Do dolphins think nonlinearly? - physicsworld.com
"Unlike solid objects such as the sea floor or fish – which reflect acoustic signals linearly, as straight-forward echoes – bubbles reflect nonlinearly. This means that acoustic echoes from bubbles contain harmonic frequencies in addition to the fundamental frequencies of the outgoing signal – producing a "clutter" that is incredibly confusing to sonar. And to make matters worse, bubbles are extremely efficient at reflecting sound."