The great thing about recording and designing sound effects is that source material is near infinite! Fortunately and unfortunately, having such an incredible variety of sound sources makes each new recording session a technical and creative challenge, requiring forethought and experience. One of the decisions we must make is the format in which we will capture the sound; mono, stereo, quad-surround, 5.1 surround and ambisonic are all valid options depending on the source at hand. Sound effects are most commonly captured in mono or stereo and today we will compare several common stereo microphone techniques for field recording.
Coincident or near-coincident?
Stereo microphone techniques fall into two categories: coincident, in which microphone capsules are aligned as closely as possible in physical space, and non-coincident, where microphone capsules are spread apart a given distance. Each has advantages and disadvantages for field recording in terms of both sound and convenience.
These techniques involve a pair of microphones with capsules spaced apart over a given distance. Near-coincident pairs are often praised for creating a wider, and more spacious stereo image. Disadvantages depending on the technique can include lack of mono-compatibility in higher frequencies and a lack of focus in the center of the image. As sound effects recordists we must also consider portability and convenience. In the field it can sometimes be more difficult to accurately position or protect from wind a spaced pair versus a coincident pair.
ORTF is a stereo technique wherein two cardioid microphones are spread at a 110° angle with their capsules positioned 17cm apart. This technique creates its stereo effect based on timing differences as sound reaches each microphone and volume differences from sounds being on and off-axis to each microphone capsule. ORTF can create a beautifully wide stereo image and still deliver some mono information. One disadvantage is that a sound source in the center of the stereo image will be off-axis from both microphones, limiting frequency response. That's one reason many use this technique to record more natural, diffuse or ambient sounds, or sounds with a lot of stereo movement rather than a subject with a lot of action in the center of the stereo image.
The definition of the A-B stereo technique is much less specific than ORTF. This technique incorporates two microphones, either cardioid or omni-directional, facing forward in parallel, spaced at a distance determined by the recordist. The spacing between the microphones can determine the width of the stereo field, since timing distances are primarily how the stereo effect is accomplished with this technique. The greater distance between the microphones the less accurately sound in the center will be captured, and the less mono-compatible the recording will be, depending on the microphone used.
Binaural stereo is a technique designed to capture sound as similar to the way a human hears it as possible. Most often two omnidirectional microphones are placed within dummy head which mimics the sound characteristics of a human head, including the Pinnae, the outer part of your ears that reflects and shapes sound into your ear canal(More about how our ears work here).
The result is recordings that sound excellent when played back in headphones, but can have issues when played back in speakers, such as a “hole” in the center of the image from the head, and lack of mono-compatibility. There are advantages though, such as the use of in-ear microphones for stealthy recordings, like capturing clean walla in an environment where the presence of recording equipment would negatively impact the activity of the subjects. You can achieve psuedo-binaural recordings without an expensive or complicated dummy head by placing a baffle between two omnidirectional mics at the correct distance. This baffle will simulate the role of the head, but the lack of Pinnae means that a listener in headphones will not be able to discern vertical dimension from the recording in the way a true binaural recording would allow.
Coincident techniques involve positioning two microphone capsules at the same position in space(realistically as close as possible) and allowing the directionality of the microphones to create the desired stereo effect, rather than timing differences, such as in near-coincident pairs. Coincident pairs are generally more mono-compatible as a result, and can be more convenient for field recording in circumstances where wind protection or fast setup are an issue.
This common stereo technique involves positioning directional microphone capsules at a perpendicular 90-degree angle from each other. X-Y is praised for an accurate and focused stereo image(though not as wide as ORTF or AB), strong mono-compatibility and the convenience of having two capsules close together. Some manufacturers offer X-Y microphones in a single microphone body, such as the Rode NT-4, which was used for these sample recordings. One disadvantage is that sounds in the center of the stereo image will not be on-axis to either capsule.
Mid-Side is a stereo technique highly praised among field recordists. This technique involves a forward facing directional microphone (the mid mic) positioned coincident to a bi-directional figure-8 microphone, the side mic. I have seen cardioid, hyper-cardioid and shotgun mics used as mid mics in MS configuration. The pair should be arranged so that the mid mic points forward to the center of the stereo field, and the side mic picks up the left and right sides perpendicular the mid microphone. Advantages are a forward facing capsule, for full frequency response in the center of the image, a post-recording adjustable stereo width, and the convenience of a coincident pair in the field. Disadvantages are the need for a bi-directional mic (not used in many other common techniques besides blumlein) the need for post-processing (if the recording device does not offer processing). Many recordists process their Mid-Side recordings down to L-R stereo files in the editing stage, so if you are delivering raw files to an editor it could be best to have your stereo processing done in advance to save a step later and not cause confusion down the line.
Mid-side is my personal favorite stereo technique. The combination of having the pair set up and ready to go inside a blimp, the ability to record clean mono spot effects with the same rig, and the possibility of adjusting stereo width when editing make this my favorite choice for sound effects recording.
When making creative decisions about which stereo technique to use, we must ask ourselves about a sound's purpose. If we are recording an ambience, trying to create a strong sense of place in a unique environment, we can use a wider stereo technique to show space, size and depth. If we are capturing massive, impactful, detailed effects we can use coincident pairs to create focus in the center and create a more versatile library file. Near-coincident pairs can have a beautiful wide sound that is great for recording ambient sounds, but it is hard to beat the grab-and-go convenience of Mid-Side or X-Y. The important thing is to experiment and find which stereo method inspires you to record, and use it to capture the world around you!