Most trumpeters play the Irons exercises too fast and too loud. Write "p" (piano) on the top of every page. Place a piece of tape over the dynamic marking. If your neighbors can hear you, you are playing too loud.
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This is the holy grail. Unlike a typical method book that mixes theory with songs, 27 Groups is a pure athletic workout for the embouchure. Most trumpeters play the Irons exercises too fast