This is the asphalt paint I used to deaden the back of the 828 bass cabinet horn flares.
Henry 107
I got the 1 gallon size. It's thick, black and easy to use. I mixed a lot of sand in with it. But it's not nearly as thick as the solvent based tars I've used. But water clean up and low smell. You need a few coats.
Don't know how practical it would be on a surface that is handled or touched a lot, like a 511 horn.