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B.M.D. - Beast Mode Devastator

19K views 129 replies 19 participants last post by  xb1032 
#1 · (Edited)
Another version of the Devastator that is geared for 20Hz to 80Hz, it should hit with a lot of authority in that range. Response rolls off around 70Hz, integration with higher crossovers should still be possible, but not as ideal. The additional height allows for a long front chamber which pushes the response of the resonator down a bit. Also the rear chamber gets added volume which helps pick up the low end. I tuned it a little higher since I would rather have the added SPL vs the extension. I can add port lengthening information if anyone is interested pushing the tune down. For this design a pillow should be added between the braces just below the sub woofer since the 48 inch mark has been breached by more than a little :) Power handling on the B&C 21DS115-8 is around 1625 watts (91 volts) which pushes excursion to its max and port velocity to 25m/s. 8.25 inches added to the port brings the tune down a bit under 19Hz if you want the added extension. Graph attached on the bottom of the post.












BMD vs Devastator 1w1m


BMD vs Stormbreaker (11cuft, 18Hz tune) 1w1m


BMD 91volts into 5.1 Ohms 19Hz 3rd order HPF pushing 20mm of excursion over under tune








Rectangle Font Parallel Screenshot Number
 

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#2 · (Edited)
This is the LaVoce SAN214.50 vs the B&C 21DS115-8 at 1w1m.

Dayton PA460 vs B&C 21DS115-8 1w1m



Alternate bracing structure using waste material














 

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#3 · (Edited)
B.M.D. Plywood Dimensions

Dimensions have been verified using FreeCad along with @Red Five. This assumes the thickness of 0.688" plywood however the difference of dimensions are also included which should make it very easy to convert to any plywood dimension you wish.

Left Side of the attachment are the original dimensions. Right side is the converted dimensions.
 

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#4 · (Edited)
Outstanding work. Design looks outstanding. Looks like I will be building two of these :)


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Looking forward to seeing some pictures :)
 
#9 ·
@superkyle


There is still more fine detail work left, but this is roughly what MDF bracing will look like. It saves 28.1 liters of displacement and uses a good amount of scrap material vs picking up additional wood. Tune then comes in at 19.8Hz and adds a slight bit more SPL down low.



 

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#15 ·
Love the new design, somehow seems a lot easier to build from this pic. Don’t cut the low end off too much. As you said chasing extension isn’t worth it in your situation, I’m still a fan of reaching for powerful 20Hz content in large rooms and hard floors.
 
#10 ·
Nice that’s basically 1 ft3 saved with same dimensions. Did you end up deciding to keep it the same size? Waiting on the final dimensions before going through and calculating and posting up the conversion to using plywood.


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#12 ·
If I took an inch off it would be down to 346 liters vs the 414 liters, so I think leaving it at 28 inches deep is a good option. Especially if permission is already there ;)

I am out and about I have to look at the notes, math seems off.
 
#13 · (Edited)
I made a typo in the unit converter 21 liters of added displacement, so about .75cuft. An inch off will take it down to 379.7 liters vs the 407.5liters. There would be a few liters added on due to shorter bracing, but this will be in the ballpark. What do you guys think? Once I get down to 24 inches deep things really go down on the low end lol.

@superkyle @Purenv


 

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#17 ·
Been tied up at work but loving the new design. I’ll compare the numbers when I convert them to plywood measurements like I did before.

Edit: I see the difference now have to zoom in haha.

Yeah permission was key. Now that I have it I want as much low end as possible while maintaining the mid bass. I’m thrilled with the new design.

Never tried window bracing so may have some questions but I plan to document everything in a build thread as how this goes together can seem a bit tricky. Kreg jig for the win!
 
#18 ·
I have to make one adjustment on the upper frame brace, I forgot that one is shorter.

All you do is mark 2 inches lines around the diameter and then two lines 11.25" from the outside edges. That will leave 2 inches of material in the center of the frame. Then cut down the boxes with a jig saw.
 
#21 ·
Another version of the Devastator that is geared for 20Hz to 80Hz, it should hit with a lot of authority in that range. Response rolls off around 70Hz, integration with higher crossovers should still be possible, but not as ideal. The additional height allows for a long front chamber which pushes the response of the resonator down a bit. Also the rear chamber gets added volume which helps pick up the low end. I tuned it a little higher since I would rather have the added SPL vs the extension. I can add port lengthening information if anyone is interested pushing the tune down. For this design a pillow should be added between the braces just below the sub woofer since the 48 inch mark has been breached by more than a little :) Power handling on the B&C 21DS115-8 is around 1625 watts (91 volts) which pushes excursion to its max and port velocity to 25m/s. 8.25 inches added to the port brings the tune down a bit under 19Hz if you want the added extension. Graph attached on the bottom of the post.












BMD vs Devastator 1w1m


BMD vs Stormbreaker (11cuft, 18Hz tune) 1w1m


BMD 91volts into 5.1 Ohms 19Hz 3rd order HPF pushing 20mm of excursion over under tune






Can a cab exist with max of 20"depth, 24 width and flexible height , I would definitely go for the build as I just received my 21s today.
 
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#22 ·
So I originally was looking for something similar and decided on this build. Basically when you change things up to those dimensions you lose a lot of efficiency and a ton of output. A couple inches hopefully won’t be too bad bc the output is on another level!


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#24 ·
Where I stand is my jbl 4722 are 18" depth so I will projecting 2" more just by going 20". So I always try to get the cab a max of 18" to not waste even a few inches.
 
#31 ·


Please go over this with a fine tooth comb. Tried to keep ports the exact same size but that’s not possible so one needs to change but only very slightly. I doubt it changes anything though. I very easily could of made a mistake so someone let me know if you want to work through it with me to make sure it’s correct.


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One of the upper window frame braces is 13.75". I will have to look at it closer later.
 
#28 ·
I will take a look later when I get a chance concerning the comparison. That would be a traditional 6th order parallel band pass, but it would not work the same way in which a Devastator does. The front chamber is a quarter wave resonator, not a Helmholtz resonator. The length and diameter effects the response, not the port length and volume, at least on the mid and upper end of the response.
 
#37 ·
@Rcris
This is my educated guess where a 60.5x25.5x24 BMD will land. I compared it against standard BMD using window frames and also the 8.3ft3 18Hz tune box you linked. There is a lot of low end loss due to the loss of displacement, but it does have its advantage against the ported box. With a 47.75" vent the first port resonance is pretty low, but that is what it took to get it down to 19Hz tune.






 

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#38 · (Edited)
@superkyle I think the conversion looks good as far as panels and fitting go. I will tinker with the Hornresp model and see what it looks like, I would not expect anything too drastic. Just again not there top window frame is the same length as the upper front to back braces. The middle brace maintains the upper brace dimensions I saw in the sheet. Looks good!


Edit: They model so closely, I cannot tell what is what on the SPL graph lol.
 

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#46 · (Edited)
The original version has an external displacement of 29.4 cuft. The dimensions you have up there come in at 30.1 cuft, so it will be very close to the original with displacement.


Edit: @Purenv

The depth of the resonator should be 10.25inches, that should bring it very close to the original version. Then just calculate your rear displacement based off that depth. You could probably use vertical standing window frame for support around the vent on the bottom.
 
#48 ·
Those dimensions are not really what I would use. I was thinking, in stand up view, “y” height x 25.5 wide x 25.5 deep and solving for “y”. So 29.4 ft3 = 50803.2 in3
25.5 * 25.5 = 650.25
50803.2/650.25 = 78.28” in height = y

Unfortunately too tall, or if laying it on it’s side like I am, too long.

I’ll try and work the numbers a bit.

I might just stick with your dimensions and switch back to the 893s.
 
#56 ·
Use the parameters he has posted above and just change the driver specifics. I would venture to guess the drive would work just fine in the BDM but I’m not sure familiar so modeling it is always recommended.


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#58 ·
I think I can fit in two of these with ports facing sideways .Is there any preference in which direction the port faces behind AT screen?
 
#59 ·
The nice thing about low frequencies they are not all that directional, pointing it sideways should have little effect if any. Exceptions, pointing it at the wall and then placing it one or two inches from that wall. You could see an effect in the response form that.
 
#60 ·
Going to break ground today cutting up the plywood. My previous builds I had the plywood ripped to smaller cuts before bringing it home so it was easy to cut to size on my table saw. This however I’m not quite sure. Thinking about building a diy rip guide or even buying a tracksaw. What do you guys use to rip down the full sheets?


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#63 ·
I was going to make some extra cuts and run the circular saw across the sheet using my 48" straight edge. The side panels will require a longer straight edge. One of the local stores has 72" straight edge for $12, I am planning on going that route.
 
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