CHAPTER I

ACOUSTICS


Telephony is the art of reproducing at a distant point, usually by the
agency of electricity, sounds produced at a sending point. In this art
the elements of two general 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