bell

In a wind instrument, the flat disc or flare at the opposite end of the tube from the mouthpiece.

In woodwind instruments, it acts as a shaped baffle controlling the expanding pressure wave emitted from the bell when all the finger holes are closed (at higher pitches pressure escapes from the highest open hole). In brass instruments, it also acts as a directional radiator, the thin metal vibrating with the variation between the escaping sound and atmospheric pressure.