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 piano strings; smaller ones of other
stringed instruments; soprano voices; small bells; and the voices of
most birds and insects.

Doppler's Principle:--As pitch depends upon the frequency at which