This disc is placed between two microphones in a stereo pair to enhance their sound field.
It was developed by Jürgen Jecklin for his stereo microphone technique known as Optimal Stereo Signal (OSS). The idea behind OSS is to use a pair of spaced omnidirectional microphones approximately 6 inches apart (roughly the distance between the human ears) with a disc placed in between the spaced pair. The disc is covered with a thin layer of foam intended to absorb high frequencies – thus improving stereo separation between the microphones.