I believe we could build a low frequency sound fence at the border that would be efficient and cheap to build. You could even use translators and boosters at certain locations. It could be towers to bounce the signal. Aiming it at the border should allow the signal to flow only that direction. Low frequency generators are simple simon circuits. Why doesn’t the government just do that?
Read the article in this link. http://www.newworldwar.org/sw.htm