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Tim F 9th March 2003 10:37 PM

Replacing the floor
I have to replace the floorboards in my living/hifi room & it creates the ideal opportunity to improve the acoustics at the same time.
The floor boards are laid across 20cm*5cm (8”*2”) joists & are to be replaced with chipboard panels. Final covering will be rubber backed underlay & a thick carpet.
Enquiries at the local builders merchant give the options of rolls of 15cm(6”) mineral wool thermal loft insulation or panels of 5cm (2”) acoustic insulation.
If I use the latter I can’t afford to put more than 2 layers down.
Can anyone suggest the best way forward or offer some economical alternatives.
The current floor is very ‘hollow and boomy’ or ‘live’.
Please help me to make the most of a once in a lifetime chance.

Pete Mazz 10th March 2003 02:45 AM

One of the best things you can do is use subfloor adhesive on the top of the floor joists and install the new subfloor with screws instead of nails. Installing fiberglass or mineral wool will help absorb some of the cavity resonance. I wouldn't spend the extra for the acoustic panels for this application, tho.


Kanga 10th March 2003 06:54 AM


If you know where the speaker will go, then it would be beneficial to increase the stiffness and/or mass of the floor in this area. This will reduce the amplitude of vibrations going into the floor. Assuming that you can't increase height of the floor (without reducing the height of the floor joists) then you will have to add mass/ thickness to the chipboard between the joists. To do this you could add some 6" x 2" noggins running perpendicular to the joists where the speakers are located. If you put the top of these say 3/4" below the top of the existing joists, then you can fix 3/4" MDF or chip board to the the top of these. The chipboard/MDF would sit between the joists and flush with the top of the joists. Then when you attach the chipboard flooring, make sure you glue it well to the sheets of chipboard/mdf that you added, maybe using a flexible glue to add damping.

This is not something that I've tried, but it should be pretty cheap and it certainly wouldn't do any harm to the sound.


planet10 11th March 2003 04:38 PM

I don't know if it will be helpful, but i paid attention to my floor for acoustic stiffness when i built it.

2x10 joists, 3/4" ply on top of this, then a layer of 1/2" ply (sheets rotated 90 degrees, glued & screwed), and then 3/4" fir flooring laid on top of that at 30 degrees rotation (it was a pain, but works). From below there is fiberglass blanket insulation and 5/8" firestop drywall. The floor does not completely kill sound transmission between the upper & lower floors, but is quite stiff.


faustian bargin 11th March 2003 05:04 PM

if all you're concerned about is the sound within the living room, then deadening the floor using the methods listed above will probably help a lot. the carpeting with a padded backing will really work well.

another suggestion is to add stiffeners to your floor joists. 2x6 or 2x8 wood blocking that spans between the joists should help.

if you're also concerned with sound transmission to a room below, you might look into different ways of mounting the ceiling of that room. resilient channels, clips, and springs are often used to minimize structure-borne impact sound. you can find a lot of information in a little book put out by the united states gypsum company called 'gypsum construction handbook'.

all that said, the only other obvious thing is to mechanically isolate your speakers from the floor. i don't know if this is acceptable however... i don't know what the conventional wisdom is regarding coupling vs. isolating.

/andrew - actually an architect.

Tim F 11th March 2003 09:14 PM

Hey Guys
Thanks for all your suggestions & help.
I am only concerned about improving the listening room acoustics & not worried about the sound that is relayed into other rooms.
Planet 10 your suggestions are great but need your phone number so that my good lady can ring you for you to break the news to her! She's bound to take it better coming from you!!

Added to this she may just start pushing for her kitchen to be finished if I go this far.
Seriously though I will fit the stiffeners between the joists & put an extre layer of MDF (glued & screwed) in the area of the speakers location.

My local builders merchant also suggested that as it is an old house the joists may well have dried out & are probably a slack fit into the walls. So he suggested a couple of squirts of PU foam around each joist end to firm them up a bit & I figure every little thing will help.

I will also lay a strip of old fashioned pipe lagging felt on top of all the joists & new stiffeners before fixing final floor panels.
While I'm at it this is the time to run copper tube ducting through the floor for future cabling isn't it. Should I earth this to improve shielding??

All your help & advice really appreciated by this newbie.

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