Sounder
You'll hear the term "sounder" used interchangeably with fish finder and sonar in most instances. The "sounder" portion of GPS/Fish Finder combo units is that portion of the unit that translates the transducer signal into useful screen content, rather like a TV that has built in tuner. So you'll see advertisements that say they are GPS/Sounder or GPS/FF combinations, pretty much the same thing. The components are the same regardless of the emphasis placed on the various items, fish, structure, bottom detected.
The black box sounder is an add-on, rather like a cable box when your TV does not not have its own digital tuner, that provides translation, like in the combo boxes, of transducer signal to a compatible display. Or it can act as a hub to provide signal from the transducer to all the compatible displays in a network on a boat like those supported by Lowrance, Garmin, Raymarine etc.
