Cane Pole,
1/3 the depth is a good rule of thumb for a 20 degree cone angle transducer and there is a formula for figuring the approximate area of coverage for transducers with different cone angle widths. We used to have some depth multiplier numbers on the Humminbird web site for all of the transducers and beam widths that we made but I could not find it when I looked today. If there were a chart available the problem with it would be that you would have to be aware of at what sound level point (db) that the transducer you are interested in was measured at. Some will tell you that the industry standard for this is at the -3db point or that this is where the US Navy measures transducer beam widths at – this is not true. Sure the Navy measures their transducers at the -3db point, but they also measure them at the -6db and -10db points as well as others. It all depends on what the system is being used for and how sensitive the detection criterion is. We specify our transducer beam widths at the -10db point since this more accurately reflects the what the depth sounder is being used for (to show the bottom, structures and any fish that may be between the surface and bottom). The – (minus) in front of the db numbers means that the sonar signal has degraded to this point with the -3db point being what is called the half power point or where the sound level is only half of what it is when directly under the transducer, and the -10db point being where the sound level is one-tenth of what it is when directly under the transducer. Directly under the transducer would be the loudest or strongest sound level area. So you can see that a transducer that measures 20 degrees at the -3 db point would actually show sonar returns over a wider area than a transducer measures 20 degrees at the -10db point. A transducer that measures 20 degrees at -3db would measure 24 degrees at -10db. A transducer that measures 20 degrees at -10db would measure 16 degrees at -3db. So when you compare one product to another, you would need to be aware of what sound level point (db) the transducer beam width was measured at.

Our (Humminbird) more popular cone angle measurements and the approximate area covered are:
20 degrees (@-10db) = depth X 0.35
60 degrees (@-10db) = depth X 1.00
90 degrees (@-10db) = depth X 2.00

This is all nice, mathematical, cut and dry – but this is not telling the whole story. The problem with transducers is that they do not always focus all of their sonar energy in one nice conical area. Although Engineers try to minimize it, they can produce side lobes of sound energy. If enough energy from these side lobes gets reflected back to the transducer, it effectively increases the cone angle of the transducer. Add to this other factors that may be out of our control that can also bring about a change in the effective sonar beam width (water quality, bottom composition and angle, composition of objects in the water, water depth, angle of the transducer to the bottom, transmit power, receiver sensitivity,…).

When attempting to explain transducer beam widths and coverage (that I may have just managed to mangle and confuse everyone with) I like the analogy of the flashlight:
Turn the flashlight on and the light goes out in a ‘beam’. We don’t see the light beam itself but what gets reflected back to our eyes – much like a transducer. We could measure how wide this beam is and use this measurement to calculate just how large of an area the flashlight beam will be covering at a certain distance – just like sonar. Even with the focusable flashlights, we have all seen how there are objects, while not directly in the main beam of the flashlight, that can be seen off to the sides – all due to those pesky ‘side lobes’ of energy and because these objects reflect enough light for us to see them. Objects that are at a right angle to the flashlight beam tend to reflect more light back to you as do objects that naturally reflect more light – same with sonar and sound. Now that flashlight beam does not go on forever and so diminishes with distance, but before it does, the area that we are able to view objects in narrows back up – just like a sonar beam narrowing up at deeper water depths. If you were to use the flashlight in an area that was smoky (or poor air quality), you would find that you could not see as far – just like water quality for sonar. Want to see farther? Let’s increase the amount of light that the flashlight produces or its power. Now we can not only see farther but we can also see smaller objects that we could not see before because they did not reflect enough light for us to notice. Light too bright for you or not bright enough to see something that does not reflect a lot of energy? You can partially close or fully open your eyes just like increasing or decreasing the relative sensitivity of a depth sounder. Maybe you are getting a lot of glare? Put some sunglasses on to block some of that unwanted light – much like a filter would work for sonar.

In our products when you adjust the Sensitivity menu setting you are not actually adjusting the receiver gain or the transmitter power. The receivers is basically wide open and all signal level adjustment, filtering other signal processing typically occurs in the computer. So while you may have the sensitivity menu on its lowest setting, the unit is hearing all that it can but only displaying what it is directed to through the unit’s programming. Think of it like this; we assign certain colors (or levels of grayscale) to certain levels of returned sonar signals through programming. Some of these may not even be shown when the sensitivity menu is set to its nominal setting. When you change the sensitivity menu setting, you are changing which color levels relate to which returned sound levels. Each color may now represent more different sound levels (if you turn the sensitivity up) or fewer different sound levels (if you turn the sensitivity down) while at the same time being skewed to either lower or higher sound levels. – Okay, I’ve retyped this like three times and it still sounds like gibberish! It all looks so clear when I draw it out on my white board…

My intention was not to confuse anyone but I feel that I may have done so.