Bitumastic damping for DIY Loudspeakers.
Bitumastic is a dense and very heavy, petroleum-based product, often used in damping applications such as automotive, marine and tanking. It is available in rolled sheet format, often in thicknesses of 2.5mm and 5mm and is a completely dead material, i.e. it has virtually no elasticity.
Sheets have a thin protective plastic film on the outer face and a self-adhesive film with a waxed backing paper on the reverse, this must be removed before use. Bitumastic sheet
The sheet flexibility varies with temperature and in cold conditions it becomes very brittle and is easy to crack and snap. In order to cut to size, use a straight edge and craft knife. Lightly score the first cut to prevent knife wander and repeatedly score more firmly a further 3 - 4 times. Reverse the material and on the underside repeat the scoring process, as above. Repeatedly bend the material along the cut line and it will snap. With a (1.5 - 2)mm drill, make some pilot holes, 5 to 6 will do, over the entire surface, 4 corners, some 25mm from the edges and a central point is good. This is for later pinning the pad to the board. The material 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 adhesive.
Position the pad carefully, as once fitted it will be difficult to remove. 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. Ensure the tacks are of appropriate length and will not protrude through to the other side of the board. This tacking will prevent creep of the pad during warm weather / high temperatures, hence job done.
Note:- It is not always necessary to treat two like-sized panels equally. This depends 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.
Bitumastic Sheet is available from DAMPING MATERIALS