Cheap Under-desk Subwoofer Designs? (40Hz under $50?)

[drneilmb]

[tl;dr Modest 40Hz subwoofer for under $50?]



I'm new to DIY audio and I had a blast with my first speaker build (full-range Planet10 bookshelfs), my first amplifier build (LM1875 gainclone), and now I'm working on a preamp to complete that system. When the preamp gets done, it all goes out in the living room and I won't have a system for the office/workbench anymore.


I'd like to stick with the DIY theme and do a pair of micro-Cornucopya for the office wall (thanks Xrk1971!), a handbuilt pre/power amp, but I also want to do a subwoofer for under the desk. The major constraint of this project is that I would like to do it for CHEAP. I'm aiming at under $50 for the subwoofer drivers and parts.



I'm building the amp, so I can design in an active crossover and a cheap TPA3116 50W mono amp, but I'm stumped on cheap subwoofer designs that are small enough to fit under my desk. I don't need much sensitivity since I'll practically be sitting on it, nor terribly low extension since this will be for casual workday music only, but I'm not finding an existing design that fits the bill.



Here's what I've turned up in my research:

1. Paul Carmody's Voxel - $43 for the W5-1138 driver and someone came up with a slot port version (voxelslot -
   
   Techtalk Speaker Building, Audio, Video Discussion Forum) so I don't even need to buy the ports.
2. Brian Steele's Boom Unit - $51 for the DCS205 and a boards-only MLTL cabinet.
3. RJB's Cerberus - $55 for the W6-1139 driver is already over the budget, but the right size and extension.
4. Paul Carmody's Pony - $110 for the Dayton RSS210 is right out, but the size would be just right for laying down under a desk against the wall.
5. Ed Schliling bucket sub - The Peerless SLS 830667 is just over budget at $55 and I could definitely fit a bucket under my desk. No WAF problems in the office either.
6. Silly's 1st sub - $25 MCM driver is priced right, but its extremely low Qts demands ports which are very, very long, so it needs passive radiators which run another $26. DIY PRs are occasionally talked about and if they were doable, this would be a possibility.
7. Other MCM 55-2421 designs - I've seen it in some tapped horns (too large?) and I think I also saw a dual sealed alignment that I can't put my finger on, but I'm not confident about any of those possibilities.
8. Some other cheap driver in a simple sealed/vented box: PE has various Goldwood, GRS, Dayton SD215 under $25.

As a complete novice, I have very little knowledge to choose between these various choices, or the possibility of designing something else. Can you help me choose a project which will be most successful? Are there details about the designs on my list that I'm not seeing because I'm such a n00b?


Thanks in advance for any help you can offer.


-Neil N0FN

 

[DPH]

Sealed aint gonna get you to 40 Hz without some serious EQ and Xmax ($$$).

Going to be hard to beat a 55-2421 alignment, or maybe a similarly beefy 8" car subwoofer. Look around and see if anyone has a decent tapered quarter wave tapped pipe (TQTP) that you can package appropriately. Or an mass loaded transmission line (MLTL), which is a hybrid TL plus vented alignment, which might get you into a size that works well. A look at Brian Steele's folding calculators can get you pretty far too. (Big props to him for making them!)

The Subwoofer DIY Page - Horn Folding

 

[Brian Steele]

One thing going for a DCS205-based arrangement is that the driver's parameters are almost perfect for an Extended Bass Shelf (EBS) or MLTL design that's tuned anyway from 32 Hz upwards. The slightly downward sloping response can end up being a great match for a subwoofer used in a corner, under a desk, etc. The box requirements also end up being large enough so you can actually get a decent-sized vent in there to achieve the required resonance frequency. And it's a 4 ohm driver, so it's a good match for those small class D amps that produce a lot more power into a 4 ohm load. Similar 8" drivers that have t/s parameters that call for smaller boxes can pose a challenge when it's time to tune them to the desired resonance frequency using a vent that's both large enough to reduce compression effects and long enough to achieve the target resonance.

I'm quite happy with my "Boom Unit" build, and this is from someone who can be very critical about his own builds, LOL. It's also an easy build, and this is coming from a pretty horrible carpenter.

Note: the "Boom Unit" was also designed to fit exactly under the drawers in my desk, thereby removing another location for dust-bunnies to collect

 

[drneilmb]

Thanks Brian, both for your feedback on the Boom Unit and the Dayton DCS205, but also for your MLTL design spreadsheet.

I've been playing around with the dimensions to try to make it an easier build with some particle board shelves that I salvaged, including a roughly square baffle so that the top, bottom, and the two inside boards are the same as the stock width of the two sides.

I'll try to post some screenshots from the spreadsheet and Hornresp when I get a chance.

Thanks again, Brian. Anyone else want to speak up for one of these other designs? Otherwise I'll go with the Boom Unit by default (possibly modified in some dimensions).

-Neil N0FN

 

[GM]

One of these or similar spec mounted OB to a 'modesty' panel in the top rear of the leg well is about as simple, cheap as you can get: GRS 12SW-4 12" Poly Cone Subwoofer 4 Ohm

GM

 

[drneilmb]

One of these or similar spec mounted OB to a 'modesty' panel in the top rear of the leg well is about as simple, cheap as you can get:

Tee hee, the term "modesty panel" is exactly right here, but it always makes me giggle.

I don't know very much about OB alignments, but my impression is that they need some space away from the wall to perform. Would that still be the case here? My desk is pushed up against the wall and doesn't have a "leg well", it's totally open underneath the fairly thick desktop.

Would I mount the panel as far forward (away from the wall) as my knees can handle and let the desktop and the thin opening at the back wall sort out the back wave from the front wave?

I'm sorry for the stupid question, I feel like I don't understand OB bass at all yet.

-Neil N0FN

 

[GM]

The gap is mostly a function of the XO point, so for just bass [< 120 Hz/2nd order IME] not too critical beyond the driver's pipe end correction or ~3.6" for a 12" and some find as little as 0.75" acceptable, ditto floor gap, so worth experimenting with and of course the baffle ideally needs to be full width and preferably add panels to the sides.

Another option is to just use the 0.75" and experiment with 'gapping' the driver, i.e. spacing it away a tiny bit from the cutout with an open cell foam, fiberglass or similar lossy gasket to leak away some of the high pressure.

Admittedly, I'm use to thinking in terms of a traditional desk with drawers, etc.; a basic workbench will probably require a bit of experimentation for optimal results.

GM

 

[CuriousOne]

When I had similar task, I went to local junkyard, where they take apart cars for parts and scrap metal, and bought Mercedes ML350 subwoofer, which is located under the seat. Put it on two bricks, and added two more from top, to prevent it from jumping. Works amazingly well, and costed me less than $20.