When any object composed of an elastic material
is disturbed, it will vibrate at its own special set of frequencies, which
together form its special sound.The frequency is the natural frequency of that
object. The tendency of one object to force another object into vibrational
motion is called forced vibration. If an unmounted tuning fork is struck, the
sound it makes is faint. Hold the base of the fork on a table top, and the
sound is relatively loud. This is because the table is forced to vibrate, and its larger surface sets more air in motion. The table top becomes a
sounding board, and can be forced into vibration with forks of various
frequencies. When the frequency of forced vibrations on an object matches the
object's natural frequency, a dramatic increase in amplitude occurs. This
phenomenon is known as resonance. A glass
has a natural frequency. Now if the pitch of the sound
creating forced vibration match the resonant frequency of glass the vibrating
air will cause the glass vibrating too. If this occur with sufficient volume,
right frequency and high enough amplitude, this will cause exceed the strength
of the glass to resist the vibrations. When the sound gets too loud for the
glass to vibrate, it breaks the glass.
