Stereo Microphone Arrays for Ambient Field Recording
Like most people in the pro audio industry, I learned over the years how to work with a handful of common techniques for creating stereo audio: AB, XY, M-S, ORTF/NOS, Blumlein, Jeklin, binaural, panning of multiple mono tracks. Each has certain strengths and weaknesses; each has its place in the grand scheme of things.
During the 1990s I experimented casually from time to time with ambient field recording using some of these traditional techniques. In 2004, after a long hiatus, I took up the challenge much more seriously, beginning with something that seemed to work fairly well for me in those early days: a pair of Shure Beta58 microphones that I owned, situated in an ORTF array. I also built a couple non-traditional experimental mic rigs that year. Those were colossal failures. I learned a lot from them, but the memories are too embarrassing to recount here.
At that time I also joined the Nature Recordists e-mail forum, where a lot of discussion soon developed about Shure WL183 lavalier microphones. These small omnidirectional electret condenser mics seemed to be well regarded by several group participants, so I ordered some and began tinkering with literally dozens of stereo mounting configurations using inexpensive, readily available materials.
Little by little, through seemingly endless sequences of trial and error and hundreds of hours of careful listening, I began to discern many of the subtleties I was hearing from these and other stereo arrays, and how they translate to both headphones and loudspeakers. The more I listened, the more I wished I could capture greater depth and spaciousness in my field recordings.
In March 2006 I posed a question about this to the Nature Recordists group. Rob Danielson, a leading participant at the time, responded with a key insight about human auditory perception that triggered an important "ah-hah" moment for me -- a critical "what if" hunch -- that soon led to a crude configuration similar to the one pictured here, using forward-facing Shure WL183s attached to small parallel wooden barriers, with the microphone capsules approximately 5.5 inches apart:
Spaciousness and depth were so dramatically improved that I immediately abandoned all previous experiments and adopted this as my new default approach.
Unwilling to spend thousands of dollars for some of the most highly esteemed microphones on the planet, but still wanting to move toward higher quality and lower microphone self-noise in my recordings, I paid close attention when members of the Nature Recordists group began discussing AudioTechnica's modestly priced AT3032 mics (now discontinued, replaced by the new AT4022). These small-diameter omnidirectional studio condenser microphones have extended frequency response characteristics, high SPL handling capabilities and very low self-noise (documented by group participant Eric Benjamin to be much lower than the manufacturer's published specs, and actually 5 to 6 db lower than many of those most highly esteemed and high-priced microphones on the planet).
I bought a pair, mounted them in a head-spaced parallel barrier array, and was very pleased with the results. Below are three variations on the theme:
Stereo localization in these arrays is derived mainly by the setback distance from microphone capsules to the leading edge of the barriers. Barrier size is also important -- too small and the desired barrier effect is diminished, too large and unpleasant mid-frequency "bumps" begin to appear. The distance between the microphones will also affect the perceived width of the soundstage; there's room for variation here, but roughly "head-spaced," more or less, makes undeniable good sense and yields excellent results. Any angle between the barriers, inward or outward, will have its own effect. Each of these variables overlap with and affect all the others, so one must season to taste... wisely.
I prefer this "quasi-binaural" type approach over coincident stereo arrays because it captures important arrival-time differences that aid in spacial imaging. In addition, these small-ish barriers seem to stabilize the stereo field, enhancing clarity and transparency by nearly eliminating those "wavy" microphone interactions that usually accompany traditional M-S, ORTF/NOS and spaced omni arrays. Being highly phase coherent, these barrier arrays also deliver excellent mono compatibility. Most importantly, this approach seems to capture considerably greater spaciousness and depth than any other stereo technique I have worked with to date.
These photos and descriptions are intended merely to suggest the basic concept and show some construction possibilities. I offer no plans or specifications, as each particular combination of microphone model and construction technique will require its own adjustments.
I always appreciate hearing from people who have come up with adaptations of their own. Some of their efforts have been very clever and successful.
Curt Olson, Audio Producer/Editor
Minneapolis/St. Paul, Minnesota, USA
August 2007 (Last updated: December 21, 2013)
1. It turns out that a solid body structure, such as the rig with 4x4 fence post material shown above, eliminates certain rare and extremely subtle resonances that have been known to occur between two spaced barriers. The trade-off, of course, is added weight.
2. Counterintuitive as it may seem, cardioid microphones can also be used successfully in these arrays. Cardioids mounted this way seem to deliver somewhat crisper articulation of specific sound sources than omnis, but also a noticeably "flatter," less spacious image. This might be preferred for certain indoor recording situations.
3. Some microphones just don't sound right when placed adjacent to boundaries or barriers. Proceed with caution and "try before you buy" if possible.
4. Once again the ancient Hebrew Scriptures prove to be correct: "...there is nothing new under the sun." Two years before I began my own experiments, musician, scientist and environmental sound recording pioneer Bernie Krause published a similar "quasi-binaural" technique in his wonderful 2002 book, Wild Soundscapes: Discovering the Voice of the Natural World. Bernie suggests mounting omnidirectional microphones on opposite sides of a tree that is approximately the same diameter as the straight-line distance between two human ears. He demonstrates the results in his companion audio CD. I highly recommend this helpful resource.
5. My "tinkering ways" have taken me a step or two beyond what I describe above. In the spring of 2008 I began experimenting with a radically unorthodox bottom plate structure that, to my ear, seems to greatly enhance spacial definition. Almost everything I've recorded since then has been captured with some variation of this "winged-rig" concept. Here are three assemblies I'm currently using...
...one of them built with dimension lumber:
...and more recently with 6061-T6 structural aluminum angle for the wings, in an effort to reduce size and weight (bottom assembly pictured with AudioTechnica AE5100 cardioid microphones):
Note: These winged arrays have a definite upward orientation. If your target sound source is in front of the assembly, tipping it forward toward the source is often helpful. Spacial definition doesn't seem to suffer from this.
6. Another concept worth considering is the partially baffled boundary approach (PBB), made widely accessible in 2010 by Rob Danielson. This is basically a "DIY" adaptation of the old Crown SASS, using low-noise omnidirectional condenser microphones.
To my ear the PBB approach, with its flush-to-boundary microphone orientation, fails to capture key spacial cues that are a specialty of the winged arrays. The PBB effect strikes me as considerably "flatter," less "natural" sounding and generally much less interesting. Nevertheless, the concept has gained traction with many nature sound recordists who appreciate the crisp articulation and tight imaging of the PBB rigs.
Here, as in most of life, we end up grappling with trade-offs and compromises, where choosing "this" means giving up "that."
Look for Rob's nicely detailed plans (PBB2N, PBB2N metric and PBMB) in the "Links" section of his DIY Boundary Mics blog. (And, for curiosity's sake, here's my own lighthearted attempt to build a mini PBB array.)
7. People sometimes ask about my digital audio recorders. I currently favor a Sound Devices MixPre compact field mixer (now discontinued, replaced by the new MixPre-D) feeding the line-input of a Sony PCM-M10 Linear PCM Recorder. To my way of thinking, this setup, though nowhere near as elegant as the coveted Sound Devices 7-Series recorders, yields virtually the same "world-class" audio performance without any unnecessary frills or expense.