A highly important feature of mouth-pharynx resonance is the forming of the various vowel sounds. Each vowel is simply a distinct quality of sound, caused by the special prominence of some one or two overtones. Helmholtz investigated this aspect of voice production exhaustively. At first thought, it seems strange that a tone on some one pitch gives the effect of a particular vowel. Yet we can readily hear this in the steam siren whistle commonly used for alarms of fire. This produces a screaming sound, caused by a gradual rise and fall of pitch through a range of several octaves. Its almost vocal effect of ‘Ooh-oh-ah-eh-ee’ gives us a clear idea of the manner in which these vowels are each the sound of a note somewhat higher than the one preceding. Helmholtz gives as the determining notes for certain of the German vowels the following table: [Illustration: Music score] For each vowel the mouth-pharynx must assume a shape and size adapted to reinforce with special prominence its particular note or notes. Anyone can readily observe for himself what the positions of the tongue and lips are for the various vowels. For _ee_ the tongue is arched high in the mouth and the lips are only slightly parted. For _ah_ the mouth is opened slightly wider, while the tongue lies flat, etc. No variation can be made in the size or shape of the nasal and head cavities. As these hollow spaces are small and very various in form, they reinforce only the higher overtones. Special prominence given to the higher upper partials[3] has the effect of making the tone brilliant and somewhat metallic in quality, and it is here that the special function of nasal resonance is seen. A certain degree of nasal resonance is therefore essential to the correctly used voice, but this must be kept within well-defined bounds. Excessive prominence of this resonance is the cause of the unpleasantly nasal sound which is absolutely out of place in correct tone production. There remains to be treated under this head the production of consonant sounds. Consonant sounds are of various acoustic characters and are formed in a variety of ways. They may best be considered as of two classes: those into which a tone produced by the vocal cords enters and those devoid of this element. The other determining factor in consonant formation is the point at which an interruption takes place in the outflow of the breath. There are a number of allied pairs of consonants, one with and the other without an accompanying vocal sound. These are: _v_ and _f_ _b_ and _p_ _z_ and _s_ _d_ and _t_ _j_ and _ch_ _th_ (as in that) and _th_ (as in think) _z_ (as in azure) and _sh_ _g_ and _k_ In each of these pairs the interruption takes place at the same point--_v_ and _f_ between the lower lip and the upper teeth, _b_ and _p_ at the lips, etc.