What is "diplophonia"

n. (context music English) The production, by the voice, of sounds of two different pitches simultaneously

Diplophonia is a phenomenon where two voices with different pitch are heard simultaneously. It has been reported from old days, but there are no uniform interpretation of established mechanisms. It is known that this phenomenon may be caused as a symptom of vocal folds polyp, vocal fold nodule, recurrent laryngeal nerve paralysis or vestibular fold hypertrophy.