[Illustration: FIG. 13 I & II To face page 47]
[Description: FIG. 13.--Diagram after Aikin.
1. To show position of tongue and lips in the production of the vowel sounds _a, o, oo_.
2. To show successive positions of the tongue in the production of the vowel sounds _a, ei, e, i_.]
I have already said that Helmholtz showed that each vowel sound has its particular overtones, and the quality or "timbre" of the voice depends upon the proportional strength of these overtones. Helmholtz was able by means of resonators to find out what were the overtones for each vowel sound when a particular note was sung. The flame manometer of König (_vide_ fig. 14) shows that if the same note be sung with different vowels the serrated flame image in the mirror is different for each vowel, and if a more complicated form of this instrument be used (such as I show you in a picture on the screen) the overtones of the vowel sounds can be analysed. You will observe that this instrument consists of a number of resonators placed in front of a series of membranes which cover capsules, each capsule being connected with a jet of gas.
Resonators, with capsules connected with gas jets]
[Description: FIG. 14.--König's flame manometer. The fundamental note C is sung on a vowel sound in front of the instrument; the lowest resonator is proper to that note and the air in it is thrown into corresponding periodic rhythmical vibrations, which are communicated through an intervening membrane to the gas in the capsule at the back of the resonator; but the gas is connected with the lighted jet, the flame of which is reflected in the mirror, the result being that the flame vibrates. When the mirror is made to revolve by turning the handle the reflected image shows a number of teeth corresponding to the number of vibrations produced by the note which was sung. The remaining resonators of the harmonic series with their capsules and gas-jets respond in the same manner to the overtones proper to each vowel sound when the fundamental note is sung.]
Each resonator corresponds from below upwards to the harmonics of the fundamental note c. In order to know if the sound of the voice contains harmonics and what they are, it is necessary to sing the fundamental note c on some particular vowel sound; the resonators corresponding to the particular harmonics of the vowel sound are thus set in action, and a glance at the revolving mirror shows which particular gas jets vibrate. Experiments conducted with this instrument show that the vowel _U=oo_ is composed of the fundamental note very strong and the third harmonic (viz. g) is fairly pronounced.