Soyuz 013 Ambisonic

Shockmount, standmount and cable are included, but there’s no wind protection as standard.

Will Soyuz’s beautifully crafted four‑capsule mic help turn Ambisonics from a niche interest to a mainstream studio technology?

The brainchild of pioneering mathematicians Michael Gerzon and Peter Craven, Ambisonics extends the idea of Mid‑Sides stereo into three dimensions. And given how much popular confusion there is about Mid‑Sides stereo, it’s hardly surprising that Ambisonics is widely misunderstood. In particular, I think many people view it as something that’s only relevant if you’re working in surround — but nothing could be further from the truth. An Ambisonic signal simply describes the 3D soundfield at a particular point in space, and the beauty of it is that this soundfield can be mapped onto any playback system you like, from a mono speaker to a vast Atmos array.

Formats such as Dolby Atmos are ‘spatial’ in the sense that they generate a three‑dimensional soundscape on playback. In practice, this is typically manufactured by panning mono and stereo sources around a speaker array. Ambisonics, by contrast, is a spatial recording format, and an Ambisonic microphone is designed to capture the 3D soundfield that impinges on it.

The Age Of Ambisonics

In conjunction with Ken Farrer at Calrec, Gerzon and Craven created the first commercial Ambisonic mic, the Soundfield, back in the late ’70s. Now part of the Rode group, Soundfield still make high‑end Ambisonic mics such as the ST450 MkII and SPS200, and were instrumental in the design of the impressive Rode NT‑SF1 back in 2018. All Soundfield mics are first‑order Ambisonic mics, which employ four cardioid or subcardioid capsules in a tetrahedral array. The same is true of models such as the Sennheiser Ambeo, Brahma Studio 4 and Core Sound Tetramic, and you can now buy second‑order Ambisonic mics with eight capsules such as the Voyage Audio Spatial Mic, Core Sound Octomic and Brahma Studio 8. In principle, a second‑order mic offers sharper localisation and thus the potential for greater realism, at the expense of greater complexity and cost.

Before anything useful can be done with the raw sound from the capsules of any Ambisonic mic, it must be processed. In the analogue domain, closely matched gain settings are used to preamplify each capsule’s output by exactly the same amount. Filters are then used to matrix the raw four‑ or eight‑channel signal into a universal pattern known as the B‑format. It’s this B‑format signal that can be thought of as the 3D extension of Mid‑Sides, and which can then be decoded into mono, stereo, or any surround format. If you’re working in stereo, this decoding can mimic any coincident stereo miking array, pointing in any direction you like. In other words, an Ambisonic mic can be thought of a stereo mic that can be reoriented after the fact. (It can do a whole lot else, too.)

These days, the filtering and decoding is typically handled digitally, and for good reason. Since the Ambisonic mic is by definition a coincident array, directional information is represented purely as amplitude differences between the channels, and a high degree of precision and consistency is important in order to maintain the integrity of this information. This is much easier to achieve in the digital domain, albeit that no plug‑in is quite as impressive to look at as the analogue decoder box from a Soundfield MkIII! Whereas some of the Soundfield models still come with a custom‑made preamp and transcoder, most other Ambisonic mics supply only the means to capture the signal from each capsule directly. It’s up to the user to provide four or eight matched preamp channels and A‑D conversion, whereupon digital processing is used to extract the B‑format magic from the raw (A‑format) signal. As a consequence, mics like the NT‑SF1, Ambeo, Tetramic and Studio 4 are more affordable to buy than the high‑end Soundfield models.

Purity Test

It’s probably fair to say that all the Ambisonic mics that have existed up to now have been marketed primarily as tools for capturing sound in an uncoloured, natural, lifelike fashion. They’re not conceived as ‘character’ mics in the way that, say, a valve mic or a large‑diaphragm capacitor mic might be, and they’re not designed to add their own contribution to what we hear. An Ambisonic mic is typically intended to provide a completely transparent — and uniquely versatile — window on the audio world.

That’s not quite so true of the latest first‑order Ambisonic mic to reach the market. Like most such designs, Soyuz’s 013 Ambisonic employs four directional capsules in a tetrahedral array. But unlike any other current Ambisonic mic, it has a transformer‑balanced output stage for each capsule, employing Soyuz’s custom toroidal transformers to add “additional depth with a subtle character that complements any source”. And, whereas many Ambisonic mics use miniature electret capsules, especially at the affordable end...