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Tuesday, April 30, 2013

Q. Can I use the front and rear sides of a Blumlein array simultaneously?

Most of the recording I do involves tracking several musicians playing together in a room. I’d like to use a stereo pair to capture the overall picture, as well as close miking, but often the musicians arrange themselves in such a way that X-Y or A-B rigs won’t work. I’ve been wondering about using a Blumlein-crossed figure-of-eight pair placed between the drummer and the rest of the group, in such a way that the front of the array captures the drum kit and the rear captures the other musicians. In other words, is Blumlein strictly restricted to the 90-degree acceptance angle in front, or is it OK to use the 90-degree space behind the array too? And if so, should I reverse the polarity of any other mics on that side?

You actually have little choice over whether to use the rear of your mics in a Blumlein array, as the mics will always capture ambient noise to the rear of the setup. This can be quite useful in certain circumstances, such as radio drama, for example, in which the setup allows the actors to be positioned less rigidly but still be picked up by the mics.
Simon Earle, via email
SOS Technical Editor Hugh Robjohns replies: 
The short answer is yes, it’s perfectly OK to use the rear pick-up region, and yes, you might need to reverse the polarity of spot mics covering sources on the rear of the Blumlein array.
The slightly longer answer is that you actually have no choice in the matter; the rear side of a Blumlein array is captured anyway, so you might as well make use of it. In an orchestral recording, for example, it will be capturing the room ambience and audience (which will make it sound rather more open than might be expected). In radio drama, both sides of a Blumlein array are often used to great effect, as the technique allows the actors to face each other across the mic for good eye-contact, while still being able to move freely within their own ‘stereo space’.
In your situation, it’s perfectly acceptable to arrange the musicians to use both front and rear 90-degree stereo-recording angles, using relative distances from the mics to help achieve the appropriate balance. In radio drama, the studio floor is often marked up with tape to identify the edges of the 90-degree pickup areas, with additional marks to show the desired positions for each performer, so they don’t wander away and upset the optimum balance.
There are two things to beware of. Firstly, don’t let any real sound sources move around to the sides of the Blumlein pair, because they will then be out of phase in the stereo image. Secondly, choose your figure-of-eight mics carefully, as many are designed with strong tonal differences between front and back. That may be quite useful in your situation, but can cause significant issues in others. Finally, if you’re planning to close-mic sources to supplement their contributions to the main pair balance, sources on the rear of the mic will be captured with an inverted polarity relative to those on the front, as you say.
Consequently, you will probably need to flip the polarity of those close mics in the mix to avoid phase cancellation issues, depending, to a degree, on the distance between the close mics and Blumlein pair, the nature of the source, and the level of the spot-mic contribution. I’d start with the rear-side close mics flipped in polarity, and check each one as you build the mix, to see what works best.  

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