It doesn't make it louder per se in an SPL competition setup. In a daily setup for music it sure does clean up the sound a whole lot. This being a street setu for a fanatic that loves bass, a good bit of sound deadening would go a long way.

Also in a jetta to stop the rear deck from rattling which also detracts from the music, jus stuff some carpet or an old t-shirt or under the rear deck via the trunk.