Reference mics can be a vital tool when it comes to audio. They’re the microphones that capture the uncoloured sound, with a flat response and almost complete transparency to let you measure the acoustic characteristics of the room in which you’re recording. They let you test rooms for echo and reverberation. They let you hear (and see, on charts) what frequencies your room naturally enhances or destroys.
Reference mics can be quite expensive, though, so GitHub user loudifier decided to design his own and make it open source. It’s called OpenRefMic and it promises a low cost with a high performance that “meets or exceeds the performance of professional reference microphones”. And while commercially available reference microphones aren’t all expensive, they usually come with pretty high self-noise levels. And none are this cheap!