By constricting the larynx, the vestibular folds can be brought together (adducted) and, under certain conditions, vibrate. It can produce an undertone exactly half the frequency of the fundamental produced by the vocal folds. Therefore, for each second vibration of the vocal folds, the vestibular fold completes a whole vibration cycle. While the larynx generates such rich sound, the mouth cavity may be shaped, just like in the manipulation of vowels, to select some particular harmonics, resulting in a sound that may be perceived as having different pitches simultaneously.