[Trombone-l] harmonics

[David Chamberlain]

Does anyone out there know about why/how harmonics sound when we play a
note?  On all my horns a C rings when I play a low F.  On one of my
horns a pedal B-flat causes both an F and a D to ring.  These harmonics
sound while I'm playing a note, and the bell continues to ring after
I've stopped playing!  When playing a slow passage, the notes that don't
sound these harmonics have a noticeably duller sound.  Have any of you
had the same experience?  Is it desireble for harmonics to ring?  Is a
horn or bell "better" if it rings?  I'd appreciate it if somebody could
shed some light on this.
Dave Chamberlain
New York, NY