Mutism, autism and dysphasia are among the severe impairments of communication abilities. Specific impairment of voice production may be due to laryngeal deficiency, artificial larynx, laryngeal palsy or from deficiencies in other speech organs. The indistinct speech that results may be characterized by drawling, mumbling, slurring, etc. Impairments of voice function include absent or irregular voice modulation, pitch, intonation or volume; and harsh or other deficient qualities. Impaired speech forms includes stuttering, stammering, and similar disorders.