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A Jecklin Disk is a sound-absorbing disk placed between two microphones to create an acoustic "shadow" from one microphone to the other. The resulting two signals can possibly produce a pleasing stereo effect.
Always, a matching pair of omnidirectional microphones are used.
The technique was invented by Jorg Jecklin, the former chief sound engineer of Swiss Radio. He referred to the technique as an "Optimal Stereo Signal" (OSS). In the beginning Jecklin used omnidirectional microphones on either side of a 12" disc of about 3/4" thickness, which had a muffling layer of soft plastic foam on each side. The capsules of the microphones were above the surface of the disc just in the center, 16.5 cm apart from each other and each pointing 20 degrees outside.
Nobody of the users wantetd to notice, that the master found the 16.5 cm ear spacing between the microphone too narrow and wrong. Now he tells us in his own papers that the disk has to be 35 cm in diameter and the distance between the microphones have now to be 36 cm. Really no measures of the distance between the ears anymore. In loudspeaker stereophony there is no use of the ear spacing. See Jecklin's paper at:
Jecklin's German from his script: "Zwei Kugelmikrofone sind mit einem gegenseitigen Abstand von 36 cm angeordnet und durch eine mit Schaumstoff belegte Scheibe von 35 cm Durchmesser akustisch getrennt." That means: Two omnidirectional microphones have a distance from one to another of 36 cm and a separation between them of a foam plated disk with a diameter of 35 cm.