Mighty:
90 deg cone angle is just a large cone that covers more of the bottom of a lake. Most cone angle are smaller. Mine is just 20 deg. Here's a more detailed explaination of how sonar works.
Sound waves travel out in an ever expanding wave. The effectiveness of that wave for finding objects on the bottom and getting sound waves to bounce off those objects with enough energy to make the return trip back to the transducer and be recorded is how they determine the cone size. Some waves don't return and are lost. A narrow cone only shows a small part of the bottom. A wider cone will show more area of the bottom. There are some limitations that one must know about when trying to interpret the recordings.
The new thing these days is to take a very narrow cone size and using multiple pings (transmission) that are moved around like a flash light beam in the dark. You actually paint the bottom with multiple narrow overlapping sound waves. We do this with radar beams also. The term is phased Array Radar and I think that they are doing the same thing with sonar beams.
I am adding some pictures that I scanned into my computer to help you and anyone else that has a hard time picturing what the sonar waves are doing.
Originally Posted by mighty