The source of a musical note is always some "system" in vibration. It can be a stretched string as in the violin, a column of air as in a flute or organ or a bent rod as in a tuning fork. These vibrations communicate themselves to the air surrounding the vibrating source and are transported from the source of sound through the air to the ear of the listener. From the physical point of view we can consider three aspects of a musical note:
But thinking about sound as we hear it, Samuel Clements alias Mark Twain, said "Wagner´s music is better than it sounds" finding it a bit cranial. But most of us can easily distinguish an awful violin from a great instrument, and the ability to hear the qualitative difference between a good violin and a masterpiece really calls for the expertise of the experienced player, maker and listener. Nonetheless most people can recognize individual voices over low-tech telephone equipment, even very similar voices. The human ear is a miraculous piece of equipment. The fact that we can hear most of the distinguishing characteristics of a double bass through a tiny transistor radio with a 2 inch speaker is fascinating. The small speaker can in no way reproduce the low frequencies of the bass. This is where the ear comes in. The ear and the peculiar way in which it is linked to the perceptive regions of the brain has the amazing capabilty of actually adding the low notes of the bass to the physical sound which we hear from the tiny speaker, deducing them from the scant information at hand. This is the realm of psycho-acoustics.
If it were not for the fact that our minds/brains have since an early age been compiling a huge database of visual impressions, we might not be able to recognize a dalmatian dog on a cobblestone pavement in this highly graphic image.