Large natural or man-made objects can block acoustical transmission under certain conditions, in much the same way that a rock blocks a fast moving current of water. Close to the backside of the rock, in this example, the current is absent and the water seems still. A short distance away, the current is flowing again. Similarly, acoustic energy in the water can be blocked if the transmitting source is close to a large object. Most Single Sideband signals are efficient enough to permit communications around blocking objects.
Reception is made mainly through surface and bottom reflections.
Man-made noise may be present underwater in busy harbors, shipping lanes and many coastal locations, particularly at lower frequencies. The outstanding characteristic of this coastal ambient noise is its great variability from place to place in the same harbor and from time to time at the same place. Figure 3 Marine organisms play an important role in underwater acoustics. They are important primarily because of the effect they have on sound transmission, but they sometimes serve as sources of underwater noise as well. Communication through indirect and direct paths.