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.

*Reenforcement of Sound Waves.*--The sound vibrations from small bodies are
not infrequently reenforced 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
reenforcement 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 reenforcing
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.)

*Value of Sound Waves to the Body.*--From a physiological standpoint, the
value of sound waves is not easily overestimated. In addition to the use
made of them in the communication of ideas, they serve the purpose of
protecting the body, and in the sphere of music provide one of the most
elevating forms of entertainment. Sounds from different animals, as well
as from inanimate objects, may also be the means of supplying needed
information. The existence of two kinds of sound instruments in the
body--the one for the production, the other for the detection, of sound--is
certainly suggestive of the ability of the body to adjust itself to, and
to make use of, its physical environment. Both the larynx and the ear are
constructed with special reference to the nature and properties of sound