Brief description of the Jecklin disc:

The Jecklin disc, briefly described, is a disc that serves as a separator for recording stereo
information in the OSS technique (OSS=Optimum Stereo Signal).

The original arrangement consisted of a damped, sound-absorbing disc with a diameter of 30cm. Microphones are mounted on it on both sides at a certain angle.
The distance between the omnidirectional microphones on the sides of the disc was 16.5 cm. The distance between the microphones corresponds to the statistical average of the ear distance of an adult human being.

The signals from the two microphones (corresponding to the ear signals) are each recorded as an independent channel. Similar to the microphone used, the human ear also receives the incident sound all around at an angle of approx. 180°.
This arrangement on the Jecklin disc was particularly suitable for recordings of orchestras, choirs or chapels 

The working principle of the Jecklin disc

In general, the setup records a stereo signal with the two separate microphones. This recorded signal contains directional information for the listener. This directional information is generated both by time-of-flight differences between the two channels and by sound pressure level differences.

Analogous to the human head, the disc causes a lateral shading of the incoming sound waves. This shadowing causes significant level differences between the microphones, i.e. the human ear. The level differences are frequency-dependent. They increase towards high frequencies. Towards low frequencies the differences decrease strongly, so that below 500 Hz only mono is perceived.

Jecklin disc modifications

Later, the dimensions of the disc were changed. From the original diameter of 30 cm, it grew to 35 cm in diameter. The distance between the two omnidirectional microphones increased to 36 cm.

Comparison of different microphone arrangements in sound engineering

Excerpt from Jecklin - Theorie der Tontechnik 3. Mikrofone, Vers. 2003/04

with separator Madson Shadow

Two microphones with figure-of-eight directional characteristics are placed with an opening angle of 90 degrees and a mutual distance of 30 cm to 50 cm in front of the sound body. In addition, the microphones are acoustically separated by a separating plate (diameter of the separating plate about 50 cm). As a result, the signals of the two channels differ additionally by angle-dependent frequency response differences.

KFM - Ball from Schoeps

In this microphone arrangement, two sound pressure receivers are mounted on the surface of a spherical separator. The sonic advantages of omnidirectional microphones come into their own with this arrangement. The necessary angle-dependent time-of-flight differences result from the distance between the two microphones, the intensity and (possibly) frequency response differences from the acoustic separation of the two microphones caused by the spherical separator.

OSS - Technique (Jecklin disc)

Two omnidirectional microphones are arranged 36 cm apart and acoustically separated by a 35 cm diameter foam-covered disc. With this arrangement, the sonic advantages of the omnidirectional microphones come into their own. The correct wavelength-dependent differences in propagation time result from
the distance between the two microphones, the differences in intensity and frequency response from the acoustic separation of the disc. The two microphones must have a linear diffuse-field frequency response. Opti-mal for this arrangement is the microphone 4006 from B+K with diffuse field attachment (black).
The aperture angle is about 60° (reason: increase of the frequency response in the microphone axis).