eneral divisions of physical science are
concerned, sound and electricity.

Sound is the effect of vibrations of matter upon the ear. The
vibrations may be those of air or other matter. Various forms of
matter transmit sound vibrations in varying degrees, at different
specific speeds, and with different effects upon the vibrations. Any
form of matter may serve as a transmitting medium for sound
vibrations. Sound itself is an effect of sound vibrations upon the
ear.

Propagation of Sound. Since human beings communicate with each other
by means of speech and hearing through the air, it is with air that
the acoustics of telephony principally is concerned. In air, sound
vibrations consist of successive condensations and rarefactions
tending to proceed outwardly from the source in all directions. The
source is the center of a sphere of sound vibrations. Whatever may be
the nature of the sounds or of the medium transmitting them, they
consist of waves emitted by the source and observed by the ear. A
sound wave is one complete condensation and rarefaction of the
transmitting medium. It is produced by one complete vibration of the
sound-producing thing.

Sound waves in air travel at a rate of about 1,090 feet per second.
The rate of propagation of sound waves in other materials varies with
the density of the material. For example, the speed of transmission is
much greater in water than in air, and is much less in highly rarefied
air than in air at ordinary density. The propagation of sound waves in
a vacuum may be said not to take place at all.

Characteristics of Sound. Three qualities distinguish sound:
loudness, pitch, and timbre.

_Loudness._ Loudness depends upon the violence of the effect upon the
ear; sounds may be alike in their other qualities and differ in
loudness, the louder sounds being produced by the stronger vibrations
of the air or other medium at the ear. Other things being equal, the
louder sound is produced by the source radiating the greater energy
and so producing the greater _degree_ of condensation and rarefaction
of the medium.

_Pitch._ Pitch depends upon the frequency at which the sound waves
strike the ear. Pitches are referred to as _high_ or _low_ as the
frequency of waves reaching the ear are greater or fewer. Familiar low
pitches are the left-hand strings of a piano; the larger ones of
stringed instruments generally; bass voices; and large bells. Familiar
high pitches are right-hand pian