I am just noticing people begging for help on here... I did this event back in 2008/2009 and am acting as an event supervisor now. If you go the Google search route, a lot of the time you will be lead to Hyperphysics, which is good for looking up individual pieces of physics but not a good tool for teaching yourself... there's too many links and no clear order in which to do them. If you are looking for textbooks, the best place to start is an algebra-based high school textbook. And go through the chapters on waves, light, and sound. If you get through that it would be a good idea to get a college level algebra-based textbook, but if you start seeing any math beyond algebra or trigonometry, ditch the book because it will most likely be much too advanced for what you will need to know this event.
In my experience, physicsclassroom.com is probably the best introductory online physics resource with great lessons and tons of practice problems with answers and explanations:
NEED TO KNOW BACKWARDS AND FORWARDS:
Also very useful:
If you go through the sections on waves, sound, light, reflection, and refraction and understand them you will be excellently prepared for the event. (For sections 3.b.i. through 3.b.v. in the rules). These topics will probably be the vast majority of the focus of your exams.
The topics in the rules that will likely not be covered in physics textbooks or physicsclassroom.com are surface waves, torsional waves, "how waves are used in communication", absorption and emission spectra still, and the special state and national topics, earthquakes and breaking waves. These topics are probably best found by a Google search or searching the index of a specific textbook. Introductory earth science textbooks are likely to have information on seismic waves, breaking waves, and tsunamis, and possibly spectra since some earth science books cover astronomy. Otherwise any good introductory astronomy textbook should cover spectra.
Also, although these topics aren't specifically called out in the rules, it would be good to have a basic idea of what lasers and radar are, and what the acronyms stand for! I have seen questions about these float around in the past.
Here are some good starting points I found just now:
This really should cover nearly everything you need, except for "how waves are used in communication". This is because in Science Olympiad, there seems to be a tradition that every event has one weirdly broad and vague topic in the rules that their is no way to properly study. My best interpretation is that this rule refers to radio communications, so check this out:
http://electronics.howstuffworks.com/radio.htm
Also these resources are great to print for your binder! When I write exams, I am a jerk and try to make the exam long enough that you don't have time to look up anything. So, know this info backwards and forwards!