Building a Decca Tree with XD Bars: dimensions, connections, and checks

Engineers at the Decca Record Company developed the Decca Tree in the 1950s. Its established form normally uses three spaced omnidirectional microphones: L and R sit at the ends of the rear bar, while C projects toward the source along the centreline, forming an isosceles triangle.[1][3] DPA gives roughly 2 m between L and R with C about 1 m forward as a common starting point; a SCHOEPS example is 2 m wide and 1.5 m deep.[1][3] These are starting points, not a universal standard. The two diagonal sides should remain symmetrical. The 110° figure sometimes applied here actually describes the typical axis angle of ORTF.[4]

DPA lists about 2.5–3 m as a common starting height, but the final position still depends on ensemble width, room acoustics, and the desired proportion of direct sound.[1] Classic Decca Trees often use omnidirectional microphones. The SCHOEPS example allows all three to face forward or the L/R pair to angle slightly outward.[3] Omnidirectional microphones can still become directional at high frequencies, and cardioid or other patterns require a fresh check of off-axis colour. XD2 mounts and positions each microphone along a bar; XD3 connects one bar to another. Their roles are not interchangeable.[7][8]

Mark the midpoint of the 2 m main bar on the floor and mount L/R symmetrically. Join the 1 m or 1.5 m forward bar with the locking XD3 and place C on the centreline. Position the three microphones with XD2, adding XD1 only when the array will be suspended.[5][6][7][8] After tightening the assembly, measure from the microphones’ acoustic centres, confirm L/R symmetry, C projection, and bar level, then add the safety cable and raise the array. Once connected, verify channel names, polarity, gain, stereo image, and mono compatibility before adjusting height and fore-aft position with the real programme.