Baffled pairs and the Decca tree

Two older ideas deal with the same problem: how to get the spaciousness of omnidirectional microphones without losing the centre. One adds a baffle between the capsules, the other adds a third microphone.

Baffled pairs: the Jecklin disc

Put a disc of absorbing material between two microphones and the baffle shadows each capsule from the far side. That adds level differences at high frequencies, on top of the time difference from the spacing. The result behaves a little like a head with two ears.

The best-known version is the Jecklin disc (also called OSS, for optimal stereo signal): a disc about 36 cm across, covered with absorbent material, with two omnidirectional microphones about 16.5 cm apart, one on each side.

What it does:

• At low frequencies the disc is small compared with the wavelength, so the sound passes around it and the pair acts like a spaced omni pair.
• At high frequencies the disc shadows the opposite capsule and provides level differences.
• The overall image is natural and spacious; the recording angle is not very sharply defined.

The sound of a baffled pair is also shaped by the baffle itself: reflections from the disc and diffraction around the edge change the frequency response, and it needs to be oriented carefully with respect to the source.

The Decca tree

The Decca tree uses three omnidirectional microphones in a T-shape:

• Two outer microphones about 2 m apart.
• A centre microphone about 1.5 m forward, fed equally to both channels at a lower level.

The centre microphone fills the hole in the middle of a wide spaced pair and anchors the central sources. The outer microphones deliver spaciousness and timing differences. For large orchestras, outriggers (further outward microphones, panned wide) are often added to pick up the outer sections.

Typical dimensions vary by hall and ensemble; the figures above are a starting point, not a recipe.

Trade-offs

Try it

Compare the time differences for a given spacing with the delay and distance tool, then check what an AB pair would do in the stereo recording angle tool.

Draft: written by an AI assistant, awaiting technical review.