On back of the speaker you'll find a grille for the bass port as well as a flap that hides a 3.5 mm analog jack, reset button, charging port and a USB - A port for topping up your phone in a pinch. (techradar.com)
On the bottom back of the speaker there's a power port and a 3.5 mm audio output jack, which means that you can connect the Echo Plus to a much larger speaker or audio system. (digitaltrends.com)
The team replaced the wooden backs of speakers with rectangles of metal gauze to focus sound toward passengers» ears. (sciencenews.org)