The directivity of a microphone is a key variable in the art of capturing any sound. Because mic placement is among the hardest things to undo or fix in the mix, choosing the best directional pattern and placing the mic the right distance from the source for the particular situation is crucial. Although there is no substitute for making good mic choice and placement decisions, the two-mic technique outlined in this article gives you a lot more flexibility than a single mic would. By combining the signals from the two mics, you'll be able to change the polar pattern and vary it continuously, even after you've finished recording.
FIG. 1: A bidirectional mic is fully pressure-gradient. A sound wave of high pressure at the front pushes the diaphragm, creating a positive voltage (a). A sound wave of high pressure at the back pushes the diaphragm the other direction, creating a negative voltage (b).
Before getting into the details, a little background is helpful. When describing the design of a microphone, there are a few different characteristics to consider. The most commonly mentioned are condenser and dynamic, which are descriptions of the way a mic gets its job of transducing air pressure to voltage done. But these transducer types don't describe the directionality of the mic. For that, I'll use the terms pressure and pressure-gradient.
Pressure mic describes how the omnidirectional polar pattern does its job. It basically functions like a very fast, very sensitive barometer, detecting the minute changes in air pressure that represent the sound around it. This happens by creating a sealed capsule with a flexible diaphragm on one end that is moved by the force of air-pressure changes. Think about it as if the mic capsule were an empty coffee can with a balloon stretched over one end. When the air pressure around the can changes, the balloon moves in or out in response to it. This movement is converted into changes in voltage. When a point of high pressure hits the diaphragm and pushes it in, a positive voltage is produced. When the pressure outside the sealed can is lower than that inside, the diaphragm pushes out and a negative voltage results. Just like any mono channel, a pressure mic doesn't know anything about which direction a sound is coming from, only that a pressure change happened somewhere.
A pressure-gradient design is quite different because the diaphragm is open at both ends, allowing sound waves to hit it from each side. It works not unlike the coffee can if it had both ends punched out and the balloon stretched in the middle of the container. If the diaphragm is pushed in from the front by arriving high pressure, the mic outputs a positive voltage. Once the low-pressure area of that arriving wave gets to the diaphragm, it gets pulled out, causing a negative voltage. If sound hits the back of the diaphragm, the reverse happens — high pressure on the back pushes it in towards the front and produces a negative voltage, and low pressure pulls it and produces a positive voltage (see Figs. 1a and 1b). If sounds are arriving at both the front and back in some combination (as usually happens in the real world), the diaphragm is moved more toward the direction of whichever one is stronger. So a pressure-gradient mic doesn't know which direction a sound came from, but it does know the difference between the level of sound at the front and back. If you didn't guess already, a pressure-gradient mic produces the bidirectional (figure-8) pattern you're probably familiar with.
These are the two basic directional designs used in most conventional microphones. It turns out that they can be combined in different ways to create the different polar patterns we know and love (although mic manufacturers typically use other techniques for that purpose). For example, the cardioid pattern is created by taking a pressure design and turning half of it into a pressure-gradient one by opening the back to incoming sounds.
Mix and Match
All this is background to help you better understand the recording technique I'll tell you about now. This method can be used to make some changes in the characteristics of a mic's sound after recording is complete. The basic idea is that if you use both a pressure mic and a pressure-gradient mic to record the same source to separate tracks, you can change the characteristics of the recording by combining the two mics' signals to different degrees. How can this possibly work? By using the simple and powerful phase relationships between the mics' polar responses.
FIG. 2: The two mics should be positioned as coincident as possible. A stereo bar with a gooseneck is one way to mount the mics in position. Ideally, the two mics are the same to achieve the best results, but other combinations can still work well (shown here with an Audio-Technica AT3032 and AT4050).
Try it: It will be clearest in a moderately live room and with a single source a foot or two from the mics, but you should be able to hear the changes in just about any space. You'll need a figure-8 mic and an omni mic. Ideally they will be closely matched, if not the same model, but again most combinations can work to some degree. Set up the two mics so they are coincident, meaning as close together as possible without touching. You want the diaphragms to be on top of each other and facing so they are both on-axis to the sound source. This is critical because as the mics move apart, the sound no longer arrives at each at the same time.
Depending on the mics and mounting options available, it can be a little tricky, but I've had some luck with a stereo bar and a gooseneck (see Fig. 2). If you have access to a stereo mic that allows the capsules to be rotated and patterns to be selected independently (such as an AKG C 426), that is an ideal way to try this.
Run each mic into its own channel. Adjust the gains so that the meters for each read as equally as possible (don't just judge by the position of the gain control), and be sure each channel is panned exactly to the center. Take a moment and solo each mic to observe its sound independently. This will obviously vary based on your situation, but the omni channel will reveal more reflections from the space you're in, and the bidirectional will likely sound a bit closer. It may also have an increased low-end response from proximity effect, which doesn't apply to the omni.
Now bring up the faders so they are at an equal position for both channels and listen to them both summed together. Have the sound source move slowly around the mic at a fairly close distance. Voilà, you have a cardioid!
Experiment with steering the faders, maintaining an equal level at the master as much as possible. As the amount of figure-8 level in the mix is decreased and the omni is increased, notice that the sound of the omni becomes more and more dominant as the back of the figure-8 (which was creating the null-spot for the cardioid) disappears. Going the other way, from equal levels, drop the omni level and raise the figure-8 slowly. The sound will appear closer and the surrounding room less and less noticeable.
The fixed polar pattern in any conventional microphone is just a specific combination of the pressure and pressure-gradient designs, so the standard patterns are really just stops along the continuum between the two (see Fig. 3). For maximum flexibility, record each mic to a separate track in any session so this mix can be tweaked later.
How does it work? Quite elegantly! Recalling the discussion of mic designs and the mention of polarity above, consider what happens when the two mics are combined (summed) by panning them the same direction: The signal from the omni adds constructively with the signal from the figure-8 in the front, so we get an increase in level on-axis (where the source usually is). Because the back of the figure-8 is opposite polarity with the omni, they cancel, creating the null spot in the cardioid.
Have Your Cake and Eat It Too
One of the toughest things to achieve is the right blend of an instrument being recorded with the space it's in, and it's not just relevant for acoustic groups. Every time you pick up a microphone, the question of exactly how far away to place it is raised. This is as true for a spot mic on a soloist in an orchestra as it is for the lead vocal in a hip-hop track. When you use the omni/figure-8 technique described here, it's almost like being able to move a microphone after recording, allowing some fine-tuning of that blend long after the physical mic position has been set in stone (see Web Clip 1).
FIG. 3: Here are some the basic polar patterns you can get with the omni-and-figure-8 technique. By varying the relationship of the faders to each other, you can continuously adjust the pattern.
The ability to make polar-pattern adjustments can be a real life-saver for ensemble recording. Often there is a need for some number of close spot mics to reinforce the main stereo pair. But getting these support mics to blend with each other and then the stereo pair is often the challenge. Bringing up the omni adds back some ambience and leakage around the instrument, improving the success of the blend. In cases where the spot mic would ideally be closer than it was able to be (support mics on vocalists come to mind) or the mic needs to be positioned in a way that makes it particularly tricky to get a good blend (such as a harpsichord or piano with the lid on a half-stick), the ability to dial in a little more reach with the figure-8 can make all the difference. Best of all, this all happens without the need for EQ, artificial reverb, or having to settle for a compromise in mic choice.
It can be very difficult to achieve an organic, cohesive mix when faced with synths, samples and dozens of tracks miked differently. This is often a problem that reveals itself most in the mix, long after the key decisions of mic choice and placement have been made. The technique of mixing with continuously variable pickup patterns, often in combination with other processing, can really shine in combating this problem. The fix to the age-old conundrum of sitting a lead vocal in a mix can be surprisingly easy when it includes the option to roll back slightly from the standard cardioid and add more of the omni element. This naturally adds back some of the openness omnis are known for while preserving the right amount of the figure-8's intimacy.
Background vocal blends can be even tougher. Optimal mic distance can be difficult to judge during the tracking process, and adding heavier fake-verb later can do more harm than good. The option of pattern adjustment can be particularly useful in group background vocals, where the fuller sound of a pressure-gradient mic is usually preferred, but there can't be as much coloration off-axis as many cardioids exhibit.
There are plenty of challenges to judging the sweet spot in a space where the precise placement of a stereo pair is critical. Whether in a live event or studio session, when this technique is brought to stereo and multichannel setups, a whole set of ways to adjust open up. You can have an X/Y with hypercardioids or subcardioids. Choose between spaced omnis, cardioids or any point in between. If the omni/bidirectional pair are combined and used as the middle channel in a mid/side pair, any pattern can now be dialed in on that mid mic.
Beyond fixes, there are creative applications to increase the sound palette and make tonal adjustments. A guitar amp can take on more of the live, bright stairwell it's in during the percussive verses, and reined in for clarity during the heavy, fuzzed-out chorus. Trying some more unorthodox combinations of different microphones can also prove useful, although the pattern summing will be less predictable.
The Josephson Engineering C700A
Drum overheads are also a prime choice for experimenting. In even a moderately live space, the range of sounds that can be dialed in is incredibly wide. To get more fullness from the toms without overdoing the cymbals, try favoring the figure-8 mic slightly and adding a judicious low-pass filter or shelving EQ cut. Also, heavy compression or limiting on one of the mics can create an interesting option. Try the figure-8 for a tighter sound and the omni for a roomier one, post squashing.
Speaking of a roomier sound, to create distant room mics where there were none, try this little trick: Take the recorded omni/figure-8 pair and make a cardioid by using equal parts of each. Now activate the polarity reverse on the figure-8 channel. The mic has just been electrically turned backward! Be sure to check this in the mix, because the polarity is now opposite from the way it was originally recorded.
Granted, this omni/figure-8 recording technique requires more setup that a conventional miking scheme, but it offers a great deal more than any single microphone can. With a little background on how they do their jobs and a little logic about how signals combine (all 1940s technology!), there is now hopefully new pioneering to be done with the simple magic of the microphone.
Brian Heller is a freelance engineer, composer and educator in Minneapolis. He teaches in the Sound Arts program at Minneapolis Community and Technical College.
Josephson Engineering C700A and Sennheiser MKH 800 Twin
Two top-quality mic developers have created models that specifically use the techniques discussed here, each with its own particular approach.
Josephson Engineering has created the C700A ($4,590, see Fig. A), which combines a pressure and a pressure-gradient element directly on top of one another in the same housing for the most convenient and accurate way to accomplish this setup. The C700S ($6,750) adds another pressure-gradient element facing 90 degrees to the side for use as an M/S stereo mic. This is basically like having three mics in one unit, and it provides an extremely efficient and sonically accurate way to record M/S stereo or surround with variable patterns.
“The 700 is our most important LDC and is a wonderful mic, useful for just about anything,” says John Vanderslice, solo artist and proprietor of Tiny Telephone studio in San Francisco. “I sold my [Neumann] U 67 and was able to satisfy all seven of my house engineers by getting the Josephson — not an easy task!”
Sennheiser recently released the Twin version ($3,199.95) of its venerable MKH 800 multipattern mic, which uses two back-to-back cardioids to accomplish the variable pattern. Instead of switching patterns inside the mic, it has two outputs: one for the front diaphragm and one for the back. These outputs can be brought separately into a mixer or multitrack and combined in different ways to create any pattern.
Although they are considered specialty items and are quite expensive, they can be amazing tools for the right engineer. They are certainly some of the finest mics for acoustic recording on the market.