These are absorption type resonators, so they are more like mufflers and reduce the volume of exhaust noise. They also increase exhaust flow much better than straight pipes. I installed them on my Hellcat which was straight, scratchy and made an annoying hum at high speeds. A couple of them got my car back into legal territory, it still sounds like Hellcat but is louder than stock and not annoying anymore. I lost about 12 decibels on cold start and 5-8 when idling compared to the straight pipe,