[Illustration: FIG. 169.--Elastic balls transmit motion.]
[Illustration: FIG. 170.--When a ball meets more than one ball, it
divides its motion.]
253. Why Sound dies away with Distance. Since the last ball _B_ is
driven outward with a force nearly equal to that possessed by _A_, it
would seem that the effect on the ear drum should be independent of
distance and that a sound should be heard as distinctly when remote as
when near. But we know from experience that this is not true, because
the more distant the source of sound, the fainter the impression; and
finally, if the distance between the source of sound and the hearer
becomes too great, the sound disappears entirely and nothing is heard.
The explanation of this well-known fact is found in a further study of
the elastic balls (Fig. 170). If _A_ hits two balls instead of one,
the energy possessed by _A_ is given in part to one ball, and in part
to the other, so that neither obtains the full amount. These balls,
having each received less than the original energy, have less to
transmit; each of these balls in turn meets with others, and hence the
motion becomes more and more distributed, and distant balls receive
less and less impetus. The energy finally given becomes too slight to
affect neighboring balls, and the system comes to rest. This is what
occurs in the atmosphere; a moving air particle meets not one but many
adjacent air particles, and each of these receives a portion of the
original energy and transmits a portion. When the original disturbance
becomes scattered over a large number of air particles, the energy
given to any one air particle becomes correspondingly small, and
finally the energy becomes so small that further particles are not
affected; beyond this limit the sound cannot be heard.
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