Click here to purchase the entire book in PDF format. Sound pressure level If you know how loud the sound source is normally, then you can tell how far away it is based on how loud it is. This is particularly true of sound sources with which we are very familiar like human speech, for example. Of course, if you have never heard the sound before, or if you really don't know how loud it is (like the sound of thunder, for example) then the sound's loudness won't help you tell how far away it is.