Basic Documentation

Chapter 2–Physics of Sound
Chapter 2 discusses the properties of sound and how sound is measured. It includes the
following topics:
• Sound wave propagation
• Sound wave parameters
• Sound measurement parameters
• NC Curves
• RC Curves
• Determining an RC rating
Sound Wave Propagation
The human ear hears or senses sound when oscillations or vibrations occur within its hearing
mechanism. Under normal circumstances, these oscillations are transmitted to our ear as
sound waves that are really air pressure waves. These air pressure waves impact upon the
ear’s sensing or hearing mechanism and cause it to oscillate or vibrate.
As sound waves travel to the ear, they may travel not only through air but also use different
mediums as well. Recall that a basic physics classroom experiment on sound consists of
putting a sound generating device (sometimes an alarm clock) under a large glass container
(typically a bell jar). The vibrations of the sound generating device cause sound waves in the
air within the bell jar that travel outward until they reach the glass wall of the bell jar. There,
they cause the wall of the bell jar to vibrate that in turn causes sound waves to be generated
in the air outside of the bell jar. These sound waves then continue and eventually reach the
ears of those in the classroom. As long as the bell jar contains room air at normal
atmospheric pressure and density, the above scenario takes place and the sound is easily
heard.
However, after a vacuum pump removes most of the air from inside the bell jar, the sound
made by the sound generating device is dramatically reduced since the density of the air
within the bell jar has been substantially reduced and thus has a much more limited impact
on the wall of the bell jar. This experiment shows that sound waves are highly dependent
upon having an adequate medium for their transmission.
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