Transmission of Sound.
a. An important fact to remember is that sound, unlike light and electromagnetic (radio) energy, must have a
medium within which to travel. Although electricity and light can be transmitted within a vacuum, sound cannot. When
sound is transmitted the medium between the source and the listener is usually air, but other mediums such as liquids or
solids may be used. The American Indian used to press his ear to the ground to detect far away footsteps. Little boys
playing around railroad tracks used to put their ears to the track to detect the sound of oncoming trains long before the
train could be heard or seen. In both cases, the denser medium carried sound farther than it traveled in air. This principle
is also used in the underwater detection of ships. Sensitive listening devices attached to the hull of the ship pick up the
sound of propeller vibrations carried by the sea from other ships in the area.
The motion of air molecules set up by a body vibrating in the air travel outward in all directions from the
vibrating source. The manner in which the molecules travel are called waves, sound waves to be more precise. This can
be better understood by considering a vibrating strip of metal such as the hacksaw blade in figure 2.
a. A hacksaw blade is fastened to a table as shown in figure 2A, and caused to vibrate rapidly back and forth.
As it makes its initial trip to the right, two events of opposite nature occur as shown in figure 2B. In the first event the
blade increases the pressure existing in the group of air molecules on its right causing a bunching up (condensation) of the
particles on that side. In the second, the blade decreases the pressure existing in the group of particles on the left causing
a dispersion (rarefaction) of the molecules on that side. Remember condensation and rarefaction occur at the same time
by a single motion of the blade.
b. The blade moves back to its vertical resting position as in 2C, but motion has been given to the molecules
(particles) on each side and their succeeding behavior is effected. The bunched up group on the right moves outward and
away from the blade pushing against the layer of particles still farther to the right.