Muffler design is the main factor. The chambered mufflers send the sound waves back and forth in the muffler so the muffler is very loud. Many of us recall the ‘69 Camaro with the hot street muffler of the day. We didn't care if it was loud inside the car or out; we wanted people to hear those horses. Today, enthusiasts like to hear the CD player or carry on a conversation while still enjoying a muscle car. And, we have to deal with both legal issues and the integrity of “do onto others as we want them to do onto us”. If we live in a gated community, the neighbors will probably not be willing to deal with a 100 db machine rumbling into the garage at 11pm. Hence the recent muffler relocation to the rear on most vehicles - an effort to keep resonance down inside the vehicles. Breaking down the sound, however you do it, will be quieter for the driver done as far away from them as possible. Effectively speaking: mufflers placed within about 18” of the header collector make the most power.