The PCL consists of the main unit and the hydrophone. The hydrophone is a microphone for use in water, and our is moulded in black rubber. It is the hydrophone that detects the clicks from the porpoise. Inside the main unit, there is a data chip. The data chip processes the sounds detected by the hydrophone, and determines if the sounds are to be saved in the memory as a porpoise click. What determines this is the length of the click, the time between two clicks and the amplitude and frequency composition of the click.