With so much attention given to mixing in surround these days, it seems like theres much less time given to the actual practice of surround recording. Even though classical recordists have been documenting a performance in surround for some time, other musical genres, as well as location recording for film and broadcast, have only begun to exploit the possibilities. Since the aim of any recording is to capture the environment as well as the source, surround miking accomplishes this goal to the extent that we have never heard before.
So with that in mind, here are a few different methods for surround acquisition. Youll find some different and, in some cases, little known approaches using microphones commonly found in any mic locker as well as some dedicated solutions.
Whichever one you choose, the followiing methods add a spaciousness that you simply cant even approximate with outboard processors or any other traditional stereo miking techniques.
Surround Acquisition Using Common Microphones
OCT Surround: All surround miking techniques start with the classic stereo techniques as a reference point and augment from that point. One of the most venerable techniques is the Decca Tree, which was developed primarily for orchestral recording. The Decca Tree is essentially a spaced microphone pair with a center mic connected to a custom stand and suspended over the conductor. Decca Records, who had a long tradition of developing experimental recording techniques (which included surround sound and proprietary recording equipment), developed the Decca Tree in the 1950s as a compromise between the purist stereo pair and multi-mic arrays. Apart from the individual engineers choice of mics, it remains unchanged to this day and is still in use in film scoring/classical orchestral and opera recording as it produces a very spacious stereo image with good localization.
Optimized Cardioid Triangle (OCT) is a modified Decca Tree that starts with three cardioid microphones in a triangle. The center mic is about 3 inches or so away from the center and the side mics 15 to 36 inches away from each other. One of the differences from a normal Decca Tree configuration is that the side mics face out toward the walls instead of forward. Although OCT has cardioid in its name, better bass response can be had if omni is substituted. By adding two additional rear cardioids 15 inches back from the L and R and 8 inches farther outside the L and R and pointing to the rear, the normal OCT becomes a surround version. (See Figure 1.)
IRT Cross: Before we can talk about the IRT Cross, we have to talk bout the ORTF technique. ORTF uses two cardioids angled 110 degrees apart and spaced seven inches (17 cm) apart horizontally. (ORTF stands for Office de Radiodif-fusion Television Française — French Broadcasting Organization.) This method tends to provide accurate localization and provides a much greater sense of space due to time/phase differences since the capsules are as far apart as your ears.
The IRT Cross configuration is in essence a double-ORTF setup (IRT stands for the German-based Institute of Radio Technology) with four cardioids arranged in a perfect-square with an angle of 90 degrees to each other. (See Figure 2.) To compensate for the narrower angle compared to the standard ORTF (which is 110 degrees), the distance between the mics is greater (8 inches compared to 6 inches with ORTF). Strictly speaking, the IRT microphone cross is best used for ambience recording. Its prime characteristic is a transparent and spatial reproduction of the acoustic environment. Its also the configuration of choice for NPRs Radio Expeditions spectacular recordings, which are some of the best Ive heard.
Hamasaki Square: The Hamasaki Square configuration is similar to the IRT Cross except that figure 8 mics are used instead of cardioids. The distance between each mic is longer as well, at about 6 feet, and the figure-8s have their nulls turned to the front so that this array is relatively insensitive to direct sound. (See Figure 3.)
Double M-S: M-S stands for Mid-Side and consists again of two microphones; a directional mic (an omni can be substituted as well) pointed toward the sound source and a figure-8 mic pointed toward the sides.
Once again the mics are positioned so their capsules are as close to touching as possible. M-S is great for stereo imaging, especially when most of the sound is directly in front of the mics. M-S has no phase problems in stereo, with excellent mono compatibility, which makes it a great alternative for room and ambience miking under the right circumstances.
For best placement, walk around the room and listen to where the instrument or source material sounds best. Note the balance of the sound source to the room, and the stereo image of the room as well. Once you have found a location, set up the directional mic where the middle of your head was.
The Double M-S method uses a standard M-S configuration with the addition of a rear facing cardioid mic for surround. (See Figure 4.)
Dedicated Surround Mics
The Holophone H2 Pro Surround Microphone System: The Holophone H2 Pro is actually the latest model in the Holophone series. The mic is essentially a 7.5- by 5.7-inch fiberglass epoxy ellipsoid that looks something like a giant teardrop. This ellipsoid holds seven DPA omnidirectional microphone elements; five in the now-standard multichannel fashion with the front center element at the tip of the teardrop, plus one on top for height and an element internally mounted in the ellipsoid for the LFE. If the internal mics are connected to wireless transmitters, the setup makes it relatively easy for a sound designer to collect samples in the field; just point the mic and the Holophone does the rest. The Holophone has seen action in a number of high profile situations, from SuperBowl 39 to the X Games to movies like Any Given Sunday to music events like the Bob Marley One Love Tribute Concert. For more information, visit the Web site at www.holophone.com.
The Schoeps KFM 360: The Schoeps KFM360 surround microphone system, designed by location recordist Jerry Bruck, utilizes a KFM 360 sphere microphone and DSP-4 KFM 360 processor.
The central unit in this system is the sphere microphone KFM 360, which uses two pressure transducers and can also be used for stereophonic recording. Surround capability is achieved through the use of two figure-8 microphones, which can be attached beneath the pressure transducers by an adjustable, detachable clamp system with bayonet-style connectors (SGC-KFM). These two microphones are then aimed forward.
The DSP-4 KFM 360 processor derives the four corner channels from the microphone signals with a center channel being created from the two front signals. An additional channel carries only the low frequencies up to 70 Hz. It is possible to lower the level of the channels, delay them and/or set an upper limit on their frequency response.
The processor unit offers both analog and digital inputs for the microphone signals. In addition to providing gain, it offers a high-frequency emphasis for the built-in pressure transducers as well as a low-frequency boost for the figure-8s. For more information, visit www.posthorn.com.
Soundfield MK V Microphone and Model 451 5.1 Decoder: While the Soundfield microphone has been around since the 70s in a stereo version, the latest version, the Mark 5, has now truly become a surround microphone with the recent addition of the model 451 surround processor.
The Soundfield microphone employs a four-element array in a tetrahedral pattern that can be electronically controlled from the supplied preamp/controller. The mic is rather small and unobtrusive, considering the number of capsules employed, and can easily placed in most miking situations, even in the supplied shock mount.
The Soundfield controller is actually a multi-function processor that combines a microphone preamp and the appropriate electronics needed to control the various parameters of the Mark 5. The heart of the processor lies within the Soundfield controls, which offers some unfamiliar parameters usually not associated with a microphone. For instance, Azimuth provides for complete electronic rotation of the microphone, Elevation allows for ±45 degrees of continuous variation of the vertical alignment, and Dominance is a form of zoom control that gives the effect of the mic either moving closer or farther away from the sound source. With the addition of the model 451 Surround Decoder, the Mark 5 becomes a true 5.1 microphone. The processor takes either previously recorded or the B Format outputs from the Mark 5 processor and delivers full 5.1 surround via balanced XLR outputs. An excellent B format VST and TDM software decoder plug-in is now available from Soundfield as well.
Essentially B Format is a sphere with four elements: an X plane (front to back), a Y plane (left to right), a Z plane (up and down), all with a central reference called W. SoundField uses B Format as its core technology for documenting and translating the four elements of real acoustical events so they can be recorded on a 4-channel audio recorder so that the microphone parameters can be manipulated later. Also, since all options are based on the same reference information, there are no phase difference issues to contend with.
For more information, visit www.soundfield.com.
SPL/Brauner Atmos 5.1 Surround Microphone System: The Atmos 5.1 closely resembles a dedicated OCT system, and consists of two pieces; the SPL Atmos 5.1 console and the Brauner ASM (Adjustable Surround Microphone) 5. The Atmos 5.1 console actually is two units; one main unit occupying a 5U rack space, and a 1U power supply. The unit features all the things youd expect on a small console and more. It starts with five high-quality mic amps, each with an illuminated VU meter, and each containing illuminated switches for input pad, phase reverse, phantom power, low cut filters, aux send, and insert. Each mic amp also employs the unique feature of having the gain trim pots motorized so all five can be linked to a master control if desired.
The unit also features a section that allows continuous adjustment of the polar pattern characteristic of each microphone from omnidirectional up to figure-8. These adjustments can be made remotely from the Atmos 5.1 console and can be monitored while recording.
The Brauner ASM 5 microphone unit consists of five matched Brauner VM1 capsules that are mounted on a machined aluminum spider. The capsules are able to be manually rotated 90 degrees and the patterns can be remotely controlled from the Atmos unit.
For more information, visit
www.spl-usa.com.
It should be noted that these different techniques and dedicated alternatives are only a starting point in any situation. Variations on any of the above might yield better results for a particular application, so dont be afraid to experiment. Remember the unofficial miking maxim: if it sounds good, it is good!
Please Note: portions of this article are excerpts from The Recording Engineers Manual, available from Thomson Course Technologies.
