i have to do a report on the principle behind how brass instruments work. i play one myself, but don't know where to find this kind of information. can anyone help?
There are purely scientific reasons why any instrument makes the sound it makes, and the scientific discipline that would give you the best start on this topic is physics. So do a google on "brass physics" or "physics of brass instruments" and read around on the first handful of sites that come back. This will give you a good start.
maintube