Why do some aircraft seem louder than others?

Each type of aircraft has a different set of noise characteristics.  Loudness is not entirely dependent upon aircraft age, size, weight, number of engines, performance, or any other single characteristic, but rather upon a combination of all.  For example, a 15,000 pound Lear 25 corporate jet is louder than a 656,000 pound 777.  A large propeller driven aircraft may be louder than a Boeing 717 aircraft.  The new A380 aircraft is reportedly 20-30 decibels quieter than the MD-80 aircraft, but both meet the requirements of FAR, Part 36 for Stage 3 aircraft.