The Raspberry Pi Foundation have blogged about this great project from Germans Holger and Henrike Körber. They are using a USB high-sensitivity microphone to pick up on bat calls. This is plugged into a Raspberry Pi which records the data and allows them to graphically represent the calls and manipulate the sound waves into something which can be heard by humans. Read the blog post at the Foundation’s site or head over to the German-language project description.