there is term ppl use : "decouple" , which means u dont want ur speaker directly contacting d metal of ur door or back dash since like i said = rattles
what i do cut a ring of mdf & apply a bead of caulking on it (which also seals any air escaping between d mdf ring & d metal) b4 i attach it to d metal - then u screw\bolt ur speaker onto d mdf ring