I recently added two Magnaflow 14218 mufflers to the car. It has a rather loud drone between 1200 and 1900 rpms. This is an idling video of the result.
I did a sound meter test (Droid app) and got the following readings (highest avg. readings shown ):
Idle – 50db (600 rpm)
Standard Acceleration – 75db (short peak at 77db, significant drone noted between 600-1900 rpm)
cruise – 63db (2200 rpm)
Idle – 55 db (600 rpm)
Acceleration – 79db (short peak at 81db, significant drone noted between 600-2000 rpm)
cruise – 68db (2200 rpm)
Exterior sound measure at idle (@4ft) – 68db
This is a low frequency sound and you feel it more than hear it.