DIY sub between floor joists

Gregory

Active Member
Moved on from buying a replacement sub to building one. Seems so much more entertaining. I toyed with a large sealed one behind the a sofa, but it causes audio placement issues (hard to get a wide accurate sound stage). Also, I then came up with a much easier thing to try.

We have a pair of removable floorboards pieces between two joists in the corner of the room. This gives enough access to construct a sub using the space under the floor as the box. It (should be) easy to build a mounting for a driver such as the XLS10 (as used in the BK XLS200). That should given a nice low frequency response, even though the large box size (almost 300 litres - approaching an infinite baffle size for that driver) will limit the power ... but that's fine - we don't use it loud, but do want great low frequency response. May need to play with the equalisation, especially since there may be resonance modes from what is a rather long thin chamber, but in principle it feels worth a play.

Best of all, it should be pretty much completely invisible when done - just an air vent in the corner of the room to show where it is.

Anyone done this before? Any vital lessons? I'm placing an order for the XLS10 anyway since I do want to play, but top tips would be great.

Cheers

Greg
 

HiFiRuss71

Distinguished Member
Have you had a look here. It'll give you some ideas about how you can shoe horn driver(s) into a space that apparently only offers enough room for one.

Russell
 

Nimby

Distinguished Member
Indeed. The problem with trying to build into the floor is that you will bottom normal drivers at the first mention of bass. You need lots of cone area to move a lot of air and still leave you some headroom. Think in terms of 2 x 15" or 4 x 12" drivers as a minimum if the underfloor is leaky.

If you can build a conventional box under the floor you can treat it as a normal sub. But you should check whether your proposed situation is optimum with a normal sub and REW before going ahead. If the situation doesn't work then you have completely wasted your time and money.

Try WinISD Pro with your potential driver choices and likely enclosure volume.
 

Gregory

Active Member
Indeed. The problem with trying to build into the floor is that you will bottom normal drivers at the first mention of bass. You need lots of cone area to move a lot of air and still leave you some headroom. Think in terms of 2 x 15" or 4 x 12" drivers as a minimum if the underfloor is leaky.

I would, but I'm really not after volume - I'm after a low range. Also, I have the option of putting a block further down under the floor to make it a sealed box of 'normal' size pretty easily if the driver looks imperilled, or there simply isn't enough volume.

If you can build a conventional box under the floor you can treat it as a normal sub. But you should check whether your proposed situation is optimum with a normal sub and REW before going ahead. If the situation doesn't work then you have completely wasted your time and money.

Done that - sounds fine, though with limited bass due to rather naff sub (Celestion S10).

Thanks for the thoughts though (Russ as well)- much appreciated.

Cheers

Greg
 

aventhusiast

Active Member
Before you read my thoughts on this I have to say I hate being negative about something because I'm not the pesimistic type but I have real concerns as to how this would workout. :(

I'd steer clear of this idea personally. The bass is likely to cause real bad resonance troughout the building as its attached directly to it. Your walls/ floor will vibrate & transmit sound alot more, along with their own resonance which is not what you should be aiming for. Its like when someone is hammer drilling in a room opposite & you obviously get the drill noise (would be your sub in this case) but then ALSO you get the resonance caused by the vibrations transmitting directly through the structure. Its often a horrible boxy mid tone. In short, attaching a driver (especially a bass driver) to a structure is exactly the opposite of how to acchieve good sound. Isolation (amongst others) is key to getting direct accurate sound.
 

Gregory

Active Member
I'd steer clear of this idea personally. The bass is likely to cause real bad resonance troughout the building as its attached directly to it. Your walls/ floor will vibrate & transmit sound alot more sound along with their own resonance which is not what you should be aiming for ... In short, attaching a driver (especially a bass driver) to a structure is exactly the opposite of how to acchieve good sound. Isolation (amongst others) is key to getting direct accurate sound.

I have had similar thoughts, and I am a little concerned to see how the long relatively thin box works in terms of resonance (and it is all a trial - easy to do and easy to reverse if it proves a failure). But, I'm less concerned about the direct connection to the structure. That is exactly how Infinite Baffle subs work, and they are reputed to have pretty much the cleanest sound you can get. However, it may be necessary to use a very narrow band filter (Beringer feedback destroyer or similar) if there are resonance modes excited by the box shape. I'd also note that the current sub is a down-firing one onto exactly the same floor, and that works fine.

Give me a week or two and I'll let you know!

Cheers

Greg
 

aventhusiast

Active Member
Thinking about the direct contact, you could try some sort of neoprene or foam gasket to sit between the driver & its mounting.

Give me a week or two and I'll let you know!

Nice one! Be interesting to see the progress of this one.

Hope it goes well :thumbsup:
 

Nimby

Distinguished Member
Sorry aventhusiast but you are completely wrong.

Structural vibration comes from the sound's air pressure waves. Not the mechanical vibrations.

My IB sub has 4 x 15" drivers built right into a rather flimsy stud wall. The wall was already there so I used it.

Despite the 1.5" solid plywood baffle the wall moves a lot when the drivers move a lot. Which isn't very often. I can get 120dB and not even see the cones moving.

The glazed double doors right beside the array shake like mad and even rattle sometimes to very loud bass. This is bass that very few commercial subs can manage in loudness or depth. Leaning on the doors with all my weight doesn't affect their vibrations at all. They are part of the IB enclosure and if they want to move to 10Hz sound waves they will.

Having the whole house shaking does not affect the quality of the bass. There is no coloration of the sound due to structural vibration. A few rattles seem natural and add to the excitement on films. Having the whole roof heave with a loud creak on LFE might be pushing it just a bit too far.

Putting a subwoofer in the floor is a free Buttkicker.

You get the desirable floor vibrations that tell you your sub has serious output.

Just as you would if you owned a big SVS and parked it anywhere on top of the floor.

I could easily build an opposed driver manifold instead of having a line array. That would cancel out the mechanical vibrations but not the sound pressure waves. I'm really not sure it would be worth the effort so I haven't bothered so far.

All this is completely academic if you don't use lots of large drivers. A little cone in the floor won't do anything but hit the end stops on the first bit of bass. You'll just have to think in terms of building a small sub box hidden between the joists.
 

aventhusiast

Active Member
We'll have to agree to disagree. Look at any half decient recordig/mixing/mastering studio (where the audio tracks we're talking about are made) & you'll see that isolation is key. Never would anyone mount a driver to the structure. Even the soffit mounted speakers are isolated from their enclosure. Wole floating rooms are created to isolate and prevent vibrations and transmission which would cause unwanted resonance & in effect mask the direct sound from the speakers. The objective should be to reduce the amplitude and broaden the frequency range of any resonance & not uncontrollably create it.
 

aventhusiast

Active Member
It's EXACTLY the same,

Person - Speakers - Room

Objective: Un-coloured, Accurate, Direct Sound.

The only difference is the budgets can be very different, but what we are talking about here can be done for £0 by just avoiding doing it in the first place.
 

HiFiRuss71

Distinguished Member
Sorry, I'm confused.

If you're right, I should bin my big box sub, because it's more capable of exciting the rooms modes, which will colour the sound?:confused: I can't have my sub 20Hz performance cake and eat it? Rubbish.

By your logic, best not to have an audio system at all, if the room isn't unimpeachable.

If he were running an opposed driver array, thereby cancelling the opposing forces, all he's left with is the pressure waves from the combined driver output. I don't see how this is any different to dumping a large box sub in your front room. Except it'll go deeper with less distortion of course.

It would be nice to employ all of the isolation and dampening techniques you describe, but they're either A) Expensive and/or B) Butt ugly. As such you have to endure the colorations your room adds unless you want it to look like some freakish laboratory, which is fine if you have a dedicated listening room or no eyes in your head.

EQ devices are commonplace in AV amps, or as add on devices and domestically, they're far more acceptable. They're not perfect, but then nor's my room. Is yours?

Russell
 

aventhusiast

Active Member
What I'm saying is attaching it to the structure will make it worse. Of course everything resonates, nothing is perfect, but this will make it WORSE. Thats why so many isolation products exist :rolleyes: .

I never said you have to employ the techniques of a studio & make your room a "freakish laboratory". All I'm saying is you can learn from WHY some of the best rooms in the world are the best (THEY ISOLATE), & be smart about what what you do with your own. i.e. dont do the opposite & fix it to the structure.

I agree no rooms perfect but whats the point of adding MORE resonance when all it will do is mask the direct sound from the speakers and skew your stereo image, clarity, definition,...& the rest

As I said in my first post I hate to sound negative but this is basic rules of good room design in terms of audio. Why not learn from Acoustic Technicians & Audio engineers who have created some of the best rooms which HAVE to be as accurate as possible (to MAKE the very audio that gets put on the DVD's?) To do the opposite baffles me, especially when its free & wont look like a "freakish laboratory".
 

HiFiRuss71

Distinguished Member
I see what you're saying about isolation, but the negatives of this style of design are outweighed by the positive benefits IMHO. We're talking about an opposed driver array. The forces will cancel. They'll exert less force on the floor than a single down firing 12" sub sitting on it. In turn, the mechanical forces exerted on the floor are tiny compared to the acoustic ones exerted by the drivers output.

A lot of respected names in AV world, designers of speakers and the like, are utilising subwoofer arrays just like this. If it didn't work, it wouldn't have developed the cult following it has.

To most people in this forum, a flat, low distortion response to 20Hz is what matters. If it makes a room buzz or two, so-be-it. It's better than no 20Hz response at all. If a structural resonance added to the sound in any significant way, then it's perfectly easy and cheap to EQ the problem frequency out. We're regularly EQing out 10-20dB peaks caused by a rooms fundamental dimensions, what you're worrying about is a fraction of that.

You're also assuming that ultimate fidelity is everyone elses goal. Your solution to the OPs problem isn't free, it's just to do nothing and that's no fun.

To most of us, the fun bit is more important and none of this is helping the OP who has already decided to do this.

Russell
 

aventhusiast

Active Member
I take your point, home cinema should be about fun afterall. We're probably coming from different angles. I'm more from the audio engineer side of things than AV, every weakness would have to be minimized before driving the first nail :boring:. I was concerned he might get the asthetic he was after but introduce a few nasty side-affects without knowing.

It's all healthy discussion & still on topic as it's just our thoughts on the issue. :smashin:
 

Nimby

Distinguished Member
There are several hundred IB subs in existence.
If structural vibration as result of mechanical excitation was a problem then you would have certainly heard about it by now. :)
 

Gregory

Active Member
All this is completely academic if you don't use lots of large drivers. A little cone in the floor won't do anything but hit the end stops on the first bit of bass. You'll just have to think in terms of building a small sub box hidden between the joists.

As noted this may be exactly where I end up. Would prefer a true IB, but the roof structure doesn't allow it, so needs must etc. May also just try a normal large unit (monlith etc.) but would rather try the invisible option first.

And, to the later posts, yes I know about the potential issues. But trying and learning is half the fun, especially if it's looking for solutions in a way that others haven't tried before (probably says something bad about me, but not sure I care) - Russell is spot on. And to repeat, low matters more than loud here, so I am hoping that max extension on the driver isn't a huge issue (though it clearly will be a constraint, and I am derating the normal ~400W to about 200W to partially compensate)

Cheers

Greg
 

Gregory

Active Member
An update.

I got the XLS10, and thought I'd test for Nimby's concern by running the driver with no baffles at all - just about as bad as it could possibly be in terms of too much extension. On large step pulses (there are several in 'Flushed away' as an aside - that happened to be what the kids were watching and it did the trick), at not huge volume, it certainly moves the cone a long way. Looked inside the safe range of the driver (just) - but a little concerning.

I am pondering putting a stop further down the joists to make a 'normal' sealed box. However, I am also tempted to put it in as originally planned and keep an eye on it. I can always fit the back of the box later (with effort!). Or, bite the bullet and put a proper IB in the roof. I'm not super keen on that at the moment through since there are three layers of roof to cut through.

In fact, as I think about it, I might also see if I can rig it up as a servo (i.e. put a displacement sensor on it, and then control for displacement, not force you get if there is no feedback - i.e. as normal), so properly protecting the driver from over-extension. That might be fun to try, and would get decent sound pressure and bass range, without risk to the driver - it would sound rubbish if it maxed out, but it wouldn't cause any damage.

A further aside - I started by using the plate amp from the existing Celestion S10. Despite the alleged 200W rating, it hardly shifted the driver at all. I suspect it just curled up and gave up at a much larger driver. The S10 is an 8 inch driver, though I'd guessed it had an 8ohm voicecoil, so thought the amp would work fine. Appears not (though I didn't measure the impedance of the driver in the S10). When I tried a standard Denon Stereo amp it worked just fine.

Cheers

Greg
 

Gregory

Active Member
Further update.

I wanted to check the sound quality, do some further driver excursion tests, and check the frequency response before burying it in the floor, so I built a test cabinet.

If there were marks for grunge style then this would win serious points, given that it uses re-used dense chipboard loft boarding, some marine ply, and is sealed using duct tape. But, in terms of construction its very solid, and at about 55 litres feels big enough to be a sealed cabinet for the peerless XLS10 driver.

Initial view, lashed onto an old stereo amp, is that it works a treat (by comparison with the ghastly Celestion S10 - don't know why I've put up with that for so long). Way more LFE capacity in films, and notably better for music. But, that's a real quick assessment since I'll be away for a few days and wanted to get a feel for it before I left. BTW, in case anyone has doubts about the need for sealing, it made a horrible hissy noise until it was sealed (air rushing in and out of holes - you could easily feel it - more movement than a bike inner tube with a puncture).

The challenge, as noted by Nimby, is to avoid blowing the driver on low stuff if mounted as an IB. The current plan is to have a BFD preset for moderate vols that allows relatively low frequencies - this is how we watch most films anyway (kids are in bed). Should also work for music that tends to have less super low freq stuff anyway (AFAIK). Then have another preset that clips off the lower stuff to a safer level that allows use at higher volumes without driver destruction. Slightly annoying in that it would be easy to use the wrong preset - not exactly fault tolerant! But, it's fun to tinker. I think I could fit 2x12 inch drivers with a lot of effort, so that might also provide a route if the 10 inch driver really isn't up to it. But, want to check first since the effort would be huge

Cheers

Greg
 

Nimby

Distinguished Member
Even 4 longthrow 12" drivers would be very risky in an IB for films!

Think about at least 8 longthrow 12" for safety.

4 x 15" or two longthrow (expensive!) 18" drivers is much more like IB film fun without worrying about trashing them on LOTR or any other fav bass fest.

Sink a suitable box for your driver into the floor if that's really where you want it.

Forget duct tape it's not going to stay airtight forever. The usual thing is to run glue or silicone around the inside of all the joints to seal them.

Personally I'd just build a properly matched box for your driver and forget all about excavating the floor.

If woodworking is not your finest skill then look at alternative enclosures.

Seal the driver baffle into something alternative that doesn't look aything like a subwoofer. The garden/DIY centers are full of large decorative pots for not a lot of money. Just don't force a badly fitting baffle into them or they might crack!
 

Gregory

Active Member
Even 4 longthrow 12" drivers would be very risky in an IB for films!

Think about at least 8 longthrow 12" for safety.

4 x 15" or two longthrow (expensive!) 18" drivers is much more like IB film fun without worrying about trashing them on LOTR or any other fav bass fest.

More than I've seen for any other IB set-up! Next time maybe (I have another room where that might work better).

Sink a suitable box for your driver into the floor if that's really where you want it.

Clearly still an option.

Forget duct tape it's not going to stay airtight forever. The usual thing is to run glue or silicone around the inside of all the joints to seal them.

Absolutely - but this is intended as a 1 month test box, so as long as it stays on it's fine!

Personally I'd just build a properly matched box for your driver and forget all about excavating the floor.

If woodworking is not your finest skill then look at alternative enclosures.

Final back-up plan it to declare that it can't be hidden and so have a clear excuse (sorry carefully reasoned argument) for simply buying something like the SVS cylinder sub-woofer

Th
anks for the thoughts - and I absolutely recognise that there is a material chance that this won't work ... but fiddling is quite entertaining :) .

Cheers

Greg
 

Gregory

Active Member
After a break on this project whilst I did a patio (not finished yet thanks to the endless rain), I have picked it up again. The 10 inch driver in the grunge box still works fine, but I have decided to (almost) follow Nimby's sage advice, and go for a driver with much much more headroom. So, a Mach5Audio IXL 18.4 driver is on it's way from Canada. This is an 18 inch driver with a huge Xmax (22mm in each direction), so provides serious horsepower for a single driver. And very reasonably priced as well - £200 inc. air-freight (£155 by sea, but that means a 2 month wait). I suspect I'll have to pay duty as well, but it still makes it pretty much cheap enough to simply buy to play with.

I have two mounting options - either hanging below the floor joists pointing upwards, or pointing downwards with the body of the driver hidden away between the joists. I am favouring the latter as it will be less intrusive in the room below, but will check it out to see that it sounds OK when using the back face as the 'listening face'.

I can also add more drivers fairly easily if I should find that this one doesn't provide enough oomph. But, with roughly twice the displacement of most 18 inch drivers, I suspect it's enough!

Of course this leaves me with the unwanted XLS10 driver - hardly run in if anyone wants one.

Cheers

Greg
 

Nimby

Distinguished Member
Congratulations, Greg! :smashin:

You'll probably have the first IXL18.4 in Europe! :cool:

I would have preferred to have seen two used in an IB for safe headroom on action film LFE.

You might get away with it in a modestly sized room if you are careful.

Two would offer about the same displacement as my 4 x 15" drivers.

I imagine you will get a bill for VAT (17.5%) and a nominal sum for customs clearance.

This new driver is going to be very big in IB worldwide.

Fortunately it has 18.4" broad shoulders. :smashin:

It was designed after questioning the IB Cult members what they wanted in a driver.

If one could get a forum powerbuy organised for these beautiful drivers the freight charges could come down considerably according to the manufacturer.

The total order would need to reach a minimum of 50 units to make this work.

Even at £200 per unit that's one hell of a driver for the price in European terms.

Two drivers for £400 + an EP1500 or 2500 pro-power amp would make a world beater sub for mere pocket money in box sub terms.

Magnet up or down is a matter of taste. My IB manifold uses both orientations. This helps reduce distortion by about 30%.

Any chance of posting some images when you get your driver? :)

You'll get them all drooling! :devil:
 

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