Stereo Microphone Arrays for Ambient Field Recording

Like most people in the pro audio industry, I've 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 group, 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 understood, 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, 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 below, using forward-facing Shure WL183s attached to small parallel wooden barriers, with the mic capsules approximately 5.5 inches apart:

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Sample audio clips:
Sandhill Cranes :: Listen (1:00) >>>
Sandlot Volleyball :: Listen (1:28) >>>

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 microhone 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; the new AT4022 looks like a possible replacement). 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 considerably 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 (with apologies for the poor photos):

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Sample audio clips:
Spring Thunder w/ Siren :: Listen (1:31) >>>
Evening Twilight Over Beaver Pond :: Listen (2:53) >>>
Freight Train and Evening Thunder in Hinckley, MN :: Listen (4:31) >>>

Stereo localization in arrays of this type is derived mainly by the setback distance from microphone capsules to the leading edge of the barriers. Overall 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 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.

The photos and descriptions above are intended merely to explain the basic idea and to show some construction possibilities. I offer no plans or specifications for two reasons: 1) each combination of mics and construction methods will require its own adjustments; 2) my own tinkering has taken me a couple steps beyond what I describe here. Bottom line: it's unlikely that I'll ever have a settled set of particulars to publish.

I always appreciate hearing from others who have come up with adaptations of their own. Some of their efforts have been very clever and successful.

Curt Olson
Track Seventeen Productions, Inc.
Minneapolis/St. Paul, Minnesota, USA
August 2007 (updated: November 2009)

Notes:

1. Rob Danielson has tested some of the variables of these and other stereo arrays -- in particular, the effects of different microphone setback distances. Click here to see and hear some of his findings.

2. It turns out that a solid body, such as the short piece of 4x4-inch cedar fence post 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.

3. Counterintuitive as it may seem, cardioid microphones can also be used successfully in these rigs. Cardioids mounted this way seem to deliver cleaner articulation of specific sound sources than omnis, but also a slightly "flatter," less spacious soundstage.

4. Certain microphones just don't sound right when placed adjacent to boundaries or barriers. Proceed with caution, and "try before you buy" if possible.

5. People sometimes ask about my digital audio recorders. I currently alternate between two configurations, depending on the circumstances: 1) A Fostex FR-2LE -- a simple, relatively ugly-looking deck with surprisingly good mic preamps, proven through painstaking testing by a number of field recordists to be in the same league as, and nearly indistinguishable from, those found in the vastly more elegant Sound Devices 7-Series recorders; 2) A Sound Devices MixPre compact field mixer, feeding the line-input of a Zoom H2 digital audio recorder.

6. Once again the ancient Hebrew scriptures prove to be correct: there is truly 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 excellent 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.