Many sounds are more compelling when captured in stereo. I use three different kinds of stereo mic techniques, all of which are applicable to field or studio work. However, don't discount the utility of "dual-mono" recordings, where two mics are used at once to capture two different perspectives on the same sound.
XY stereo micing is one of the most common techniques used. Although there are several variations on this theme, the basic idea is to cross a pair of cardioid-pattern mics at approximately at a right angle to each other (actually, anywhere from 90 to 130 degrees). The resulting pattern gives a wide, directional coverage in front of the mics while attenuating sounds coming from the rear.
Keeping this crossed pair of capsules right next to each other improves mono-compatibility; separating them increases the perceived stereo "width," but introduces more phase anomalies between channels.
In field recording, XY micing is a good choice when a directional pattern is required. However, the variation in a cardioid capsule's off-axis frequency response tends to color ambiences somewhat. So, while I wouldn't reach for an XY mic to capture the ambiance of a crowded train station, I would use it to focus on the lazy squeak of an overhead ceiling fan.
M/S, short for middle-side, is an altogether different stereo micing technique. Like XY, two capsules are used. However, rather than picking up sounds from the left and right, one gathers sound from the front or "middle," the other is sensitive to sounds coming from the sides. This is done by combining a sideways mounted figure 8 capsule with an omni or cardioid capsule looking straight ahead. Then, an electronic circuit splits the figure-8 into left and right channels, reversing the polarity of one side. When the middle mic is mixed with the in- and out-of-polarity side mic, a remarkably natural stereo image is created. The level of the figure-8 mic signal controls the overall stereo "width."
M/S stereo has excellent mono compatibility. When collapsed to mono, the out of polarity side signal cancels itself out completely, leaving only the middle sound behind. However, this can slightly "dry up" the sound, as a significant part of the ambient information present in the side signal is now gone.
While single-point M/S mics are the most convinient, two mics, one figure-8, the other cardioid or omni will also do the job. In this case, you'll have to use a M/S decoder to convert the results into a stereo image.
A third type of stereo micing uses spaced omni-mics. To my ear, omni-directional mics often provide clearer, more natural and open sound than directional mics. However, being non-directional, placement becomes one's only tool for controlling what sounds do (or don't) end up on tape. For mobile recording, a pair of spaced microphones is difficult to move around. An alternative is the use of binaural recording, where a dummy head is fitted with a pair of mics located within anatomically correct ear canals. This technique works best for headphone-only playback. A variation this is the quasi-binaural technique made possible be clipping a pair of small mics to your glasses. This provides recordings suitable for headphone or loudspeaker playback.
Spaced-omni recordings don't have as strong a center image as XY or M/S recordings (sometimes this is referred to as the "hole-in-the-middle" effect). If you're recording a foreground effect, other stereo techniques may deliver a sound with greater detail, prescence, or impact. However, if you are gathering a background sound that must co-exist with other foreground sounds in the final mix, spaced omnis can be perfect. Here's why: when layering background and foreground elements, a slight "hole-in-the-middle" in your ambient track makes it much easier to place a foreground sound in the center, without having it compete with the background.
While stereo is fun, a dual-mono approach has its own advantages. For example, I'll often use two different mics on the same sound, each on their own track. This gives me two unique mic colorations of the raw sound. Later, I can choose either one or blend them as needed.
In other cases, I'll use one directional mic to capture as much detail on the primary sound as possible. Then I record a second, omni-directional mic on the other channel. This gives me the ability to control the perceived "distance" from the sound source during mixing. The directional mic alone gives you the closest possible perspective. Mixing in the omni mic gradually "zooms back" away from the sound source. If you want a lot of distance, you may want to set the omni on a stand some feet behind you and move in closer with the directional mic. In this case, you are creating a time offset between mics, as the sound reaches the closer mic first. This can lead to phase cancellation when mixing the two sources later, but this can easily be fixed by time-aligning the tracks on a hard disk editing system. Another dual-mono application is in location film sound recording, where one track contains close-up dialog and the other just has ambiance or room tone. In this case, the ambient mic is positioned to get as little of the direct sound as possible.
Wind Noise is a fact of life outdoors. While a simple foam windscreen is a start, it may not be enough on a blustery day. A low-cut switch can help, although using this costs you tone which you may or may not be able to afford to lose. If you've got the foam screen and low-cut switch engaged and are still getting too much rumble, what can you do?
Field recording pros often carry large "zeppelin" windscreens, optionally covered with long, silky fur, a.k.a.a "shaggy dog." Zeppalins and shaggy dogs vastly improve on the performance of foam screens by creating a layer of still air around the mic. The best known supplier in this area is the UK company Rycote, who makes specific model clips, zeppelins and furry-slip covers for a variety of popular microphones. I bought the zepplin or "windshield" for the Shure VP-88 and other larger stereo mics. This, along with the shaggy dog, have served me well in difficult, windy conditions.
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