You are in a park when you spot your friend 100 m away. At what lowest decibel should you shout your greetings so that your friend can just hear you? Assume that your friend has normal hearing. My prof never gave an example of how to do this and I can't figure it out. The correct answer is 40 dB. We know that β = 10log(I/Io) and I = P/A = P/4∏r^2 I really just have no idea how to do this because they gave no baseline to compare the 100 m distance to.