Note: Spectrograph displays vertical striations to represent pitch of vocal cords. Narrow and wide spectrographs are used for different purposes. Different resonance chambers are excited by different fundamental frequencies. The introduction of new resonance chambers requires adjustments on part of singer based on feedback. Articulation habits must vary with pitch. The spectrograph also shows breathiness patterns and harmonics. Resonance distortion or reverberation from the room in which the recording takes place also shows up on the spectrograph. Trubey notes that there are at least nine recognizable categories of nasality: these include the alveolar nasality of French and Portuguese, the open nasality of a singer in a mosque, and nasality in which the tongue is thrust up into the nasal cavity and nostrils are contracted, as in some Asian classical singing. Pop singers such as Al Jolson employed nasality for added resonance (to hit the galleries). Clear enunciation takes place in the front of the mouth. In opera singing intelligibility is minimal. On a spectrograph vowel sounds sung by opera singers are sometimes scarcely distinguishable from each other.