Pressure waves strike your eardrum and generate nerve impulses that your brain interprets. It's all done within your ear, and you don't even realize it's occurring. In the ocean, however, it's impractical to use our ears. We use devices called transducers instead. They're very similar to speakers and microphones that you have in your car radio or home stereo.
Transducers convert pressure waves to electrical signals and vice versa.
When an electrical current is applied to a transducer, the transducer flexes. We use another transducer to listen for these reflections we call echoes. When the pressure wave from an echo strikes the receiving transducer, it causes the transducer to flex slightly, and this generates an electrical current that we can measure. The reflection of sound from a siphonophore. These animals have a small gas bubble which is the main source of this echo.