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Discussion Starter #1 (Edited)
OK, Kevin has talked me into (read upsell (also read - he didn't have to push too hard) buying a 21" Maelstrom-X and building a sealed box over going with a ported 18". My current setup is a Wicked One folded horn with a pair of JL Audio 10" drivers...which sounds great, but it drops like a rock below 30hz. The 21" Mael should easily replace the folded horn, especially when pushed with the EP2500.

My current box, being a dual folded horn, is basically a coffee table format (but placed in the room like an end table next to a couch and wall). It's sized at 36" X 36" X 14" It looks like this:



The 21" is going to be in a similar format, simply because I like that shape, and it works well in my livingroom/theater. Kevin suggested 10-12 cf sealed, believe it or not! So my box is going to be looking like 36" X 36" X 20" which should give me about 12 cf after bracing and sub displacement. (correct me if that sounds wrong). I could go bigger, but the man himself said no need. (an old photo, quickly chopped in MS Paint)





I haven't learned Google SketchUp yet, and I typically do 90% of my designing in my head. Maybe, if I have time, I'll see if I can figure this out and do a digital drawing...

I have a decent, basic shop in the garage, and I'm a capable (though out of practice) woodworker and I enjoy fun projects.








So, for my Mael-X 21" build, I'd love to do something lighter then my MDF monster of a folded horn. That thing is a beast to move around. I was sore for days after tumbling it around the living room to measure and listen for the best sub location...So I'm hoping to make the 21" box a bit lighter. I'd also love to do something fun like install an Acrylic side panel on the sealed box, with a small cold cathode tube inside to show off the bracing I plan on building....but I don't think my budget would allow for a $100 piece of plastic, just to show off my fancy wood working.

That said, I'm thinking I might have to mix MDF and plywood to lighten the box and keep costs down at the same time. I'm thinking doubled up 3/4" plywood for whatever face I put the driver on (I'm still debating narrow face or wide face), MDF on all other panels, and 1/2" plywood bracing throughout.

Does anyone see any issues with mixing MDF and Baltic Birch Plywood?
What about using 1/2" bracing throughout? Since bracing is used to break larger (lower resonating) panels into smaller (higher resonating) panels, I don't see the need for 3/4" bracing. I think a better design should take the place of MOAR WOOD!

I'm also planning on making the bracing interlock into the side panels. Just cutting some dato slots that the bracing would fit into. A little glue, and it's be solid like a puzzle piece.

Again, this is all just thinking out loud. Feel free to chime in, or ignore completely. Chances are I'll do whatever floats my boat...but I'm always open to ideas and thoughts that would help me avoid mistakes.
 

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My comments:

1. You gonna give us a good shot of your Christmas socks this year or what? :bigsmile:

2. Those diffusers sure are purdy!!

3. Resonance

I had this discussion with another member here a while back and it is my current understanding that putting in braces to reduce panel size does not change the resonant frequency of the panel. Resonant frequency is determined by density and stiffness.

I have been told that resonance is described by mass spring theory. This seems to be backed up by formulas given for panel absorber design. You change the tuning of the panel not by changing the area of the front panel, but by changing the depth of the air cavity (the spring) or the thickness (or density) of the front panel (the mass).

All this leads me to wonder why cross bracing is any more effective than just increasing the thickness of the subs panels.

I'm looking forward to comments from those more knowledgeable than I on the topic.
 

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My comments:

1. You gonna give us a good shot of your Christmas socks this year or what? :bigsmile:

2. Those diffusers sure are purdy!!

3. Resonance

I had this discussion with another member here a while back and it is my current understanding that putting in braces to reduce panel size does not change the resonant frequency of the panel. Resonant frequency is determined by density and stiffness.

I have been told that resonance is described by mass spring theory. This seems to be backed up by formulas given for panel absorber design. You change the tuning of the panel not by changing the area of the front panel, but by changing the depth of the air cavity (the spring) or the thickness (or density) of the front panel (the mass).

All this leads me to wonder why cross bracing is any more effective than just increasing the thickness of the subs panels.

I'm looking forward to comments from those more knowledgeable than I on the topic.
In terms of physics, all mechanical systems have a natural resonance frequency. That is a frequency where the system will maximally absorb energy, when stimulated with that given frequency. If you look a simple system, like a transducer, it is reflected by the resonance peak you see in the response. It is easier to see if you look at the impedance curve, which shows a rising impedance at the resonance.

Panels in subwoofers are the same. There are multiple resonant frequencies, for the various parts of the enclosure based upon the physical properties of the panels. Those properties are length, mass, and stiffness of the panel. Maybe an easier analogy is that of a guitar string. The frequency a guitar string will play (its natural resonance frequency) is based upon the length of the string, the tension (stiffness), and its mass. A lighter, shorter or tighter string all translate to a higher natural frequency. All you have to do to convince yourself is play a guitar string, and tighten it, play the thinner (lower mass) strings and compare them to the thicker (higher mass) strings. If you play a string and put your finger down on part of the fret (making it shorter), you get a higher note.

All of this, directly translates to panels used in loudspeakers and subwoofers. You use braces, to shorten the panel, and raise the natural resonance of that panel. Why raise it? Because you don't want the device that you are designing, to play at a frequency that will stimulate the natural frequency of the materials that make it up. That has the effect of storing energy, and releasing it in ways that do not coincide with accuracy. So we add braces to INCREASE the resonant frequency of a given panel. What other things can we do to achieve the same thing? We can use lower mass, which is counter to what most people are thinking about when building a sub but lower mass materials, will have a higher natural resonance. We can also use stiffer panels, by choosing a different materials that is stiffer than MDF.

The one factor I've left out is damping. Damping is anything that reduces the amplitude of the oscillations in a system. It is the ability to stop the oscillations quicker. A mechanical system that most people understand and can relate to is the shock absorbers in your car. The system has a spring, and a shock absorber that moves a piston through a cylinder filled with oil to create a frictional loss. This system damps the spring oscillation. Take the shocks off a car, and it will bounce happily down the road in and uncontrolled manner. The damping nature of the shocks create a frictional loss, that damps the oscillation. That is the same thing that happens when you add damping to any system, acoustical or mechanical.

MDF has good damping properties, even though it has high mass. Adding thicker panels, actually lowers the resonance frequency, but it increases the damping. Using braces, to effectively shorten the panel raises the resonance frequency.

Make sense?
 

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Kinda. I need to look up my references to panel absorbers at home to make sure I didn't miss something.

I wish I could find something that laid out the math in a 'for dummies' kind of way. I always find it easier to understand stuff when I can work through examples to see how different variables affect things. I need a WinISD for panel resonance. :bigsmile:
 

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Discussion Starter #5
Panels in subwoofers are the same. There are multiple resonant frequencies, for the various parts of the enclosure based upon the physical properties of the panels. Those properties are length, mass, and stiffness of the panel. Maybe an easier analogy is that of a guitar string. The frequency a guitar string will play (its natural resonance frequency) is based upon the length of the string, the tension (stiffness), and its mass. A lighter, shorter or tighter string all translate to a higher natural frequency. All you have to do to convince yourself is play a guitar string, and tighten it, play the thinner (lower mass) strings and compare them to the thicker (higher mass) strings. If you play a string and put your finger down on part of the fret (making it shorter), you get a higher note.
Keven, this is exactly how I understood it, and I use the same analogy as well. So when you take a 36" panel, and put a brace in the middle, you are effectively splitting it into two 15" resonating panels...like fretting a guitar at the 12th fret...raising the pitch of how it resonates.

So, my question still holds, why can't I use 1/2" panels in place of 3/4" panels for the bracing?

All that said, this is academic for me right now, as I found a sale on 3/4" birch ply and bought two full panels of it for the cost of two 1/2" panels. And they weigh a good 40lbs less then equivalent MDF. I did however pick up several 1/8" and 1/4" MDF panels for future sound diffuser projects I'll be working on!

I have all next week off work, I took the week off for my birthday...so if the Maelstrom 21" shows up Friday, I can actually start the build process ASAP! :sn:
 

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Discussion Starter #6
I forgot to respond directly to Fred.

1: I have none, but that would be a fun follow up photo. This photo shown is old, and I've upgraded to a MUCH larger screen, got the speakers up off the floor, and cleaned up a bit in there. I should literally update the photo with my legs in PJs and X-mas socks on. Thanks for the idea!

2: Thank you! More to come!

3. I think we are addressing this.
 

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Keven, this is exactly how I understood it, and I use the same analogy as well. So when you take a 36" panel, and put a brace in the middle, you are effectively splitting it into two 15" resonating panels...like fretting a guitar at the 12th fret...raising the pitch of how it resonates.

So, my question still holds, why can't I use 1/2" panels in place of 3/4" panels for the bracing?

All that said, this is academic for me right now, as I found a sale on 3/4" birch ply and bought two full panels of it for the cost of two 1/2" panels. And they weigh a good 40lbs less then equivalent MDF. I did however pick up several 1/8" and 1/4" MDF panels for future sound diffuser projects I'll be working on!

I have all next week off work, I took the week off for my birthday...so if the Maelstrom 21" shows up Friday, I can actually start the build process ASAP! :sn:
You can use 1/2", but I saw a study once where it was shown not to be as effective. It was one study so it may not hold for all cases but I use 3/4" just to be safe. It doesn't take up that much more space.

Kevin Haskins
Exodus Audio
 

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Fascinating thread.

For bracing I've found Oak 1" x 2" or 1" x 4" to be very effective. Couple that with 4" rockwool on the panels and you have a very good box.
 

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Discussion Starter #9
You can use 1/2", but I saw a study once where it was shown not to be as effective. It was one study so it may not hold for all cases but I use 3/4" just to be safe. It doesn't take up that much more space.

Kevin Haskins
Exodus Audio
I'm just trying to keep the weight down a bit. Using 1/2" and a good lattice style cutout pattern I figure would save several pounds over a 15cu box. But as I said, I got the 3/4 at a great price, so no worries there.

So, going foward, assuming Mal-X 21" with 3/4" birch plywood, somewhere between 12-15 cu feet.

1) I assume I would double up the material on the baffle?

2) Do I line the panels with anything?

3) Should I be stuffing the box with anything, and are those Deep Pockets pillows I see people using a good idea?

4) Anything else I'm forgetting to prep?

I've already started cleaning up the garage/shop and unloaded the wood into the shop. I have a bunch more cleaning to do before I start cutting, not to mention drawing out some plans, maybe picking up some foam or pillows...waiting for the driver and cutting and gluing!
 

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Discussion Starter #11 (Edited)
With bracing I like to keep it asymmetrical.
If your panel is 30 inches I wouldn't use one brace in the centre to create two 15 inch panels (which have the same resonance) but instead offset it if possible, or put in two braces.
I was actually daydreaming about that today. I assume this holds true for subs as well as it does for full range speakers; I've always felt that using primes in the inner dimensions would help with standing waves or resonances.

I remember reading something about using the Fibonacci Sequence somehow in room design for audio (size) as well as speaker design (airspace).

The least I could do is be asymmetrical in my build, and build the box size to a prime rather then a square. Which would mean my internal dimensions should be 37" or 41". Or, come to think of it, I'd have to do one of each so it wouldn't be a square...hmmm. OK, this gives me a plan.

Kevin, are there any issues if I make the height of the box 23", and I put the 21" driver on that 23" face?
 

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Just my two cents, but in a room that open to the rest of the house, you're not really going to get much if any room gain, and with the sub placement, not a lot of boundary gain. You'll need to get an EQ unit to boost the low end if you have any hopes of staying reasonably flat into the teens (another couple hundred dollars).
 

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Discussion Starter #13 (Edited)
Just my two cents, but in a room that open to the rest of the house, you're not really going to get much if any room gain, and with the sub placement, not a lot of boundary gain. You'll need to get an EQ unit to boost the low end if you have any hopes of staying reasonably flat into the teens (another couple hundred dollars).
I would have needed that regardless, no? I figure I'll get one eventually, but for now, this is what I have.

Just reposting this line so Kevin sees it:

Kevin, are there any issues if I make the height of the box 23", and I put the 21" driver on that 23" face?
 
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