7. Wave length. Frequency in wave motion is related to wave length. The wave length of a water wave is the distance between the crest of one wave and the crest of the next wave. This distance remains the same as long as the wave continues, even though the wave becomes so small as to be hardly perceptible. All waves produced do not have the same wave length. A small pebble dropped into a pond will produce a wave of short wave length, but a large stone will produce a wave of correspondingly longer length. In sound the wave length is dependent upon the frequency of the source. Similarly, in sound the wave length of a sound wave is the distance between the point of maximum compression of one wave to the point of maximum compression of the next wave. These facts are illustrated graphically in Figures 4 and 1. Sound travels at different speeds in different substances, thus it travels at a much higher speed in water and steel than in air. We are interested only in sound traveling in air, where it travels about 1100 feet a second. An illustration of the fact that time is required for sound to travel from one place to another is shown by a steam whistle at a distance of several hundred yards. If it is observed when blown, it will be noticed that the steam can be seen coming from the whistle a considerable length of time before the sound of the whistle is heard. Sounds of all frequencies, or pitches, travel at the same speed. Therefore if we divide the speed at which sound travels by the frequency, we will obtain the wavelength of the sound wave.

8. Speech. The sounds of speech are divided into two classes, vowels and consonants. The vowel sounds are used in the pronunciation of the letters "a," "e," "i," "o," "u," and sometimes "y," in the formation of words. These letters are also used in combination to indicate other vowel sounds. The pitch frequencies of the vowel sounds in male voices range from 110 cycles to 140 cycles. For female voices the range is from 230 to 270 cycles. The characteristic frequencies, or overtones of the vowel sounds, however, reach frequencies of 3300 cycles. So important are these overtones that the pitch frequency can be entirely eliminated without noticeably changing the sound sensation produced on the human ear. The full range of frequencies used in vowel sounds is from 110 cycles to 4800 cycles.

The pitch frequency of the vowel sounds are produced when air is blown through the vocal cords. The vocal cords are two muscular ledges in the air passage of the throat. When these muscles are taut there is a narrow slit between them, which sets the air passing through into oscillation. The sound produced by the vocal cords is changed by the cavities of the mouth. The shapes of the cavities continuously change as a person speaks, making it possible for him to produce a wide variety of sounds, all of very nearly the same pitch frequency.

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