Building a DIY sub under/into the stairs

TB Rich

Prominent Member
Jan 17, 2019
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Just mulling over ideas and wondering on the feasibility/how easy it would be to build a sub in to the following space, where currently sits a single BK XXLS400?


The shape of the space is:
Screenshot 2021-11-24 at 17.27.14.png

The internal volume is about 14 cubic feet. The width is about 88cm and front height is about 85cm - so there is plenty of room for a pair of 15" drivers.

Before I go on a huge discovery path and ask a million questions on what driver/amp/materials etc.... Could do with fiurst addressing the challenge of how on earth I would go about it? And ruling the project in or out!

Main build thoughts I had were:
  • Build it as a standalone unit to roughly the same dimensions - allows for it to be moved in or out if ever needed, and could sit it on SVS isolation feet or similar.
  • Or build it into the structure of the room, piece by piece in situ? Would provide lots of support so less need for bracing? But would this transfer too much resonance in to the fabric of the building?
  • Sealed seems a lot easier to design than ported? I haven't decided if this will be cinema use only, or if it'll replace the BK entirely and do music as well (another reason I think sealed should be fine).
  • It's big enough for a pair or 15" drivers (hell could even get 4 there!), but was also wondering if a 15" driver and 15" passive rad would be better? More bass depth (?) and half the amp demands than 2 drivers.
  • Where could I locate the amp? Guess it would be unsafe to install inside (needs to breath/cool), however if the front panel didn't come all the way to the front and was recessed in by an amount (say 3-4 inches), no reason I guess a plate amp couldn't be attached and I could cover the whole thing with a nice acoustic fabric hiding it all from sight (might be preferable anyway actually).
Think that's it in terms of the main feasibility things I can think of? Not sure yet if this is a project that's biting off more than I can chew!? But I think it looks ok in theory, also I am not concerned with a spraying/finishing type of operation after to make it look nice, all I would need to do is on the visible front just paint with emulsion to match the walls, or as said use the acoustic fabric as a screen.

Also should I decide this is too difficult, anyone had this type of thing done as a commissionable service (probably not given I'm in the DIY section :D)? I've had a brief look and it seems car audio places offering custom sub work - don't suppose it makes too much difference, MDF is MDF I guess?! :)

Thoughts and advice welcome.
seems doable to me, I would build it as a self contained unit though rather than just a baffle as I'd guess that sealing that would be harder. Bracing that adequately is probably an interesting challenge in terms of how you actually assemble it & physically manoeuvring it into position should be fun too :)
Probably is easier as self contained, I suppose I could make it slightly less wide the than the space and attach the plate amp to that surface.

When doing a DIY sub, do you need to be accurate with the internal volume sizing? Or is it case of just make sure its bigger than the minimum and that'll be fine?
When doing a DIY sub, do you need to be accurate with the internal volume sizing? Or is it case of just make sure its bigger than the minimum and that'll be fine?
Depends on the design really, some are more sensitive to volume than others
I’d be keen to keep it seperate so it avoids transmission of noise to the staircase, be very annoying if you have resonances

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