[Illustration: FIG. 175.--The hollow wooden box reenforces the sound.]
258. Borrowed Sound. Picture frames and ornaments sometimes buzz and
give forth faint murmurs when a piano or organ is played. The waves
sent out by a sounding body fall upon all surrounding objects and by
their repeated action tend to throw these bodies into vibration. If
the period of any one of the objects corresponds with the period of
the sounding body, the gentle but frequent impulses affect the object,
which responds by emitting a sound. If, however, the periods do not
correspond, the action of the sound waves is not sufficiently powerful
to throw the object into vibration, and no sound is heard. Bodies
which respond in this way are said to be sympathetic and the response
produced is called _resonance_. Seashells when held to the ear seem to
contain the roar of the sea; this is because the air within the shell
is set into sympathetic vibrations by some external tone. If the
seashell were held to the ear in an absolutely quiet room, no sound
would be heard, because there would be no external forces to set into
vibration the air within the shell.
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