Spent a bit of time working on quieting the cab. Worked with a combination of the self adhesive aluminium/rubber mentioned in an earlier post, fire resistant acoustic foam, reflectix insulation and some heavy rubber matting I had left over from another project.
I rolled the aluminium/rubber on with a large spark plug socket and sealed some of the edges with aluminium roofing tape. The butyl rubber compound won't off-gas and so I didn't worry about sealing all of the edges. Also, as the intent is only to prevent a harmonic resonance, there is no need to cover the entire surface with this material. The reflectix and acoustic foam will provide thermal insulation and attenuate reflected sound. As well, I pulled the bench seat out and cut some heavy rubber to cover the cab floor. It will be under the stock matting.
I ended up insulating all 3 of the cab doors as well as the floor, bulkhead behind the rear bench seat and driver rear side of the cab. Eventually, I'll pull the dash and do that area as well.