In modern times, orchestras or singers have begun to introduce Program or Backing Track, which gives songs multi timbre , such as the elements of electro-acoustic, string and other performances . But this requires all musicians to bite very tightly with the metronome, and one shot cannot be missed. Therefore, the sound of the metronome and whole band are usually played in IEM. You know that it is impossible to put the metronome on the monitor speaker . How embarrassing if audience find that !