TC 9-64 _________________________________________________________________________
the words sonic, ultrasonic, and infrasonic are used. Sounds capable of being
heard by the human ear are called sonics. The normal hearing range extends
from about 20 to 20,000 hertz. In the Army to eliminate confusion when
referring to different ranges of frequencies, an arbitrary dividing line has
been established at 10,000 Hz. Even though the average person can hear
sounds above 10,000 hertz, it is standard practice to refer to sounds above
that frequency as ultrasonic. Sounds between 15 hertz and 10,000 hertz are
called sonic. Sounds below 15 hertz are known as infrasonic (formerly
referred to as subsonic) sounds.
1-51. Recall that sound waves are compression waves. The existence of
compression waves depends on the transfer of energy. To produce vibrations
that become sounds, a mechanical device (the source) must first receive an
input of energy. Next, the device must be in contact with a medium that will
receive the sound energy and carry it to a receiver. If the device is not in
contact with a medium, the energy will not be transferred to a receiver, and
there will be no sound. Thus, three basic elements for transmission and
reception of sound must be present before a sound can be produced. They are
A medium for carrying the sound (e.g., air, water, metal).
The detector (or receiver).
1-52. A simple experiment provides convincing evidence that a medium must
be present if sound is to be transferred. In figure 1-12, an electric bell is
suspended by rubber bands in a bell jar from which the air can be removed.
An external switch is connected from a battery to the bell so the bell may be
rung intermittently. As the air is pumped out, the sound from the bell
becomes weaker and weaker. If a perfect vacuum could be obtained, and if no
sound were conducted out of the jar by the rubber bands, the sound from the
bell would be completely inaudible. In other words, sound cannot be
transmitted through a vacuum. When the air is admitted again, the sound is
as loud as it was at the beginning. This experiment shows that when air is in
contact with the vibrating bell, it carries energy to the walls of the jar, which
in turn are set in vibration. Thus, the energy passes into the air outside of
the jar and then on to the ear of the observer. This experiment illustrates
that sound cannot exist in empty space (or a vacuum).