These animals have a small gas bubble which is the main source of this echo. This echo is very large compared to echoes from animals of similar size that lack a gas bubble. Based on the tens of thousands of echoes that we've measured on this cruise, there are a lot of siphonophores in these waters. This explains how we can estimate the size of an echo from the voltages we record. To answer this, we need to know something about how sound travels in water.
Sound travels much farther, and much faster in water than it does in air.
After producing a sound pulse, we can measure the time that it takes for an echo to return to the transducer with great precision. These two pieces of information can be used to tell us how far away the animal is from the transducer. By multiplying the time that it takes to receive an echo by the speed of sound, we can estimate the total distance traveled by the sound pulse. This is twice the distance between the transducer and the target because the sound first has to travel to the target, and then it must return. Therefore, we need to divide it in half to arrive at the distance between the target and the transducer.