Legal noise level for cars built before dec 1983 is 96Db's. cars built after that its 90Db's.
And i pretty sure the Db is measured to 3000rpms,i havn't heard of it being varied to the car. also its measure only on the up-rev.ie. when u let off the gas at 3k, that measurement doesn't count.
This is for Victoria btw, i dont know if there is a difference in other states.