*How Sound Waves act as Stimuli.*—Any sound wave represents a small but definite amount of energy, this being a part of the original force that acted on the vibrating body to set it in motion. The hammer, for instance, in striking a bell imparts to it a measurable quantity of energy, which the bell in turn imparts to the air. This energy is in the sound waves and is communicated to the bodies against which they strike.(117) Though the force exerted by most sound waves is, indeed, very slight, it is sufficient to enable them to act as stimuli to the nervous system. *How Sounds Differ.*—Three distinct effects are produced by sound waves upon the nerves of hearing, and through them upon the mind. These are known as _pitch, intensity_, and _quality_, and they are dependent upon the vibrations of the sound-producing bodies. _Pitch_, which has reference to the height, or degree of sharpness, of tones, is determined by the rapidity of the vibrations of the vibrating body. The more rapid the vibrations, the higher the pitch, the number of vibrations doubling for each musical interval known as the octave. _Intensity_ is the energy, or force, of the sound waves. This is recognized by the strength of the sensation and is expressed by the term _loudness_. Intensity is governed mainly by the width of the vibrations of the vibrating body, and the width depends upon the force applied to the body to make it vibrate. _Quality_ is that peculiarity of sound that enables tones from different instruments to sound differently, although they may have the same pitch and intensity. Quality depends upon the fact that most tones are complex in nature and result from the blending together of simple tones of different pitch. *Reënforcement of Sound Waves.*—The sound vibrations from small bodies are not infrequently reënforced by surrounding conditions so that their outgoing waves reach farther and are more effective than waves from larger bodies. This is true of the sound waves produced by most musical instruments and also those produced by the human larynx. Such reënforcement is effected in two general ways—by sounding boards and by inclosed columns of air. Stringed instruments—violin, guitar, piano, etc.—employ sounding boards, while wind instruments, as the flute, pipe organ, and the various kinds of horns, employ air columns for reënforcing their vibrations. In the use of the sounding board, the vibrations are communicated to a larger surface, and in the use of the air column the vibrations are communicated to the inclosed air. (See Practical Work.)