rar·e·fac·tion 
(râr′ə-făkshən)
(râr′ə-făkshən)n.
1. The condition of being rarefied.
2.
a. A decrease in density and pressure in a medium, such as air, caused by the passage of a sound wave.
b. The region in which this occurs.
rar′e·factive adj.