Most of the industry in the beginning used 3db point to make these beam measurements but as time went on they got larger cone angles by stated what the 10db points were. (nothing really changed) For me most of this is not for the average fisherman and I've always tried to explain using our every day approach. If you are in design of xducers fine or just interested in the subject that's fine. I know that the graphic example is really for explanation only and of coarse sound continues until it fully dissipates. (0db)