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Discussion Starter · #1 ·
OK, I've built the box 7.2 cu. ft., H22.5" x W22.5" x D24.5". Material is 3/4" MDF. Box is glued and screwed with additional reinforcing strips of 1.5" x 3/4" MDF along all wall, top, bottom, back, and sides. There are additional interior 1.5" x 3/4" MDF braces installed from sidewall to sidewall and top to bottom interconnected at the inside center of box with glue and screws.
The TC Sounds LMS-5400 18" speaker is mounted in the center of the H22.5" x W 22.5" front baffle piece which leaves 3" from speaker-edge to box side edge and 6" from speaker-edge to box corner. That's NOT a lot of wood to vibrate or flex here. The largest unreinforced area on the top, sides, back, or bottom are no larger than 11" x 12".
The box weighs about 50lbs. Speaker and box together weigh 130lbs. It doesn't walk or vibrate it's way across the room or anything like that.
What I've noticed since I recently powered it up for the first time with a Crown XTi 4000 amp (1200 watts per channel) is that the box flexes when the speaker is driven close to it's maximum . All the walls flex or at least vibrate. The front speaker baffle, has a different resonant frequency than the sides, top, and back. Each wall piece has it's own resonant frequency from what I can feel with my hand.
This vibration is very noticeable at frequencies in the 20Hz to 70Hz band and not at 10Hz where the speaker cone's movement is easily visible and startlingly huge. I don't know if the box vibration contributes to distortion or not. I assume it does however it must be at a much lower spl than what the speaker is delivering at the same frequency of vibration. I can't "hear it" but I can feel it with my hand.
If the box and speaker vibrate at the same frequency IE: Frequency Hz of the box matches and changes with the frequency Hz of the speaker is there any harm done to the overall reproduction of the speaker box combination.
This speaker and amplifier combination is so powerful that I don't know that doubling the thickness of the box walls and adding more and thicker braces on the inside will totally eliminate box wall flexing.
Is there a way to measure the amount of flex in inches or milimeters. I'm curious if the amount of flex I feel is in the hundredths of an inch or thousandths.
Any thoughts?
 
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I can't offer much advice on the subject other than to say :yikes:

I can feel box vibration with my tiny tc1000 12" sub running off 200watts in a quick 3/4 ported cab I built for testing. I can only imagine what that beast is doing. :hail:

For subs not quite as capable as yours, I've seen many use 1" - 1.5" mdf/plywood for the sidewalls with bracing. Though as you said, that's a lot of power you're trying to tame.

My suggestion (not worth much compared to these guys) would be to double up on mdf and shelf brace across your cabinet (22.5x22.5x.75 with holes). The distance for spacing them apart is another question for those more knowledgeable than myself. :scratch:
 

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The force created by a specific driver is a constant, with the maximum being created when the driver moves at full excursion limits. What you are left with in regards to how much pressure is being exerted in the enclosure is the volume of the enclosure itself, as P = F/A. This means that the smaller the enclosure you build for a given driver, the more rigid it has to be - the larger it is, the less rigid it needs to be. At roughly 23"^3 using single layered 3/4" MDF without any window bracing (from what I can tell) using a LMS 18", you are obviously gonna be in trouble.

You'll probably need double thick walls, a triple thick baffle, and two window braces (one in each direction) to get that thing to be reasonably inert. More work, but that's what happens when one goes small and sealed. You sealed guys....:daydream: :sarcastic::bigsmile::innocent:
 

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You're going to need some sort of laser vibrometer to do that with any accuracy.
Just hang an old fashioned micrometer dial guage rigidly on something which will place the measuring tip on the panel in question. The tip may not follow the vibrations accurately but it will probably read peak to peak for you if high accuracy isn't necessary. :nerd:

ThomasW would probably suggest you line your MDF box in 3/4" OSB (flakeboard) or 3/4" plywood. Bracing is another way. (which has already been mentioned) Screwed rod between opposing panels with big washers and nuts inside and out would stiffen up the box nicely if you don't mind the Frankenstein's monster look.:gah:

I haven't noticed much vibration in my 4x 15" IB manifold despite using a single thickness 3/4" veneer plywood. My box used every square inch of an 8' x 4' and was already a bit too heavy for me to slide it safely up the ladder and in through the window! :dumbcrazy:
 
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You'll probably need double thick walls, a triple thick baffle, and two window braces (one in each direction) to get that thing to be reasonably inert. More work, but that's what happens when one goes small and sealed. You sealed guys....:daydream: :sarcastic::bigsmile::innocent:
What he said ^

(Window Braces! That's the term I was looking for.)
 

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Discussion Starter · #8 ·
Window Braces. Yes like this:



In a later assembly step I also added a bunch of cross braces for the other direction.
That's a beautifully constructed and designed box. Was that for an 18" with two passive radiators? Why did you add braces later? Did the added braces reduce some kind of vibration? Were they effective in some way?
 

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Discussion Starter · #9 · (Edited)
I built the box assuming I might have to add more bracing or even double up all the surfaces on the outside. I didn't expect the power of the speaker and amplifier to produce the kind pressure in the box that it has. I'm still not sure if I'm looking at vibrations within the unsupported pieces of the box or if the entire box is being puffed up like a balloon and deflated the same way.
I'll start by adding a magnet brace to hold the speaker magnet snuggly in place against the force of gravity and also a brace from the rear/back panel to the back surface of the speaker magnet.
Final step will be to line the outside of the box with another layer of MDF or plywood.
Is there anyone (and this is a serious question) who has built a sub enclosure out of other material than wood? I've built some thin walled concete projects that I placed foam billets in to save weight. I've also seen boxes with sand filled wall cavities.
 

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That's a beautifully constructed and designed box. Was that for an 18" with two passive radiators? Why did you add braces later? Did the added braces reduce some kind of vibration? Were they effective in some way?
It was for a 15" driver (currently a CSS SDX15) and two 18" passive radiators.

The cross braces just contribute to making the enclosure very solid and reduce front baffle and back wall flexure. What I did was nothing special for a DIY project (just followed many other's great projects and guidance). Granted it is overdesign and makes it very heavy. Would be difficult to justify doing for a commercial subwoofer vendor. But DIY'ers get to do overkill and eek out that extra bit of no-added resonances performance. When you make the enclosure uber solid then any remaining resonances are much higher in frequency and do not get excited in a sub. No extra added unwanted sounds -- less local color added. :cool:
 
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