The RVPR (Rountree Voice Phenomena Recorder) is a 4 track voice recorder (using the Tascam voice recorder) with 2 electret microphones and 2 dynamic microphones. The difference in the microphones is key. The design is based on experiments carried out  by David Rountree. His experiments show that a true EVP is made of EMF and will therefore be picked up by the dynamic microphones but not the electret. With a 4 track recorder these can be easily identified by looking at the waveforms of the recordings.