Google "mayco industrial noise control with lead" and then follow their recommendations for sound control?

Xian had a very well built ITA 4door civic and had interesting rear bar mounts for his speedway bar in the back. Not absurd in size but quite stout.

This is where it gets iffy. Gran's example of the bulkhead is a good one. Aluminum foil may meet the letter of the rule (don't recall a min/max thickness for it) and surely 2" thick we would agree is for other than intended purposes.

But what thickness in between is the cross over from too little to too much? Is it defined in the GCR? My personal opinion is the max thicknesses would be something like 2x the metal I am welding to.

But that is because if I get too much difference then I burn holes in the thinner material.