I know that inserting a mute into a brass instrument will sharpen it slightly (don't know why this happens either) but I wonder why; if a harmon mute, particularly with tube extended doesn't flatten the instrument? the mute itself has a circumference cork (i.e. it seals completely around the bell) so even with the tube in, the length of the tubing for the whole instrument I extended. Surely, it should flatten the instrument - with a trombone harmon mute, the extra "tubing" equates to about 1 slide position (or about a semitone). I guess it must be something about the pitch being derived somewhere in the bell area and anything that happens after that is irrelevant to the pitch.