Sound in nature often comes from every direction. It usually encompasses a 360º horizon of distant sounds, to which can be added closer sources above; birds in flight or calling in the tree canopy overhead. So it can be conceived of simplistically as a dome of sound, only in certain circumstances presenting anything from the ground below.

Turning this into a left-right stereo field for a soundscape recording is not as straightforward as it may seem, and many rigs have been tried and championed by various recordists. The one I’ve been using over the last decade is a Crown SASS, modified to hold a pair of Sennheiser MKH20 omni-directional microphones. It does a pretty spectacular job, yet recently I’ve been curious to explore a different approach.

The SASS’s strength is the barrier effect that amplifies signals by up to 3dB, and the wide stereo field it captures. However, whilst the SASS captures nearly 270º, there is a significant ‘shadow’ behind the microphone housing, resulting in a slight muffling and attenuation of any sounds from that direction. This can be useful, for instance when you have undesired sounds from one quarter, maybe heavy ocean wavewash or a distant highway, as you can soften it by pointing the mic away. But on other occasions this ‘shadow’ is a limitation. In a rich environment, surrounded by birdsong, some of it will invariably come from ‘behind the mic’, and thus not be captured clearly. Tilting the SASS upwards avoids this to some extent, but there is only so far one can angle up before the horizon in front begins to soften, and the stereo landscape is shuffled off to left and right to the detriment of the centre.

All this is not surprising, as the SASS was designed for studio rather than field applications. These issues, and some additional ones, also plague other familiar stereo field recording rigs, such as Jecklin discs, spaced microphone pairs and MS configurations.

Thus I began thinking about a mic rig that would equally capture 360º and the space above. An egalitarian mic rig, you could say. If you think in terms of ‘front’ and ‘back’, then my objective was to create a configuration which would capture them symmetrically, while maintaining a wide stereo field left to right. I wanted to maintain the barrier effect to some degree, but maybe soften it from the strong directionality inherent in the SASS.

Additionally, I wanted the whole rig to be simple and light. I don’t have workshop skills or equipment for making complex components, and I have always found the SASS at the upper limit of what I wanted to carry, either in the field or my luggage.

This mic rig is the result.

This Iso-binaural array symmetrically renders sound sources from front and rear equally, unlike traditional binaural in which model ear pinae give an audible front preference.

The iso-binaural is based on a pair of omni microphones (in this image Audio Technica 3032s, but I now use Sennheiser MKH 8020s) mounted at 180º in clips on a T-bar. Of all the microphone capsule types including various cardioid pickups, omnis have a reputation for giving the quietest noise floor, being robustly built, and having the least sensitivity to humidity and temperature changes. This makes them preferred for nature recording applications.

To the microphones are added a pair of baffles, which slide snuggly over the barrel of each, with the surface of the baffle coming to level with the diaphragm. These baffles are largely responsible for the stereo separation required, and offer a moderate barrier effect. The curved profile is to encourage sound to reach the mics clearly from directly front and rear.

The baffles have been sculpted from high-density foam, sourced from yoga blocks – a friend with a lathe assisted in shaping these and boring the centre holes. The use of yoga blocks was inspired by Australian recordist Vicki Powys’s pioneering experiments, and Paul Jacobson’s detailed measurements confirming that this material is very suitable acoustically.

I chose a 120mm diameter for the baffles, somewhat arbitrarily in the beginning, to fit the end caps off a Rycote blimp, which I cannibalised to provide a domed air space for wind protection. The plastic end caps slide over the foam baffles with enough tightness to stay in place.

The caps on their own don’t provide significant wind protection. So I made ‘socks’ of a thin, somewhat fluffy fabric, with an elasticated drawstring. This fits over the rycote end caps and tightens up behind the mic.

For more extreme wind, I made up another pair of faux fur covers which can be substituted or even added over the top as conditions require. Whilst these fabrics were chosen from what was available at a suburban craft shop, they seem to be satisfactory, but more acoustically transparent materials could certainly be explored.

Soon I’ll return the Rycote caps to their intended use, and replace them with a simple plastic mesh dome. An open mesh toy ball from a pet supplies store, cut in half, should be perfect for this. This will hold the fabric covers away from the diaphragms, and create the still air space required for wind attenuation.

So what about the sound? My initial expectation was that this rig would not sound as good as the SASS. However after using it in many different situations, I am pleasantly surprised.

Firstly, it does capture a balanced and spacious stereo field. Here is a recording made in temperate rainforest, about 20 meters from a roadway. A vehicle passes by, and you can hear it cross evenly over the stereo field from left to right. You can also hear how the vehicle’s sound very slightly precedes its echo.

Here is a comparison of SASS and Iso-binaural. The two were placed side by side in the field, and recordings made simultaneously on a pair of Sound Devices 7 series recorders. This clip presents the two synchronous recordings. The SASS is the first, followed by the Iso-binaural. You can find the same sections in each, and thus directly compare the way each array renders the same dynamic soundscape into stereo.

My assessment is that the SASS gives a subjectively wider stereo field, possibly due to its ‘wings’ shifting rear sources far to the side. However this SASS effect creates an unnaturally wide image. The Iso-binaural has a more balanced stereo field. I also sense that it renders sources with more natural depth in the landscape, but some may prefer the ‘reach’ that the SASS offers, subtly amplifying sounds in front.

To me the real advantage of Iso-binaural is that it transforms what I do in the field. With the SASS, I aim to set the rig back, so as to locate the action ‘in front’. Where sound in the environment is concentrated, this is predictable, such as from a small wetland, or birdsong in a strip of gallery forest.

But in practice it is often impossible to guess where the action will come from, particularly in woodland or forest habitats where sound comes from all around. It is often easier to anticipate the hotspots – places where activity is greater, such as a group of feeding trees or a drainage line with rich understory. With the Iso-binaural, I aim to put it in the very centre of the action, where it will capture sound equally from whatever quarter it comes. Whatever happens after I switch on my recorder and walk away, I know this system will be rendering a 360º soundscape into balanced stereo.

This lack of the subtle extra ‘reach’ of the SASS is compensated for by being able to confidently locate the mic closer to the sounds themselves. The directionality of the system is also flexible. Having the mics mounted on a T-bar offers the option of adjusting their orientation as the sound source requires, so one could turn this rig into an approximation of a SASS simply by angling the mics inward slightly.

An additional benefit of this rig are its portability. The whole setup is light enough to be carried on an ultra lightweight tripod. I use a Zipshot TR 406, which has legs in the style of elasticated aluminium tent poles, and packs down to just 350mm in length. Total weight of mics, T-bar and tripod comes in around 800g. The whole setup can also be disassembled into mics, baffles, wind covers, clips, T-bar, cables and tripod, and packed separately or into a tiny bag, unlike the awkward and somewhat fragile object that a SASS housing represents.

The final, and not insignificant, advantage of this system is that it is inexpensive, allowing you to put more of your budget into microphones and recorder. It is easy to make and flexible to experiment with. The SASS unit is no longer made by the original manufacturer, Crown, and even if one can be purchased second-hand, it will require skilled modification to mount your chosen microphones. I’ve seen homemade SASS-style units, and they are often both heavier and more cumbersome than Crown’s original plastic box design.

Many other DIY boundary array designs have been developed, such as Curt Olsen’s ‘wing’, John Hartog’s ‘wooden bowl’ array and Rob Danielson’s partially-baffled boundary (PBB) designs. All require carpentry, which result in a heavier and bulkier unit, although I suppose many of them could equally be fashioned from high-density foam. I am grateful to these recordists for sharing the outcomes of their experiments and designs – they have encouraged me to have a go at developing my own.

So, if you are looking at doing ambient soundscape recordings, you may like to experiment with this system. If you have any comments, or even better, come up with an improvement on this idea, I’d love to hear of it. Meanwhile, happy recording!

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