Until the publication of Helmholtz's work vocal theorists had known practically nothing of acoustics. The fact that the tones produced by the vocal cords are increased in power and modified in quality by the resonance of the air in the mouth-pharynx cavity came as a distinct revelation to the theoretical students of the voice. Helmholtz confined his experiments and demonstrations to the mouth-pharynx cavity, and investigated in particular the influence of this cavity in producing the various vowel and consonant sounds. But vocal theorists at once extended the idea of air resonance, and connected it with the well-known sensations of vibration in the chest and head. It was assumed that these sensations are caused by vibrations of the air in the chest and nasal cavities. This assumption has been accepted without question by the great majority of vocal scientists. Both the chest voice and the head voice are now believed to owe their distinctive qualities to the reinforcing vibrations of the air in the chest and nasal cavities respectively. The mere fact that these vibrations can be felt is held sufficient proof of the statement. "In every true chest tone the resonance can be distinctly felt as a vibration (fremitus pectoralis) by the hand laid flat on the chest." (_Die Kunst der idealen Tonbildung_, Dr. W. Reinecke, Leipzig, 1906.) It must be observed that this is by no means a satisfactory scientific proof of the doctrine of chest resonance. This feature of the subject is reserved for discussion later. The doctrine of resonance is now generally accepted as one of the basic principles of Vocal Science. It is stated, in substance, by almost every authority on the voice that "The tone produced by the vibration of the vocal cords, even when the laryngeal action is correct in every way, is weak, of poor quality, and without character. This tone must be strengthened and made of musical quality by utilizing the influence of resonance." The subject of resonance is always treated in theoretical works on the voice under the three heads of chest, mouth-pharynx, and nasal resonance. To these a fourth is sometimes added,--the sounding-board resonance of the bones of the chest and head. _Mouth-Pharynx Resonance_ Considered strictly in its bearing on tone-production, the resonance of the mouth-pharynx cavity does not receive much attention from theoretical observers of the voice. The form assumed by this cavity is of necessity determined by the vowel to be sung. Aside from its function in the pronunciation of words, the influence of mouth-pharynx resonance on the tones of the voice is seldom discussed by vocal scientists.