Diplophonia

Diplophonia, also known as diphthongia, is a phenomenon in which a voice is perceived as being produced with two concurrent pitches^[1] (not to be confused with overtone singing, in which singers actually, and deliberately, produce two tones at the same time). Diplophonia is a result of vocal fold vibrations that are quasi-periodic in nature.^[2] It has been reported from old days, but there is no uniform interpretation of established mechanisms.^[3] It has been established that diplophonia can be caused by various vocal fold pathologies, such as vocal folds polyp, vocal fold nodule, recurrent laryngeal nerve paralysis^[3] or vestibular fold hypertrophy.^[4]

The Voice Quality Symbol for diplophonia is V̬‼.