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Blumlein Miking: Guitar and Vocals

December 10, 2014

Sam Stauff records his acoustic guitar and vocal for the latest Wess Meets West release. Two Audio-Technica AT 4050 microphones are set to figureeight, placed head to head and crossed at 90 degrees.
This week, the Recording Academy / GRAMMYs announced that Alan Blumlein will be honored with a Technical Grammy in 2017. One of the most significant audio inventors of his time, Blumlein received 128 patents—the most noteworthy was for the stereo in 1931. In his honor, we show you how to capture guitar and vocals using the Blumlein miking technique.

Way back in the June 2013 issue of Electronic Musician, we examined a variety of stereo microphone techniques, one of which was Blumlein stereo. This technique employs two matching figure-eight microphones arranged as a coincident pair— meaning that they are placed as closely together as possible, with their diaphragms crossed at an angle of 90 degrees. As shownin Figure 1, neither microphone is “on axis” to the sound source; they are both 45 degrees off-axis, but since their patterns overlap in the middle, we still get a strong center image. Blumlein technique provides a realistic soundstage, with the amount of room ambience dictated by the distance between the microphones and the source. Somewhere along the way, engineers adapted Blumlein stereo to solve a very persistent issue typically encountered when recording a person who is singing and playing the acoustic guitar at the same time.

SOLVING THE BALANCE PROBLEM

Let’s suppose we’re recording a musician who is playing the acoustic guitar and singing at the same time. We’d probably place one microphone on the guitar, use another for the vocal, and record each mic to a separate track. Later we could balance the two tracks in a finished mix, perhaps adding delay or reverb to one track or the other. There are problems with this arrangement, mostly due to the physical proximity of the singer’s mouth and the guitar. First, even with cardioid or hypercardioid microphones, it’s very difficult to control leakage. There will often be a fair amount of acoustic guitar in the vocal mic and vice versa. This means that when you mix, it will be tough to make the vocal louder or add an effect to it without influencing the sound of the guitar. For example, if you add delay to the vocal mic, you may hear delay on the guitar as well. If you raise the level of the guitar track, you’ll probably hear the vocal get louder too.

Fig. 1. Diagram of Blumlein configuration. Note that the two microphones are 45 degrees off-axis to the source, but since the patterns overlap in the middle, we still get a strong center image.
The other issue we encounter is phase cancellation. If we place a mic at the vocalist’s mouth and a second microphone in front of the guitar, the mics are separated by roughly one to three feet. This distance creates a time delay of 1 to 3 milliseconds between the mics, resulting in a perfect storm that yields a flange when the second mic is mixed with the first. It usually sounds awful, but we can literally twist the Blumein technique into a solution.

Take a look at our opening image, which shows artist Sam Stauff recording his vocal and acoustic guitar for “I Will Break You” from the next Wess Meets West release. The mic placement is an adaptation of the Blumlein pair: The front of the vocal microphone is aimed toward Sam’s mouth, and the front of the guitar microphone is aimed toward the area of the guitar were the neck meets the body. So far this configuration isn’t much different from using typical cardioid pattern mics.

However the Blumlein configuration enables us to exploit the figure-eight pattern and reduce leakage to low levels that can’t be achieved when using other pickup patterns. The guitar is sitting in the “null” of the vocal mic’s figure-eight pattern, and is rejected from that mic. If we make level or EQ changes to the vocal mic, the guitar is minimally affected. On the flip side, the guitar mic is placed so that its null point is facing the vocal—rejecting it. This isolation provides us with far more control over each sound when mixing.

PHASE COHERENCE

Fig. 2. A close-up of Stauff singing, miked with an adapted Blumlein technique.
One of the most important benefits of this technique is that it takes advantage the fact that coincident microphones yield little (if any) phase cancellation. The diaphragms of the mics in a coincident pair are very close together (almost in the same physical location), so sounds arriving from any direction reach both microphones essentially at the same time. Why do you care? Because time delay between two microphones capturing the same sound produces phase cancellation— and phase cancellation results in timbral changes or a flange-type of sound. When we use the Blumlein arrangement, even if there is some guitar leaking into the vocal mic, mixing the two mics (or tracks) together will not cause phase cancellation.

The amount of leakage at the null of a figure-eight mic depends upon the integrity of the microphone’s figure-eight pattern, and will vary with brand and model. Mics possessing a more symmetrical figure-eight pattern provide greater rejection on the sides (at 90 and 270 degrees off-axis). If there is leakage, then yes, raising the vocal track might make the guitar a bit louder, but it’s less of a problem because it’s not destructive. Note that the Audio-Technica AT4050 mics used on Sam for the Wess Meets West session are set to figure-eight, and are side-address (see Figure 2). (Meaning that the side is really the front. Got that?)

Since you’ve been paying close attention, you’re now saying, “Ah, but what about sound hitting the back of the microphones?” It is true that the figure-eight pattern captures as much from the back as it does from the front, but it’s not really a problem. The rear of the vocal mic is pointing toward the floor but is not capturing much, because this mic is close to the performer’s mouth and the mic preamp gain is adjusted for the vocal, not the relatively quiet reflections bouncing off the floor. Also, we’re going to make sure the floor is carpeted.

The rear of the guitar mic is aimed at the ceiling. Preamp gain for this mic is optimized for sound hitting the front of the mic (the guitar), not what is bouncing off the ceiling (relatively low-level reflected sound).

If possible, record in a room with a relatively high ceiling and/or one that has absorptive properies. The results sound very natural and allow quite a bit of control when it comes time to mix. You and your artist will be very happy.

Steve La Cerra is an independent audio engineer based in NY. In addition to being an Electronic Musician contributor, he mixes front-of-house for Blue Öyster Cult and teaches audio at Mercy College Dobbs Ferry campus.

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