http://hyperphysics.phy-astr.gsu.edu...dio/cross.html
http://www.lalena.com/Audio/FAQ/XOver/
http://www.lalena.com/Audio/Tables/XOver/1stOrder.aspx
http://www.lalena.com/Audio/Tables/XOver/2ndOrder.aspx
http://www.lalena.com/Audio/Tables/XOver/3rdOrder.aspx
http://www.lalena.com/Audio/Calculator/XOver/