30 March 2015 Last updated at 14:57 BST
The animals hunt with clicks and buzzes - and pick up the echoes from the animals they hope to eat.
Researchers at Denmark's Aarhus University say they switch from a narrow to a wide beam of sound - "like adjusting a flashlight" - as they home in on a fish.
The researchers believe this could stop the prey of porpoises, whales and dolphins avoiding capture.
They also think that by revealing these acoustic secrets, they could come up with new ways to prevent porpoises, and other toothed whales, from becoming trapped in fishing nets.
Footage courtesy of Jakob H Kristensen
Porpoises change the beams of sound they use to hunt fish for food.