High wool content gasket felt is a good material for controlling diffraction effects on the front baffles of loudspeakers. For those with a technical mindset, DLR on his website, has very good documentation as to the benefits of using this type of material. High wool content gasket material is available online from McMaster-Carr Those people wanting to use a local supplier should look in the yellow pages under gasket material/custom gasket makers. This material is quite expensive, $8.00/sq ft and up. The higher the wool content and the thicker the material, the higher the price.

In this example we start with a single piece of 1/2" thick material. This is then split to make 2 identical pads

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In this example a Masonite template was made to elevate the tweeter, to avoid destructive interactions with the grill cloth frame. The same template was used to mark the felt pad.

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The pad was marked with a Sharpie for the cut out. The cutout for the dome should be square not round. 

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This is a test fit of the pad before splitting begins

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Starting from one edge, the pad is torn into 2 sections of equal thickness.

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This is an end view. It's fairly easy to pull the pad apart. With reasonable care, the tear will result in sections of equal thickness . Pull slowly and equally. 

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The pad just prior to separation.

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When separated, there's the factory finished surface (on the right), and the torn surface (on the left). Attach the factory finished surface to the baffle. It has a slicker surface and isn't as absorbent as the torn surface

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I attach the pads with spray adhesive, spraying only the the back side of the pad. This has adequate holding power, but allows for easy removal if necessary

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