1.http://www.bkw.ac.th/content/snet3/saowalak/wave/wave.htm
2.http://blake.prohosting.com/pstutor/physics/wave/wave_concept.html
3.http://www.icphysics.com/modules.php?name=Content&pa=showpage&pid=54
4.http://www.bs.ac.th/lab2000/physicweb/sonic.htm