Source
Boundless vets and curates high-quality, openly licensed content from around the Internet. This particular resource used the following sources:
"Rory Adams, Free High School Science Texts Project, Mark Horner, and Heather Williams, Sound - Grade 11. December 24, 2012."
http://cnx.org/content/m32834/latest/
OpenStax CNX
CC BY 3.0.