Acquiring sound in stereo is more than just putting up a couple of microphones and designating one left and the other right. Because of the physics in how sound waves relate to one another and with the microphones within the acoustic space, improper techniques can result in poor stereo imaging or even unusable sound.
Stereo microphone techniques
Over the years, several techniques have been developed for capturing a stereo image using conventional microphones. The three most common methods are spaced pair, coincident or X-Y, and mid-side (MS).
In a spaced pair arrangement, two microphones (usually omnidirectional) are spaced between 0.9m and 3m apart and panned left and right accordingly, resulting in a wide stereo image. The spacing distance is usually related to the size of the sound source, with smaller sound sources requiring a closer spacing to maintain a stereo image. Spaced pairs are also susceptible to phase-cancellation issues caused by the differences in sound arrival time at the microphones. They do not collapse down to mono well and if spaced too far apart, they can leave a “hole-in-the-middle” in the stereo image.
The coincident or X-Y arrangement uses two cardioid microphones, with the capsules placed as close together as possible. For best results, the microphones should be identical models from the same manufacturer. The angle of the capsules can vary from 90 degrees to 135 degrees, depending on the size of the sound source and the width of the stereo image desired. The pair of microphones faces directly at the sound source, and the microphones are panned left and right. Because the microphone capsules are in close proximity to each other, phase cancellation issues are minimized. Stereo separation when using an X-Y pair is good but can be restricted on a wide sound source. Mono compatibility is fair to excellent depending on the spacing (near-coincident vs. coincident).
The mid-side technique uses a cardioid microphone to pick up the on-axis sound (mid) and a bidirectional or figure-of-eight microphone oriented to face left and right to pick up off-axis sound (side). The two microphone outputs are connected to an MS matrix decoder, which allows for a variable controlled stereo image. By simply adjusting the level of the mid signal in relationship to the side signal, the stereo image can be made narrower or wider without moving the microphone.
What is a stereo mic?
Typically two microphones with appropriate mounts are used for stereo. In many broadcast applications, however, setting up stereo pairs can be cumbersome and time-consuming. Plus, pairs of mics don't always adapt well for camera mounting. A stereo mic is basically a single microphone body with two distinct capsules configured in one of the arrangements explained above. In its simplest form, a compact field interview recorder with two built-in microphones (one on each side of the device) is a basic spaced pair. More often, stereo microphones are built with their capsules configured for X-Y or MS. Some X-Y microphones incorporate two physical capsules mounted on a handle, whereas others use proprietary engineering techniques to achieve the X-Y arrangement inside a more traditional headcase.
A mid-side stereo microphone uses a common headcase to house its two capsules. This configuration allows for single-point (cardioid) or line-gradient (shotgun) versions. Some models include a built-in selectable MS decoder matrix, enabling users to choose from a wide or narrow stereo output along with discrete mid and side outputs to feed an external matrix.
Many stereo mics terminate in a five-pin XLR-type connector and include a fan-out cable terminating in three-pin XLRM-type connectors. It is often a good idea to have a spare fan-out cable in the production kit … just in case.
Finally most stereo microphones are marked as to proper orientation. Making certain “this end is up” maintains the L-R perspective.
Stereo microphones in use
The most obvious use of these microphones is to provide a stereo point of view perspective. Often they are camera-mounted in sports or on a boom pole, or stand for other programs. Stereo mics positioned over the studio audience in a talk show can provide additional depth. In live programming or reality shows, using stereo microphones helps establish perspective and localization of sound sources. They can widen the sound field to make viewers feel as if they are in the midst of the action. MS mics enable sound engineers to vary the width of the stereo image, allowing them to narrow in on the close-up action and widen the sound field for establishing shots.
In sporting events, stereo mics are used to provide side-to-side perspective of the action (hockey, basketball, football, etc.), as well as localization of specific sound sources, such as coaches and players, to spread them out in the mix. Placing stereo mics on handheld cameras provides a visual reference to the stereo image.
However, be aware that if, for example, the camera pans toward the crowd, the overall stereo perspective changes, which can confuse the viewer. Using an MS microphone with its ability to be narrowed down to mono by sound mixers is a good choice for these situations.
When producing a music-based program, the audience is often shown from the musician's perspective, while the sound is presented from the audience perspective. To minimize the effects of a “reversed sound field,” stereo ambient microphones are used in conjunction with the normal close-miked vocal and instrument microphones. Thus, sound mixers can create a pleasing soundtrack that fits the visuals regardless of angle.
Stereo mics are often used when capturing sound effects for use in post production. Their ability to maintain L-R perspective adds depth to the effect. With their rugged construction and high sensitivity, a good stereo microphone is an indispensable tool in the location recordist's bag of tricks.
In one application, we needed to capture a college marching band in surround. Because of the band's physical location, and because of the crowded space, it was nearly impossible to put mics up in front of the musicians. To accomplish this with a minimum number of microphones, we used two stereo mics, with one located on each side of the band. The individual outputs of each stereo microphone were routed in the console to provide Lf /Lr and Rf/Rr. The result enabled the A1 to position viewers in the midst of the band.
Like other audio tools, the proper use of stereo mics can enhance the overall audio experience for listeners or viewers. Try different placements and mic positions. Think outside the box, and don't be afraid to experiment. However, always remember that somewhere, someone will be listening to the program in mono. Make certain that the technique used is verified for mono compatibility before going to air.
Steve Savanyu is director of educational services for Audio-Technica.