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Sources of sound such as a drum, a
guitar and the human voice have some part which vibrates. The sound travels
through the air to our ears and we hear it. It becomes clear that air is an
essential factor once we, for example, pump out a glass containing a ringing
electric bell. The sound disappears though the striker can be seen hitting the
gong. Evidently sound cannot travel in a vacuum like light can.
Sound also gives interference and diffraction effects. Because of this
and its other properties, we believe it is a form of energy which travels as a
wave, but of a type called progressive longitudinal.
Longitudinal
In a longitudinal wave the
particles of the transmitting medium vibrate to and fro along the same line as
that in which the wave is traveling and not at right angles to it as in a
transverse wave.
A sound wave produced for example
by a loudspeaker consists of a train of compressions and rarefactions in the
air. The speaker has a cone which is made to vibrate in and out by an electric
current. When the cone moves out the air in front is compressed: when it moves
in the air is rarefied (‘thinner’). The wave progresses through the air but
the air as a whole does not move. The air particles vibrate backwards and
forwards a little as the wave passes, and this causes the sound you hear when
the wave reaches your ears.
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