As I remember, the AMA guidelines are as follows:

96 or 98 db.

This is measured at 9 feet from the plane and about 12" from the ground. Take a measurement in front of the plane, both sides and the rear of the plane.

You also need to remember that different types of engines can sound quieter or louder. A prime example is comparing a 4 stroke to a 2 stroke. Both can have the same db reading, but the 4 stroke will sound quieter.