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MJ-18 - Box / Amp specs? (1 Viewer)

bawward

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Brian Ward
Hey there - new to the forum but I've lurked for over a decade.

Background:
I bought a Mach 5 Audio MJ-18 almost ten years ago for an IB install in my wagon (seen at the bottom of this post)

Now, I'm moving the MJ-18 into the house - my reliable Polk Audio PSW505 had it's amp crash. I have a lot of experience with wood, fiberglass, etc - but a wise man knows when to ask for advice!

The Room:
The media space is about 15'-0" x 11'-6", 8' ceilings (1380 cu ft.) (See image below) I really don't know what the room gain is like, or at what frequencies. Subwoofer location shown - but I can adjust if it would help. I love the idea of IB, but at this time it just won't work - so I'm building a box!

The Purpose:
This will be in a room where 90% HT use, 10% music. As such, I'm planning on going ported. I'd like to tune it between 15-20hrz (flexible on this)

Sound Profile Desired:
SPL desires slightly outweighs SQ, however I have a feeling there's a nice 'middle ground'.

--------------------

Driver (Already Purchased)

Mach 5 Audio MJ-18 - 4ohm
Fs = 28 Hz
Re = 3.2 Ohms
Qes = 0.34
Qms = 4.99
Qts = 0.31
Mms = 229 grams
Rms = 8.1 kg/s
Cms = 0.14 mm/N
VAS = 208.4 litres
Sd = 1029.2 cm2
Xmax = 15 mm
Power Handling: 600 Watts
SPL (2.83v/1m): 93.4 db


Amp:

iNuke DSP1000

General concern - being able to get information below 20hrz to the sub - this amp has a high-pass at around 20hrz. I believe the iNuke DSP's can be adjusted to allow for this.

----------------

Questions:

Box: (Please provide input)

-I can go as large as I need to. roughly rectangular (equal depth/height preferable) - I'm assuming min. 5 cu ft, but thinking 7 cu ft might be better?
-Could be down-fire / front - doesn't matter (and I know this is arbitrary)
-Slot or 3"/4" flared tube

Possible / Recommended Box Sizes (this is from Mach 5 site):
Sealed: 80 litres (2.8 cu. ft.)
Ported: 128 litres (4.5 cu. ft.) 30Hz tuning
**EBS: 250 litres (8.8 cu. ft.) 18Hz tuning** (This one is the most interesting to me)
 

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bawward

Grip
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Hey guys, any suggestions/thoughts on the sub enclosure design, tuning, etc would be great!

If I wasn't detailed with the goals/equipment/desired use, just let me know! Hoping to start cutting some wood tonight!
 

Robert_J

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I was going to suggest another forum but I see you already posted there. Give it another day or 2 for a response. With the holiday, I have been off-line for a full week.

For anyone reading this, just Google "Full Marty subwoofer". I have custom 18" sub from Fix My Speaker dot com and I'm at the 90% sure I'm going to build a full Marty using it. It's a very flexible design. If the response isn't completely flat, then you (and I) have the ability to tweak it with DSP.
 

bawward

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Just got finished doing a bunch of modeling in WinISD for the first time.


250L (8.8 cu ft) is the 'recommended' size (250L) - shown in 'red' on the graphs.

566L (20 cu ft) box - shown in 'blue' - is the size I think I'm going to build


Modeled with:
-600rms to the sub (despite the iNuke putting out 1000rms)
-(1) 4" round port (ends up being 7.36 cm long)
-Box tuned to 21.50 Hz
-Xmax is almost reached at 15Hz and at 28Hz, but not surpassed

Question:
Could I get some quick confirmations that this design is 'safe' for the equipment, and that I'm not missing anything obvious before I cut the wood?
 

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Robert_J

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It's good that you modeled with 600 watts because that's about what the amp puts out. It's well known that the Behringer amps do about 60% to 80% of their stated power.

One 4" port will chuff like crazy. You need at minimum three 4" flared ports. 4" ports have 12.5 square inches of area while the Marty designs at that other site use a slot port with 63 square inches of port area. That's slightly more than 5 ports and will eliminate all chuffing.

If you are going with the 20 cu ft box then you want to tune much lower. I mean like 12hz tuning. Read a little bit on LLT (Large Low Tuned) subwoofer design.
 

bawward

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SO - changes! (Haha, are we surprised?)


I made a card board template of the 550L box - wife said NO!

So - we're going to go down the 250L (8.8 cu ft) road.


Here are the most recent graphs. I think I'm going to go with 250L, 20hz tuning (Green) instead of the others.

Green = 20hz
Red = 18hz
Blue = 15hz


Constants:
Mach 5 Audio MJ-18M
600 watts power (limited)
250L
High Pass filter @ 15hz
Low Pass filter @ 80hz

Ports:
I'm good with adding more ports -
(1) 4" (10.20cm) port gets 1st port resonance of 1011.91 Hz
(2) 4" (10.20cm) port gets 1st port resonance of 414.77 Hz
 

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bawward

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^ That was a great recommendation. I'm going to go with the 'Johnny Cube' - and that size/tune ends up being nearly the exact same thing I came up with.

It's good to know that the direction I was heading makes sense! Thanks for your input Robert
 

bawward

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Well, I ended up getting doing some work on the box this weekend (along with getting a Christmas tree and roughly 12" of snow)


Final Box Specs:


Size
33"x24"x24"
= 9.229 CuFt (263.0 L) - 0.5CuFt (bracing/driver) = 8.79 CuFt (248.9 L) net volume

Tune
18.5Hz

Vent
2.5"T (7.62cm) x 22.5"W (57.15 cm) x 53.5"L (136cm)
 

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bawward

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Pic 1
As with any good project, it starts with a plan (cut list / layout) and Cheez-It's! (too early for a beer - that was later)

Pic 2
Engine cart serves well as a tool-cart - first box I've ever built with a nail gun, it went SO FAST!

Pic 3
It starts!

Pic 4
Three walls and vent installed w/ supports

Pic 5
Braces cut up - they got a quick sanding before install (easiest to simply screw them together and make one cut so each window is identical)

Pic 6
Braces installed, keeping support roughly every 10"-11"

Pic 7
All sides complete and box set for clamping/drying.


Triple-baffle is yet to be installed, along with the casters / finish work.
 

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bawward

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Made a couple hours of progress last night - Got a round-over bit for the router and got out the random orbital - it's looking awesome!
 

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Mike Frezon

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I ran back through the thread but couldn't find a reference...

Are you using MDF to build the box? Because it looks really nice.

Maybe it's that orbital sander that is making the difference! :D
 

bawward

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Sure enough - I suppose I never mentioned that!

3/4" MDF for the walls / bracing / vents. The a triple-thickness baffle (the sub won't be recess mounted, but surface mounted)

That new router bit I got cuts very nicely, could help! And the random orbital was primarily for the wood putty @ the nail hole locations. However, it got the entire box to a nice consistent 'smoothness'.


For the Finish:
Planning on actually rolling on oil-based wood primer. Then a couple coats of black-ish paint over that. Probably going for a matte/flat finish (don't want reflections off the enclosure during dark-room movie viewing)

There will be some sort of hardwood element wrapping around the box - along with maybe some 16ga metal panels / inserts - not sure exactly yet - here are some phone doodles
 

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Mike Frezon

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Nice.

Industrial is very "in" right now.

Can't wait to see your finished project.

Yeah...that "consistent smoothness" was what caught my eye. Looks like it's ready to take a nice oil wood stain to become a beautiful, natural-wood piece of furniture.
 

bawward

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Yeah - it's sorta 'industrial' - but without any exposed fasteners / heavy gauge metal. Trying to make it rather modern-ish / clean (but also 'mountain) - Below are two examples of homes I've work on - kind of explains my goal.

I've never stained MDF... I'll look into it!
 

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Mike Frezon

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Wow. Those are beautiful homes.

I'm sure the matte black finish on the sub box is the right way to go. I was just sayin' that the finish on that MDF looked good enough to hold a stain and become a beautiful heirloom-type piece. But I could be way off base with how it would take a stain.
 

bawward

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Yeah, even if I had the money, not sure I would build a second home :blink: - most of these are for a family of (2) anyway!

In regards to the stain, I don't think you're off base. I did a bit of research last night about staining MDF, and there seems to be two ways folks do it.

1. Use Gel stain, put it on inconsistently, creating your own 'graining'
2. Go for a consistent, almost 'flat' looking stain coloring without graining

Or, you could always do veneer, etc. Not sure which way I'm heading honestly.

I've seen people rip-off this bluetooth speaker cube!
 

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Mike Frezon

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I saw a fellow last night on HGTV's Fixer Upper show make a nice woodgrain finish on an exterior concrete patio slab! (It involved sprinkling some brown powdery substance onto the concrete as it was curing and then using a press to form the "grain."

So I guess anything's possible.

But I digress and am veering off-topic! I'm sure your original plan to use a dark paint is the best way to go.
 

bawward

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Finished up the baffle the other night, got it all put together, glued, etc. Then primed it up! (2 coats, light sanding between priming coats)
 

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