dowels, but square or rectangular are OK as well.

Bill, would that much be ok even for half inch mdf walls? Also, would you make any modifications if the walls are about ~2-4 feet away from each other at points?

Bill Fitzmaurice 02-03-2016 08:38 AM

Quote:

Originally Posted by chadamir (Post 41220753)
Bill, would that much be ok even for half inch mdf walls? Also, would you make any modifications if the walls are about ~2-4 feet away from each other at points?

With 1/2 inch MDF I'd have brace spacing no more than six inches. That's the equivalent of 1" MDF with braces spaced twelve inches or a 24 inch span of unbraced 2" MDF. The distance between panels doesn't make much difference as far as bracing goes, but four feet could be long enough for a 1/2" thick brace to bow if you're using leftovers, so I'd make them T or L shaped using a pair of inch and a half wide wide strips.

bob_m10 02-03-2016 12:10 PM

1 Attachment(s)
Quote:

Originally Posted by Bill Fitzmaurice (Post 41217809)
They can't touch it. If you use shelf braces there's a right way and a wrong way. In the pictures below the left image is the wrong way, as it uses more material but is less effective than the right image. A complication of window braces is the added difficulty of having them connect the top and bottom as well as the front, back and sides.
http://www.billfitzmaurice.info/imag...indowbrace.jpg

I see what you are saying, still not as good as the one at the right but if you change the squares to circles would that be a bit better for the shelf on the left?

Bill Fitzmaurice 02-03-2016 12:39 PM

Quote:

Originally Posted by bob_m10 (Post 41228249)
I see what you are saying, still not as good as the one at the right but if you change the squares to circles would that be a bit better for the shelf on the left?

You still have only one connection of the opposing panels, not two, and most of the brace material is in the corners of the cab, where it's of no benefit, as that's where the cab is most resistant to vibration. What matters is the span between panel to panel connections.

fbov 02-03-2016 01:56 PM

There are multiple approaches to bracing. Bill's advice is highly pragmatic; most bang for the brace.

I have done both ways, and see no clear winner until it comes time to move the box, then Bill wins.

Have fun,
Frank

chadamir 02-03-2016 03:26 PM

Quote:

Originally Posted by Bill Fitzmaurice (Post 41221041)
With 1/2 inch MDF I'd have brace spacing no more than six inches. That's the equivalent of 1" MDF with braces spaced twelve inches or a 24 inch span of unbraced 2" MDF. The distance between panels doesn't make much difference as far as bracing goes, but four feet could be long enough for a 1/2" thick brace to bow if you're using leftovers, so I'd make them T or L shaped using a pair of inch and a half wide wide strips.

Thanks. So it is not necessary for one wall to connect to a parallel wall via braces? My height is only 13 inches. Should I brace every 4 inches or just one smack in the middle? Should they all be an inch and a half wide?

noah katz 02-03-2016 04:42 PM

Quote:

Originally Posted by Bill Fitzmaurice (Post 41217809)
It isn't. You could have the panel frequency right at Fb with no ill effect provided that the combination of mass and stiffness was sufficient that it would not vibrate. But that combination is far less practical than using methods which also happen to raise the panel frequency.


Right, so since we need to be practical, it is important.

The purpose of stiffness is to reduce vibration amplitude, and if the panel resonance is in the passband it's increased many times.

corradizo 02-03-2016 05:48 PM

I don't see allot of people using an app like boxnotes to predict their cabinet resonances. Would be good to use so you know what you're dealing with.

http://www.subwoofer-builder.com/freesoft.htm

Bill Fitzmaurice 02-03-2016 07:56 PM

Quote:

Originally Posted by chadamir (Post 41233993)
Thanks. So it is not necessary for one wall to connect to a parallel wall via braces?

Only if it's six inches or less across.
Quote:

My height is only 13 inches. Should I brace every 4 inches or just one smack in the middle?
What's the full panel dimension?
Quote:

Should they all be an inch and a half wide?
If the braces are 13 inches long 1 1/2" x 1/2" should be fine.

chadamir 02-03-2016 08:09 PM

Quote:

Originally Posted by Bill Fitzmaurice (Post 41240913)
Only if it's six inches or less across. What's the full panel dimension? If the braces are 13 inches long 1 1/2" x 1/2" should be fine.

Panels are 13x47.5 13x27.75 and 27.75x13.

michaelddd 02-03-2016 08:26 PM

101 Attachment(s)
Quote:

Originally Posted by corradizo (Post 41238121)
I don't see allot of people using an app like boxnotes to predict their cabinet resonances. Would be good to use so you know what you're dealing with.

http://www.subwoofer-builder.com/freesoft.htm

Thank you for linking this. I've been on that site many times and never saw that software. It is very useful, especially the box resonances part of the program. :cool:

corradizo 02-03-2016 08:53 PM

Happy to help!

Bill Fitzmaurice 02-04-2016 06:05 AM

Quote:

Originally Posted by chadamir (Post 41241177)
Panels are 13x47.5 13x27.75 and 27.75x13.

You'd need braces centered width wise on the panels spaced every six inches.

chadamir 02-04-2016 07:25 AM

Quote:

Originally Posted by Bill Fitzmaurice (Post 41247577)
You'd need braces centered width wise on the panels spaced every six inches.

So one in the middle of the 13 should be ok or do I need one at 6 and one at 12 inches?

Bill Fitzmaurice 02-04-2016 09:49 AM

Quote:

Originally Posted by chadamir (Post 41249057)
So one in the middle of the 13 should be ok or do I need one at 6 and one at 12 inches?

If you have one brace in the middle of a 13 inch panel that leaves less than 6.5 inches to either side of it. That should be adequate. If you wanted a higher level of bracing you'd use two braces evenly spaced, leaving less than four inches between them and between each and the corner joint.

chadamir 02-04-2016 10:00 AM

Quote:

Originally Posted by Bill Fitzmaurice (Post 41253265)
If you have one brace in the middle of a 13 inch panel that leaves less than 6.5 inches to either side of it. That should be adequate. If you wanted a higher level of bracing you'd use two braces evenly spaced, leaving less than four inches between them and between each and the corner joint.

Thank you so much, Bill. Would any of your answers change if I said that I'm using these as speaker stands and they'll have about 50-60 pounds worth of speakers on them?

Bill Fitzmaurice 02-04-2016 01:22 PM

Quote:

Originally Posted by chadamir (Post 41253697)
Thank you so much, Bill. Would any of your answers change if I said that I'm using these as speaker stands and they'll have about 50-60 pounds worth of speakers on them?

Are they speakers or are they stands? There's no internal pressures on stands.

chadamir 02-04-2016 01:51 PM

Quote:

Originally Posted by Bill Fitzmaurice (Post 41260113)
Are they speakers or are they stands? There's no internal pressures on stands.

I plan to use the subs as speaker stands. Also, would one inch round poplar dowels work just as well? I don't really want to deal with mdf dust everywhere.

Bill Fitzmaurice 02-04-2016 03:57 PM

Quote:

Originally Posted by chadamir (Post 41260961)
I plan to use the subs as speaker stands.

Sub and mains placement in the same place usually ends up badly. One of the reasons why the sub was invented was so they could be placed where they work best and mains could be placed where they work best, and very seldom the twain do meet.
Quote:

Also, would one inch round poplar dowels work just as well?
They would. I use leftovers from the build because they're free.

rhodesj 02-04-2016 04:45 PM

Quote:

Originally Posted by chadamir (Post 41260961)
Also, would one inch round poplar dowels work just as well? I don't really want to deal with mdf dust everywhere.

I'd use something square, cheap 1x1 or whatever, so I can glue them to the other braces where they meet.

chadamir 02-04-2016 05:02 PM

Quote:

Originally Posted by Bill Fitzmaurice (Post 41265529)
Sub and mains placement in the same place usually ends up badly. One of the reasons why the sub was invented was so they could be placed where they work best and mains could be placed where they work best, and very seldom the twain do meet. They would. I use leftovers from the build because they're free.

There will be four woofers in the front and two right behind the couch. All of them will be placed slightly off center. It's where they can go *shrug*

bob_m10 02-04-2016 05:11 PM

Quote:

Originally Posted by Bill Fitzmaurice (Post 41229217)
You still have only one connection of the opposing panels, not two, and most of the brace material is in the corners of the cab, where it's of no benefit, as that's where the cab is most resistant to vibration. What matters is the span between panel to panel connections.

Hi Bill curious where the 6" rule came from? Thanks -Bob

Bill Fitzmaurice 02-04-2016 05:41 PM

Quote:

Originally Posted by bob_m10 (Post 41267489)
Hi Bill curious where the 6" rule came from?

Experimenting.

ronny31 02-04-2016 07:18 PM

I find it funny every time someone tries to brace mdf with mdf, as well as trying to brace mdf with other materials.
It has the properties of wet toilet paper, the only way to achieve true rigidity is to completely coat (dope) the inside surface with for example polyester resin and glass fiber mats. Even then the glassfiber enclosure with and without the mdf on the outside is practically identical in rigidity (might be 25% MPa more tensile strength with the Mdf sheet on top of the 100+ MPa tensile strength of the glassfiber composite itself). Mdf is used for practicality and price, not db output nor sound quality. Every time you increase the rigidity of the enclosure you increase db output and lessen noise, as well as make the difference smaller between the simulated enclosure and the real thing because the simulation software assumes infinite rigidity. In a perfect world we'd use glassfiber composite sheets. But the rarity of the use of such material in this way means its expensive to find pre-made sheets of that material.


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