The "Quiet-er" reverse cones are just like the 18" straight cores, except they feature a patented chamber design to baffle some of the sound and reduce the overall noise.
Unlike the Max Performance and Open Straight Cores which "scream" at high revs, these chambered cones create a deeper tone and lower sound level.
That makes for more of a "rumble" as opposed to a "scream". So they are indeed "quieter" than the open-core cans, but definitely still louder than a stock exhaust.
Unfortunately, I do not have any decibel diagrams, etc. to share.
For more, you can always reach out to Cone Engineering.