This video from the Netherlands says about itself:
10 June 2016
At the SOS Dolfijn rehabilitation center we work together with several scientists to learn more about harbour porpoises. The harbour porpoises in our rehabilitation center carry a lot of information about their species that is still unknown. Of course all research is only possible when it is not affecting the animals. When a stranded animal is successfully rehabilitated and is almost ready to return back to sea, we ask scientists who are linked to the University of Hannover, Germany to test the animals’ hearing. During this test the animal is lying gently in the arms of one of its caretakers and a hydrophone in the water is sending out signals on different frequencies. Suction cups are placed on the harbour porpoise which measure what the animal can hear.
Whales use their hearing for several things, for example hunting and navigating. So it is very important for their survival in the wild! For example the sound of industrial shipping, sonar use by navy and windmills are all factors that can disturb the whales. Knowing on which frequencies harbour porpoises can hear can help to discover how these disturbing elements can be reduced for them. The results of the hearing tests showed that the hearing of Sven and Nena, who are both now in rehabilitation, were good.