ok..to try and simplify this,sound comes in waveforms, causing an oscilation of molecules, and is subject to harmonic distortion (the amplification and degradation of the wave form) as the wave moves thru a variety of molecules. Any number of a zillion things effect this as everything vibrates differently. the angel of the exhaust exit effects the way the sound wave reflect and refract off the water and swim platform, the density of the water in the exhaust. the composition and thickness of the material the block heads and exhaust are made of, the ambient temp of the air, the relative humidity of the air, RPM of the motor, other ambient sounds in the air (electronic noise cancelation works by creating sound waves that have the effect of 'canceling' other sound waves in a specific frequency) the properties of the fuel used can effect sound etc. even the individual auditory anatomy and physiology play a part. So what makes an engine with the same exhaust louder than another is dependent on an incredibly large amount of variables. understanding and being able to discern the subtle variables are what make some tuners so good as they can tell by listening whether a motor is running hot, cold, rich, lean, whether the cylinders are firing balanced etc. like a seasoned medic being able to tell the condition of a patient simply by looking at them. I realize the causes more questions than answers about your post, but the simple answer is that it could be on thing or any number of things. Harmonics are are also very individualized. as a good sonar tech can identify a large vessle, determine rpm and it's speed just by listening. As far back as the late 70's early 80's the us navy had sound profiles on most russian warships computerized allowing submarines to indentify what it 'heard' (sound also travels much more efficently under water) another explination would be to use guitars, 2 guitars same model same manufacture, same materials and same strings.. will require slightly different intonation settings and tunings to sound exactly the same, think about tuning a guitar by ear, when the tuning is close you hear the sound "worble" untill it is perfectly in tune than the harmonics and sound waves synch. yet the tonal qulities of the 2 guitars may be very different.
