Wave Motion in Air
المؤلف:
GEORGE A. HOADLEY
المصدر:
ESSENTIALS OF PHYSICS
الجزء والصفحة:
p-196
2025-11-13
81
Sound is transmitted by means of waves, but the particles of air do not vibrate transversely as in the coil in they vibrate longitudinally that is, in the same direction as that in which the waves are moving. The molecules that are put in motion by the first forward movement of a sounding body are suddenly pushed ahead of it and crowded nearer together, forming a condensation; but their path is not long, since they strike other molecules, which in turn set the molecules next to them in motion. When the sounding body moves back, it leaves a partial vacuum, or rarefaction, behind it, and into this the molecules we have been considering rush back. This sets up the to-and-fro motion of the air that constitutes a sound wave. The condensations and rarefactions move rapidly outward in all directions from the sounding body, and follow in regular succession as long as the sounding body continues to vibrate, at intervals that depend upon the rate of the vibration. In Fig. 1 the dark rings represent condensations and the light rings rarefactions in a train of sound waves proceeding from a sounding body at the center.
Demonstrations. - Hook one end of the wire spring as before and stretch the spring somewhat by pulling on the other end. Put a knife blade between two of the turns of wire and draw it toward the end held by the hand, pushing a few of the coils together. Remove the knife suddenly, and the wave will run the length of the spring and be reflected by the hook back to the hand. Tie a piece of thread to the spring at the middle, and the longitudinal vibrations will be shown by the sudden, jerking, to-and-fro motion of the thread.
That a mechanical impulse can be sent through the air as a wave form can be shown by the use of a tube 8 or 10 ft. long and 3 in.
in diameter. One end of this tube is capped with a cone having an opening an inch in diameter at the small end, while the other end is covered by a sheet of thin rubber tightly stretched and tied in place. A short piece of candle is lighted and so placed that the flame comes opposite the end of the cone. When two wooden blocks are struck sharply together near the closed end, the flame suddenly flares away from the end of the tube. The same thing occurs when the rubber diaphragm is tapped lightly with the finger. There is no passage of air through the tube, for the end is closed; hence the movement of the candle flame is the result of the blow received from the condensed wave sent out by the movement of the diaphragm.
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