they are a chambered muffler.
I had been running pypes violators for 3 years and was ready for something different.
$40 ea was half the cost of any of the others so the price was good. Summit and LMR both carry them for the same price. lots of positive reviews too. The only drone for me is right ar 55mph and it's only barely noticeable.
Sound is similar to flowmasters, maybe a little more mellow but I'm running cats so i'm sure that is part of too.
I did have a friend txt me after I passed him on the hwy. He knew immediatly my car sounded different.