Good points Detlof and Shadorne. The real instruments are dynamic and have overtones and (don't forget) undertones depending upon which instruments and notes you are talking about. Cello or Acoustic guitar will sound louder in lower mid, since undertones extends in the bass region. Similar to violins (as Shadorne mentioned) have overtones sounding louder than fundamentals. So then it goes back to proper and 'complete tones' reproduced cleanly by a system ( that can do that) will sound actually louder while listening. When you measure actual SPL, it is not that loud (numbers) at all. I measure when my daughter plays a guitar. the sound fills up a room, but when measured, it is a 'low' number relative to what you thought it might be. Drums (and some wind instruments) are different story. They do go high in SPL in real life, when struck hard. But when using light strokes, it sounds loud due to tones/undertones/overtones SPL differences.
This phenomena is one of the main reasons why some system sounds (close to) real and most don't.
This phenomena is one of the main reasons why some system sounds (close to) real and most don't.