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21" Subwoofers design suggestion
21" Subwoofers design suggestion
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Old 22nd May 2021, 06:07 AM   #41
JRiv is offline JRiv
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Hi GM, thanks for the tips about the decorative weight, definitely seems like an effective/attractive solution.

PS I've used the term relatively lightweight since I've chose Light MDF which is about 15~20% lither than standard MDF, but apparently that's not the case, since the material used on those sheets are radiata pine fiber(a very soft wood) according to the manufacturer, now I decided to make the front baffle of triple 5/8" sandwich to hold the 21SW152 and stay in the safe line, so I expect the box will be about ~140Lbs without the driver.

Most likely the next project will be a hand-truck to move this coffins.

Regards
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Old 22nd May 2021, 07:45 PM   #42
GM is offline GM  United States
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You're welcome! Yeah pretty heavy for me nowadays, so have a decent large appliance hand truck; but by 'lightweight' I meant thin no void plywood with scrap pieces or similar for panel bracing and all [6] panels + driver[s] tied together with threaded rod to keep it from 'breathing' as well as for added weight support.
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Old 23rd May 2021, 01:05 PM   #43
JRiv is offline JRiv
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Thanks GM for the clarification on the lightweight technique you've mentioned, that seems like another solution if carefully executed indeed.

Between I've found the old Sketchup Make 2013, the early version when Trimble took over, this old version run flawless under an Windows 7 VM since I'm on Linux, unfortunately latest Sketchup don't work in VirtualBox since it need HW video acceleration, also they don't work in WINE either.

The models I've made are a very simple/basic representation of the sub as it was my first time using that sketch software, so I still need to learn alot. about it.

Model-Front.jpgModel-Down-Angle.jpgModel-Top-Angle.jpg

Changes since the initial idea:

*Side walls top & bottom will be 1.25" tick, 2x 5/8" sandwich.
*Front baffle will be 1.88" thick, 2x 5/8" sandwich plus a later added 5/8" panel for speaker flush.
*Black matte laminate to match my diy main speakers
* Top & Bottom will have rib style bracing, and the center too.
*Final enclosure measures are 42"H x 24"W x 27.15"


Observations that should be considered:

Initially I wanted the port to be 36" long, however since the enclosure will measure 39.5" internally from top to bottom, then subtracting 1.25" from the 36" port equals 34.75"(port will be inserted on the bottom), so 39.5" minus 34.75" will bring a port clearance of just 4.75" from the top, this seems too close for an 8" dia port and may ran into audible noise/compression.

So my solution so far is to sacrifice 2" from the port and make it 34", this will bring a port clearance of 6.75" from the top with just a tad higher port tune, still non ideal but I hope experienced users ports position/clearance may chime in, since I've read that the rule of thumb is to give a port clearance as big as the port diameter.

Really hope I can go with the 6.75" port clearance(closer by just 1.25") with no major noise/compression issues, as this boxes will not be pushed to the max between.

P.S. The port has a 3" space from the back wall, this is to have clearance to spare from the speaker neodymium magnet.

For reference here's the WinISD model with the port cut to 33", in comparison with my current UM18-22 sealed:
Screenshot from 2021-05-23 08-58-43.png

Expected usable frequency response after EQed:
Screenshot from 2021-05-23 09-09-00.png

Regards

Last edited by JRiv; 23rd May 2021 at 01:11 PM.
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Old 24th May 2021, 04:22 AM   #44
weltersys is offline weltersys  United States
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Quote:
Originally Posted by JRiv View Post

Attachment 954094Attachment 954093Attachment 954095
... a port clearance of just 4.75" from the top, this seems too close for an 8" dia port and may ran into audible noise/compression.

So my solution so far is to sacrifice 2" from the port and make it 34", this will bring a port clearance of 6.75" from the top with just a tad higher port tune, still non ideal but I hope experienced users ports position/clearance may chime in, since I've read that the rule of thumb is to give a port clearance as big as the port diameter.
You could reduce the port clearance to just 2" from the top without reducing the area of the port inlet, audible noise/compression would be no problem.

The reason for the "rule of thumb" is a port clearance less that a port diameter will effectively begin to lengthen the port, lowering Fb while also virtually reducing cabinet volume.
The port within 3" from the back wall and the brace structure ring you depict surrounding the duct do the same.


As Josh Ricci wrote about his 21SW152-4 build: "The target tuning was 28Hz, but the proximity of the vent opening to the back wall of the cabinet lowered the effective tuning to about 25Hz instead. "

Data-Bass: Subwoofer Measurements

I'd suggest testing the Fb before gluing the speaker baffle or port brace structure ring in place so you can adjust length to the frequency you want.

Art

Last edited by weltersys; 24th May 2021 at 04:27 AM.
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Old 24th May 2021, 12:28 PM   #45
JRiv is offline JRiv
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Hi weltersys, thanks for the clarifications on the port clearance rule of thumb and its effects wen in close proximity to walls, learned new thing though I still need a long road to learn.

Great info on the Data-Bass indeed, I've been reading on subs performance on Data-Bass for a while, in fact his extreme testing on the B&C's made me choice this subs for my 2-CH music setup.

Looks like the DATS V3 is still in back-order on PE, however since you've denoted that the effect in wall close proximity and the port reinforcement ring may lower the Fb rather than making noise/compression, guess it will be fine as the initial wanted tuning was around ~17 but had to raise it a tad due the wanted box size, so that's a relief since this is for 99% music playback.

P.S. I've read the lower the Fb is preferred to keep the group delay at minimal possible but guess is not a big deal on subs, OTOH that port wall proximity effect effectively lowering cab internal volume is another compromise.

Regards
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Old 25th May 2021, 05:16 PM   #46
weltersys is offline weltersys  United States
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Quote:
Originally Posted by JRiv View Post
...guess it will be fine as the initial wanted tuning was around ~17 but had to raise it a tad due the wanted box size, so that's a relief since this is for 99% music playback.
OTOH that port wall proximity effect effectively lowering cab internal volume is another compromise.

Regards
Since you listen mainly to rock, oldies and electronic music, choosing an Fb near an octave below the fundamental frequencies used in most of that music is another compromise to consider when it comes to SPL output in the 30-60 Hz range.
Can't have everything, so we all get to choose our own compromises

Art
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Old 10th September 2021, 04:06 AM   #47
JRiv is offline JRiv
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Small update!

Been few months since my last entry in this project, just wanted to update that my plan are to finish this project by October hopefully.

So far I've glued the boxes back and the side panels totaling 6 x 24"W x 42"H sandwiches, as usual I just used TB3 for the glue layers and a lot of old weights and old concrete blocks for set them for 24 hours.

Next move is the top and bottom sandwiches and the port assembly, the last part is the now triple-baffle to flush mount the subwoofers.

IMG_0040_ed.JPG IMG_0049_ed.JPG IMG_0053_ed.JPG IMG_0055_ed.JPG IMG_0057_ed.JPG


Early build progress can be found in the project Google Photos
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Old 16th September 2021, 06:13 PM   #48
JRiv is offline JRiv
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Update:

I just started to cut the 20" baffle circles with a homemade Router Jig made of some scrap 1/4" plexiglass(acrylic) I had lying around, it worked just fine so far.

I will cut one hole to make a double 5/8 sandwich first then route the other side after glue sets, then cut the bigger hole to flush mount the drivers and glue them after woofer dry-run fit test.

More to come, here are some shots of the dusty work:
IMG_0078_Small.jpgIMG_0085_Small.jpgIMG_0089_Small.jpgIMG_0092_Small.jpgIMG_0094_Small.jpgIMG_0114_Small.jpgIMG_0115_Small.jpgIMG_0119_Small.jpgIMG_0120_Small.jpgIMG_0124_Small.jpg
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Old 18th September 2021, 08:41 PM   #49
JRiv is offline JRiv
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Here is another update on this 21" subwoofers "build".


Since this thread was initially for design "suggestion" between straight slot-port vs round-port(opted for the round-port), I will start a new build thread on this subs design after the internal enclosure assembly is ready.


Today I just did a driver dry-run test on the baffles to test the flush panel measures and to mark the bolt circle to install the T-Nuts, tomorrow after release the T-Nuts holding screws I will glue the third 5/8" flush panel then start with with the rest.


IMG_0159_Small.jpgIMG_0161_Small.jpgIMG_0166_Small.jpgIMG_0167_Small.jpgIMG_0171_Small.jpgIMG_0185_Small.jpgIMG_0192_Small.jpgIMG_0198_Small.jpg


All images in the Build Google Photo Album
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Old 26th September 2021, 06:23 PM   #50
JRiv is offline JRiv
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Just finished the triple baffles sandwiches and also cut the 12x top/bottom panels, 4 of them will works as brace + port support.
IMG_0211_Small.jpgIMG_0226_Small.jpg


Here I've used some formica leftovers to make a tape as a cutting guide, I had to sand the edges to smooth the cuts regardless.
IMG_0231_Small.jpgIMG_0232_Small.jpgIMG_0234_Small.jpg


Here are 2 cuts already flattened out(the port bottoms).
IMG_0237_Small.jpgIMG_0238_Small.jpgIMG_0240_Small.jpg


Here sanding the port edges and applying a little spread of wood glue to reinforce the cardboard port edges grain, still deciding whether to paint them inside/outside, or just the port inside.
IMG_0254_Small.jpgIMG_0255_Small.jpg


More to come: Photo Build Log
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