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Blumlein Pair Stereo Miking

July 1, 2007
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While working as an electrical engineer for a Bell Labs subsidiary, and later for EMI, Alan Dower Blumlein (1903-1942) had a profound effect on the fields of telecommunications, TV, radar, and audio recording and reproduction. He developed the first “weighting networks” to compensate for the non-linearity of our ears, designed moving coil mics, and much more.

But probably Blumlein’s biggest impact was that he essentially invented stereo recording and playback. Back in 1931, he also created the Blumlein Pair, one of the most useful stereo mic techniques ever (UK patent 394,325; “Improvements In and Relating to Sound-Transmission, Sound-Recording and Sound-Reproducing Systems”).

THEORY

The Blumlein Pair is a crossed coincident pair of figure-8 velocity (ribbon) mics, each placed 45° off-axis from the sound source, while 90° off-axis from each other — similar to setting up two cardioid mics in an XY stereo configuration. Placing two figure-8 (“bi-directional”) mics in this “X” position provides a very detailed stereo image, and the rear lobes of the two figure-8 mics pick up a significant amount of “room tone” or ambience and reflections. As with the Mid-Side technique (Studio Trenches May ’07), you place the mic capsules as close together (“coincident”) as possible; but unlike M-S, no polarity inversion or decoder is required.

A Blumlein Pair requires two closely matched figure-8 mics to insure balanced stereo recordings. While the patent calls for a pair of bi-directional ribbon mics like the two RCA 74b Junior Velocity mics shown in Figure 1, two identical, multi-pattern condensers with each set for figure-8 polar patterns are often used instead. While preferred, you don’t have to have a factory matched mic pair; you can use any two mics of the same type and model, as long as each has (or can be set to) a bi-directional polar pattern. If you already own one multipattern condenser mic, or one bi-directional ribbon, consider purchasing a second mic of the same model so you can exploit the Blumlein Pair technique.

PRACTICE

Set one bi-directional mic so it’s aimed 45º to one side of the sound source’s center (“aim point”), and place the other mic directly above the first, aiming it 45º to the other side of the sound source. Make sure each mic’s front side points forward, toward the sound source, so that each is in phase. Referring again to Figure 1, orient the mics so they’re “aimed” at the sound source; the camera that took the photo is positioned at the sound source location.

Route each mic into a separate mic pre and record each to a separate track, or to a single stereo track in your DAW, and pan the two tracks hard left/right. As with M-S recordings, you’ll pick up a significant amount of room ambience, so adjust the mic placement for the desired ratio of direct sound to room ambience (closer to the sound source for more direct sound, further back to pick up more reflections and reverb).

Despite its age, the Blumlein Pair technique still has significant advantages. Unlike spaced pair stereo recordings, phase issues are generally not a problem due to the coincident placement of the two mics; and as Blumlein Pairs rely on amplitude differences as opposed to differences in wave phase to generate stereo information, mono compatibility is quite good. Blumlein Pairs also provide “realistic”-sounding stereo techniques, where the stereo imaging is very similar to what you hear if you stand where the mics are placed. In fact, one of my favorite applications is to set the pair just behind and above a drummer’s head, pointing forward toward the kit. This captures an “as the drummer hears it” perspective of the entire drum kit, along with a healthy amount of room reflections.

Blumlein Pairs are also suitable for live ensemble recordings, brass sections, and stereo recordings of individual instruments (e.g., acoustic guitars and classical piano) — basically any situation where more ambience is desired than what an XY stereo configuration or close miking provides. So the next time you’re looking for a sense of space and ambience in a recording, say hi to Mr. Blumlein.

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