How to Use Bitumastic Damping Sheet in Loudspeakers
Bitumastic Damping Sheet is a dense and very heavy, petroleum-based product.
As an almost completely dead material, i.e. it has virtually zero elasticity, it is very good at Damping the cabinet panels
of loudspeakers and wooden enclosures which suffer from mechanical vibration.
These bitumastic sheets are available in three material thicknesses of 1mm, 2.5mm and 5mm and so are able to cover a variety of damping applications within loudspeaker cabinets.
Manufactured in flat sheets of around 1000mm by 600mm the bitumastic has a thin protective plastic film on the outer face which should remain attached. A self-adhesive film is on the reverse side with a waxed backing paper. This backing paper needs to be removed before use and is easy to peel off when required. Bitumastic damping sheet is easily broken if mishandled. Its flexibility or stiffness greatly varies with temperature and so in cold conditions it becomes very brittle and is easy to crack or snap.
For damping of loudspeaker enclosures the bitumastic needs to be cut from large sheets into more user friendly sized pieces and we supply the material pre-cut in pieces of 200mm by 300mm. Larger pieces can be cut, but due to fragility these need to be transported flat packed and so users of these are required to customer collect.
In order to cut to your required sheet size or shape the use of a straight edge and craft
knife is recommended. The bitumastic is far easier to cut when the material is warm, i.e. when it is in an in-house environment.
It can of course be warmed with a hairdryer or similar "warm" hot air blower. Do NOT use hot air-guns as used for
wallpaper or paint stripping, they are too hot.
Lightly score the first cut to prevent knife wander and repeatedly score more firmly a further 3 - 4 times. across the sheet.
Reverse the material, marking the edge where the first cuts extended to, and on the underside repeat the scoring process,
as above. Repeatedly bend the material along the cut line and it will snap. The very thin, 1mm sheet, can usually
be cut straight through from one side only.
If you break the sheet by accident it is still possible to use the broken pieces.
With a (1.5 - 2)mm drill, make some pilot holes, 5 to 6 will do, in the damping sheet over the entire surface, 4 corners, some 25mm from the sheet edges and a central point is good. This is for later pinning the pad to the board. The damping sheet should be warmed first, prior to fixing. A hairdryer is useful for this purpose, as this warming makes the material more pliable. This also enhances the effectiveness of the self adhesive film.
If you are working on flat wooden panels, pre-assembly, ensure you leave enough clearance around the bitumastic sheet to permit the correct fitting of the remaining enclosure panels. The bitumastic sheet will be difficult to remove once it is in position. So..... Position the pad carefully, Press the pad firmly onto the wood panel. Using a hammer or mallet lightly tap the pad all over to ensure the adhesive makes good contact with the wood panel. Fit the tacks into the pre-drilled pilot holes and tap them home. For 18mm MDF, 16 or 19 mm Carpet Tacks, these have larger heads than nails, and will not pierce the outside of your enclosures. During warm weather the damping sheet adhesive softens and the bitumastic sheet may creep. The tacks prevent this and so job done.
If you are working inside a cabinet enclosure, then you need to give serious consideration to the logistics. Hammering tacks into place in a small confined space is not going to be easy, you may need to be inventive. Of course if you're working on large bass enclosures then it's less of a problem.
Note:- It is not always necessary to treat two like-sized panels equally. This depends on whether you want to spread resonances or move them totally. For example, the over-enthusiastic use of damping in Band-pass Bass Bins can move panel resonances down from the out-band region, where they sit largely unexcited, into the more active in-band / pass-band, with unwanted results, so - - - - use carefully.