I have had some band directors try and pick my brain about how to make their trombone players sound better. This will serve as both an instruction to trombonist and educators alike. Send me your thoughts after reading this and let me know your thoughts.
First, I want to spell out how I address tone. I try to avoid giving physical suggestions like "open your throat," or "drop your jaw." I've experienced too many students that become frustrated by feeling they are doing the physical action without improving their sound. Instead, I encourage my students to focus on: relaxing, resonance, and moving their air. I've found over the last year, positive suggestions to "allow the air to move faster," "blow farther," "blow with energy," or "focus on the resonance of the instrument" to be effective.
A good trombone sound is created by a mixture of efficient air and a relaxed body. As the pitch rises, the speed of the air must increase. Air volume affects the size and dynamic of the tone. The aperture of the lips affects the 'core' of the sound. If the lips are too close together, the sound is pinched; too far apart and the tone is diffuse or airy.
Any unnecessary tension in the body will contribute to a pinched sound.
One of the first skills I address when students begin lessons is their ability to manipulate the size and speed of the air. If they can identify, change, and expect different combinations of air-speed and size, they will have more success on the trombone. I give them exercises off the instrument like blowing a sheet of paper or blowing on their hand to hone these skills.
Spend some time to see if you can change one of these variables: airspeed, air volume, or lip aperture. Explore what sounds you can make by focusing on these changes. Go to the extremes on each variable to see what new sounds you can make. I listened to Andy Chappell, bass trombonist of the Minnesota Orchestra, perform a recital in February. I left that recital in awe at the variety of timbral colors he created on stage. In the months after the recital, I've been exploring colors using the method listed above and I've found some success. Imagination helps, too! If you can imagine a sound, you're one step closer to producing it on your instrument. This is why listening is important in the development of a musician: the clearer image in your head of the sound you want, the easier it will be to duplicate the sound.
Let me know if you have questions, I'm always happy to help. Happy practicing!