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200L Barrel Subwoofer

Adam_M

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It's been touched upon a few times, but the barrel itself is going to be a significant source of distortion. Rap on it with your knuckles or a stick. Does it sound like a drum? Probably. That will all be excited by the subwoofer unless heavily braced. When you knock on the enclosure, it should sound more like knocking on a stone or brick (in the extreme - it won't get there even in the most expensive, extreme subwoofers) than a drum.

When I build subs, based on research from Bill Fitzmaurice who has extensively studied AES papers, I brace so that there's no unsupported span longer than about 6" in the walls. That's using either 3/4" MDF or 3/4" Plywood. Bill does it with 1/2" plywood. YMMV with plastic, which may ring more than plastic.
 
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kiwifi

kiwifi

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I would like to thank the ASR members on this thread that encouraged me to measure my speakers. The spec sheet for the Pyle PL1090BL proved far from accurate. Here are my DATS/WT3 measurements from my two examples of the PL1090BL...
PL1090BL.PNGPL1090BL-2.PNG

Based on the measured T/S parameters I have adjusted the tuning for the barrel sub to 23Hz
BarrelSub2.PNGConeEx.PNG

I realize that the Pyle 10" driver is not ideal for this size of enclosure and that the barrel itself may ring terribly, but now at least, the sub (driver/vent combo) is operating efficiently. There was no advantage in an isobaric pair, it just consumed more power for zero gain in SPL.

I am glad that I posted my plans here first. I have learnt a lot from your feedback and the design of this sub is better as a result. Now to build it...
 

Adam_M

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I'd reconsider that tuning (probably tuning higher). I don't have time to run the numbers, but I'd try to adjust the roll off so it's smooth without the big peak at the tuning frequency. You would typically want an uneventful roll off with no peaks - the sim should look more or less like a textbook 4th order rolloff. Dips are better than peaks though. If you want more bass and you have the X-max to do it, you could always lift it back up with a DSP.

The other option is to make the box functionally smaller by filling it with something- anything that occupies volume that isn't porous from an audible perspective could work, as could walling off part of the bottom. You should play with box size as well as tuning and see what works best.
 

Chrise36

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I would like to thank the ASR members on this thread that encouraged me to measure my speakers. The spec sheet for the Pyle PL1090BL proved far from accurate. Here are my DATS/WT3 measurements from my two examples of the PL1090BL...
View attachment 166309View attachment 166310

Based on the measured T/S parameters I have adjusted the tuning for the barrel sub to 23Hz
View attachment 166311View attachment 166312

I realize that the Pyle 10" driver is not ideal for this size of enclosure and that the barrel itself may ring terribly, but now at least, the sub (driver/vent combo) is operating efficiently. There was no advantage in an isobaric pair, it just consumed more power for zero gain in SPL.

I am glad that I posted my plans here first. I have learnt a lot from your feedback and the design of this sub is better as a result. Now to build it...
Vibration deadening sheets:
 

Adam_M

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Chrise36

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In my experience, that isn't going to solve this issue unless you use something like constrained layer dampening. What's needed in this situation is pure mechanical stiffness to raise the vibration frequencies of the sidewalls above the region of concern and to reduce the amplitude to something that's palatable.
Some wood pieces would help also but it is supposed to be a cheap fun project.
 

Godataloss

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Yep, exactly. I use 1" dowel rods. Window braces aren't really better.
Yes the rigidity in the barrel (likely made of HDPE) would primarily be the to resist expansion from the uniform force of the liquid it contained. Some cross brace dowels to resist compression and deformation of the empty container would help a lot, but it won't stop the sides from resonating. I think OP is confusing this materials shear strength with rigidity. If injection molded subwoofers were practical, someone would be cranking them out already.

That said, I support you in your bold endeavor. I think it might be workable if you buried it. The earth would damp vibrations significantly. It would also dress up the cosmetics considerably.
 
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kiwifi

kiwifi

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Yep, exactly. I use 1" dowel rods. Window braces aren't really better.
I think that end to end bracing will definitely be needed. Although the top of the barrel is 4mm thick, there is still a small amount of flex in the top with the holes for the driver and ports cut. Three 1" x 90cm dowel rods, top to bottom, should do it I reckon.
 

Adam_M

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I think that end to end bracing will definitely be needed. Although the top of the barrel is 4mm thick, there is still a small amount of flex in the top with the holes for the driver and ports cut. Three 1" x 90cm dowel rods, top to bottom, should do it I reckon.
4mm plastic is really thin. I brace the heck out of 19mm plywood. You will need a network of braces on the round sides too. Think spokes of a wagon wheel, parallel to the top and bottom, spaced every 6" horizontally and vertically.

Please consider the standard for high quality diy subs used to be 2x 3/4 MDF walls = 50mm, heavily braced, and still is used in some cases. You walls at 4mm and made of an inferior material for audio aren't going to perform.

As godataloss mentioned, you are mistaking the ability to hold a uniform liquid load with stiffness and rigidity, which is what is necessary to get the vibrations out of the pass band.
 

alex-z

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Will do absolutely nothing in this case. Adding limp mass to a structure raises the resonant frequency, but doesn't prevent the internal air pressure from flexing the walls. A common misconception is that a port/vent acts as a hole in the box, this is only true below the port tuning frequency. At higher frequencies, there is still internal pressure.

A proper solution would be internal dividers, or external steel bands, such as those seen on wine barrels. That goes against the cheap theme of this build though, which is shaping up to be an interesting academic exercise.
 

voodooless

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or external steel bands, such as those seen on wine barrels.
how would that work on plastic? In wood barrel it works because of the pressure the wood planks exert on each other. A plastic barrel does not have this feature.
 

Chrise36

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Will do absolutely nothing in this case. Adding limp mass to a structure raises the resonant frequency, but doesn't prevent the internal air pressure from flexing the walls. A common misconception is that a port/vent acts as a hole in the box, this is only true below the port tuning frequency. At higher frequencies, there is still internal pressure.

A proper solution would be internal dividers, or external steel bands, such as those seen on wine barrels. That goes against the cheap theme of this build though, which is shaping up to be an interesting academic exercise.
A 4mm plastic will resonate even without flexing some some of this will be needed together with some bracing.Added mass lowers the resonant frequency.
 
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Doodski

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Another idea is to put the 200l barrel in a bigger one and fill the gap with cement it will be dead quite if it goes well.
There are additives that improve the hardness of cement. I don't know what they are called but that type of cement is offered by cement delivery companies. It's used in critical applications. I think it is better at less cracking, less spawling and crumbling.
 
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kiwifi

kiwifi

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I measured the thickness and it is 6mm not 4mm.

I couldn't wait to hear it, so it has no bracing, just 12" of hollow fill in the bottom. It actually sounds really good and it's not properly calibrated yet. I'm just running the same signal to it as another of my subs.

IMG_20211119_140457_013.jpgIMG_20211119_152243_431.jpgIMG_20211119_153729_628.jpg
 

Adam_M

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kiwifi

kiwifi

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Well, cool! Glad you are happy with it and to have more DIY'ers around. Measurements?
I'll see what I can do. What measurements would you like to see?
 

Adam_M

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I'll see what I can do. What measurements would you like to see?
For a sub, i'd normally run a nearfield of the woofer and splice it with a nearfield of the ports. Jeff Bagby wrote a white paper on the process that's a great read.

If you have access to a parking lot or similarly large space, a groundplane or other far field measurement would be very helpful. I'd run this higher than the forecast pass band to check for issues from the barrel.
I'd also run a distortion measurement at 85 & 95 dB for h2-h5 + thd.

And I'd also grab an impedance measurement to check the tuning frequency and for any resonances.

You want to check all of the most problematic areas extra thoroughly in any design. In this design, I'd suggest that will be from about tuning frequency and 1.5 octaves up and quite honestly most of the pass band and even above because of the barrel resonances.

I've never done this, but it may be interesting to get a Nearfield from the side of the barrel. Put the mic a couple mm from it and fire it up. It may turn out to be meaningless or could give you information about what the side of the barrel is doing to a few hundred hz.
 
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