It’s a common enough experience. You’re happily playing away when suddenly your tone splits into two different pitches. Usually it’s only a simple missed attack and you can instantly correct and hold the pitch stable. Sometimes this happens around a particular note and starts to get impossible to hold the the pitch without the tone splitting. It might even get to the point where you’re so worried about this that every time you get to that pitch you mentally or even physically flinch, which just makes the problem worse.
Having had this problem myself a couple of different times I can really empathize with brass players who are having this trouble. Not understanding what exactly is going on can make it challenging to figure out what to do. Sometimes the solution that seems obvious only makes things worse and sometimes it goes away on its own, only to come back later.
While I’m sure there are many possible culprits, in my experience a double buzz is likely to be caused by one of the following scenarios.
