DIY sub between floor joists

[Gregory]

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

[Russell.Williams]

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]

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.

[suniil]

thanks russel for this, loads of ideas

Originally Posted by russ.will

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

[Gregory]

Originally Posted by Nimby

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.

Originally Posted by Nimby

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]

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]

Originally Posted by aventhusiast

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]

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

Originally Posted by Gregory

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

[Nimby]

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]

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.

[Nimby]

A studio has little to do with an average living room or HT.

[aventhusiast]

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.

[Russell.Williams]

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? 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]

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 .

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".

[Russell.Williams]

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