A brass player's sound travels out of the bell away from our ears, reflects off surfaces and returns to our ears. Sound waves are longitudinal, meaning they travel in a straight line until they reflect off of a surface. This is the sound brass players spend years listening to, and eventually it becomes their mental image of what their sound is. Variables in the size of the room, the material of the walls and the floor, the use of amplification, and number of people in the room can have a tremendous impact on our sound. What is everyone else hearing when we play, and how is it different than our mental image?
This distinction is important to explore in order to understand that what our audiences, conductors and fellow musicians are hearing can be very different that what you are hearing.
As an experiment play a few notes at a normal volume and listen to your sound. Listen for air, resonance, volume, timbre and anything else that you can identify. Now stand a few feet from a wall and point directly at the wall with your instrument angled horizontally. Play the same thing and listen to the change in sound. How is it different? Possibly brighter, especially if you're pointing at a wall made of stone or wood. Take a few minutes and point your bell around the room at various surface with various angles and get a feel for what changes.
When a mic is placed in front of our bell and we listen on headphones we're he