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Why decouple subs?

Discussion in 'Subwoofers' started by Londondecca, Aug 5, 2005.

  1. Londondecca

    Londondecca
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    I have seen a few threads where they talk about decoupling subs from the floor. As far as I can tell this seems to be done to reduce the amount of pressure which is transmitted into the building structure. Surely it is the acoustic pressure and not mechanical pressure which transmits most of the noise/music into the floor/walls etc, or I am misunderstanding the basics
     
  2. Nic Rhodes

    Nic Rhodes
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    no it can significantly reduce coupling to the building (and this means the neighbours). It means the subs works the acoustic 'air' and not the floorbords and therefore seems to be more at ease. I like it.
     
  3. philmate

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    A few questions here.. does that mean if floorboards are present a barrier ie stone/marble/mdf has to be put between the driver and floorboards and if yes would rubber feet for the stone or spikes for the mdf be the way forward.As for a concrete floor with carpet then spikes ?. As for an SVS they have 4 short (a guess) mdf rods connected to a mdf base, is this de coupling to both floorboards and a concrete floor ? or would further de coupling between this base unit and the floor concrete or floor boards be beneficial.
    Philmate.
     
  4. eviljohn2

    eviljohn2
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    It's worth remembering that there are two relevant methods of transmitting energy into the room. There are the mechanical sound waves induced by the pistonic driver movement and the mechanical waves transmitted from the speaker cabinet.

    There is nothing we can do to minimise the induced sound waves which pressurise the room - in fact that's exactly what we're trying to perform.

    We can prevent the cabinet vibrations from transmitting into the surrounding floor (floor vibrations are very annoying to me and make the sub direction localisable as they reach each foot at a discernable time in my experience). In principle this is the idea behind "decoupling" although a unit properly coupled to the floor will exhibit similar properties. The mass loading of the speaker helps to control cabinet resonance and nothing else in my opinion.

    I don't think that the argument of the driver going forwards whilst the cabinet travels backwards carries much weight either. Whilst it's true that this will happen, it is also true that momentum will be conserved. In the free-space idealised case where this effect will occur more than when the unit is actually resting on something it still becomes negligible after a few basic calculations:

    Considering my XLS200 subwoofer's as an example, we need to consider the forward travelling momentum of the driver which is specified as the MMS in it's parameters. In my case this is 135.3g, we'll also consider that the unit has travelled 6mm forwards (Xmax is 12.5mm so this is quite feasible).
    The remainder of the unit weighs about 17000g. It follows that the unit will recoil 6mm x 135/17000 = 47.6microns. Since this is about half the width of a human hair I for one don't think it's worth worrying about.

    Just a few more thoughts on the subject.
     
  5. binbag

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    I've just gone through this with the arrival of my XLS200 DF. I had my Celestion S80 on a big 2" thick marble slab with a 1/2" marble chopping board on top to tighten its response.

    My XLS is on its spikes on the chopping board which in turn is on 2 halved squash balls. This sits on a wooden floor 1' above the ground. The surprising thing to me was how ineffective the thick slab was at stopping the transmission of truly room shaking sonics. Even just the chopping board (which sits on 4 feet) gave a much cleaner result than the slab when AVia's LFE sweep evaluation tone was used.

    I don't know how much this will change at reference levels though - I'm still running it in and I don't think the room will take much more. :eek:
     
  6. eviljohn2

    eviljohn2
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    BK have just added the Superspike's to their list of subwoofer offerings. Forum member Lowrider thinks very highly of these for anyone that's interested. :)
     
  7. binbag

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    Further setup runs at higher levels all round have led me to turn the chopping board over so the subs spikes go into the squash balls (now filled with silicon). This really has taken the room out of the sound. The previous version had the wife telling me she could see the door vibrating :D

    If anything it's too smooth now. I'm going to give it a couple of weeks to get used to it before I make any another changes as all the Avia sweeps are telling me this is how it should be.
     
  8. Nimby

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    The SVS are downfiring so need space for the driver cone to breath its magic. Otherwise there'd be no sound at all! I have my big cyclinder sitting on a round concrete slab 18" dia x 2" thick resting on carpetted floorboards. I can't really say I noticed much difference in performance though. The slab was primarily to level the sub which was looking like the leaning tower thanks to a raised joint in the floorboards just where I wanted the sub to sit.

    From reading the SVS threads on other forums some owners expressed a preferenece for a concrete floor. Personally I can't see the problem. The floor may or may not be beneficial to the sound. Only the owner of the floor and the sub can decide for themselves. Fortunately slabs are dead cheap so experimentation is mere pocket money compared with kit and cables. I must say I like the feeling of the floor melting under my chair. LOTR, Matrix and Constantine had some of those great moments. :D

    Nimby
     
  9. b33k34

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    I'm only using a MA 360 sub so nothing hugely powerfull but firing directly at victorian floorboards the sound was pretty horrid - boomy, slow and uncontrolled. Effectively the whole floor was acting as a drum.

    I bought a granite slab on isolation spikes/cups from ebay (for about £40 so it wasn't worth the effort of trying to make something). i then used decent sized lumps of blu-tack to level it up and isolate the cups from the floor. Completely solved the problem and changed the sound for the better. I see that some subs already have a 'floor' at which they fire so it's something some sub manfs have already addressed.
     
  10. binbag

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    This isolation business must be trickier than I thought - the room directly above the sub has started to 'pop' the plaster covering the screw heads. :rolleyes:
     
  11. sonic65

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    My music room is triple glazed and built on two metres of stone and concrete.
    My house is detached and some twenty metres from my neighbour who works
    on night shift. He says he can sense (rather than hear) :rolleyes: the bass line from my Monolith. Fitted some vibrapods which appear to have solved the problem.
    Will have to do some placebo tests to ensure compliance.
    Decoupling from the building structure has to be the way for me.
     
  12. chris

    chris
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    Another vote here for decouplling.

    Noticed a vast improvement with my duel servo 15's on gramma platforms.......the house no longer shakes and only the living room get's filled with bass now.
    Made a very big improvement with both movies and music.
    No more door rattles or glass's or pots shaking in the kitchen. :D
     
  13. SBanga

    SBanga
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    I would've thought decoupling was more useful for music. I think the feeling of the floor moving underneath you adds to the impact of movies, and immerses you into the picture. But I'm guessing I'm wrong having never decoupled my sub!!
     
  14. b33k34

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    You can still 'feel' the bass when theres a big 'thump' in a movie. What you don't get is all the nastiness
     
  15. JorisFRST

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    My Kef PSW3000 has conic rubber feet, about 2,5" long, so I guess that's decoupled enough on its own ??
     
  16. stevedster

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    anyone know where i can get a nice square marble slab from?
     
  17. stevefish69

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    Try your local stone merchant that makes gravestones ect.

    I bought a 1" thick slab 14" square which was polished with the edges bevelled for £25
     
  18. stevedster

    stevedster
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    cool, did you put any rubber feet etc underneath it or just put it to the floor?
     
  19. stevefish69

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    I bought 3 mitchell's tenderfeet, which are like large spikes.

    info@michell-engineering.co.uk

    IIRC they were about £13 for a set of 3 delivered.

    Gotta admit that i'm getting better results from a Gramma now though :D
     
  20. stevedster

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    you mean one of these??

    [​IMG]

    you know where i can get one for and how much??
     
  21. stevefish69

    stevefish69
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    They are available for aroud £50. A quick search on google for "Auralex Gramma" will give you some suppliers.

    I managed to pick mine up on E-Bay for £29 :D so that's another choice, or stick a Wanted thread up on here.

    Hope this helps

    Steve
     
  22. Buckster

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    is decoupling only useful for Wooden floors or for concrete ones too ?

    My room is tile (carpet isn't there where sub is) - with concrete floor underneath - presumably decoupling wouldn't help in this case as it already is ?

    thanks, MArk.
     
  23. Arendal - Russell

    Arendal - Russell
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    It'll make very little difference on a concrete floor. It doesn't vibrate like a suspended wooden floor and therefore add to/colour the sound. The energy transmitted from the subs cabinet is effectively sunk into the concrete's mass.

    Having a concrete floor is one of the few times spikes are actually effective, as you want them to do exactly what's described above. Sinking the energy out of the subs cabinet helps reduce colouration from the cabinets panel resonances.

    As to whether it makes an audible difference, you decide.

    Russell
     
  24. Buckster

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    thanks Russ :)

    I did find last night though that my Sub was part on the concrete floor and part on carpet ... with only an inch gap between carpet and sub, can't have helped, so sub is now totally on concrete floor - and now a larger gap between the driver/port and the floor :)
     
  25. RichardG73

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    I currently have a suspended laminate floor on top of concrete but am about to move into a house with a hardwood floor over concrete - should this improve the sound from my sub - foward firer.
    Also I currently use a granite plinthe but its an eyesore - with the flooring could I get rid of it.
    Just how good are Gramma platforms?
     
  26. micb3rd

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    Subwoofers produce vibrations (mechanical energy).

    It is a intersting debate, to couple or decouple.

    The question is do you want to ridgidly couple using feet transfering the vibration energy to the room floor and keep the subwoofer it self stable OR do you decouple the subwoofer using foam or the like and not transfer engery to the room floor and consentrate on pressurising the air in the room.

    Want to know something interesting, SPL testing in car has often shown securing the subwoofer enclosure using staps or bolts to a solid medium gains in overall DB output.

    So a higher SPL may be achived but is it at the cost of added room noise/buzz dirtying the sound :suicide:
     
  27. Nimby

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    If anyone has a rough old concrete floor you could try bolting your sub down with strong brackets fixed to the sub's feet.
    Then you can tell us if the sub performs better. Don't try this on a nice floor though! :)
     

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