Originally Posted by
CatFan
One important thing to keep in mind is that the beams aren’t neatly defined areas like the drawing would possibly lead you to believe. The angle given is the 3dB or half power beam of the transducer. The picture of the beam represents the area where signal is 50% or more as strong as the strongest signal. Outside of that area, the signal is lower, but not non-existent. There is no hard line at the edges, and the signal strength is not the same everywhere within the area shown on the beam diagram. You can see this clearly in the formation of fish arches on 2D sonar. The arch shape is the result of geometry as the transducer gets closer to and moves away from the fish. The arch being thicker in the middle and tapering is the result of the fish being out near the edge of the beam, moving into the center, then back to the edge.