classics and new contenders

Ambisonic microphone Round-up

JON THORNTON looks at the growth of ambisonic mic options, and the models worth looking into.

Relative to the history of audio production, Ambisonics is an old technology, dating back to work carried out by Michael Gerzon and others in the 1970s. It’s had something of a renaissance over the last five years, though, as the potential of capturing audio in three dimensions and being able to decode that in different ways has been turbocharged by developments in DSP power and the rise of Virtual and Augmented Reality applications.

Put simply, Ambisonics captures a 360o , spherical soundfield, using multiple microphone capsules. It’s perhaps easiest thought of as an extension of MS stereo recording, where a figure-8 microphone’s output is added or subtracted from a central cardioid microphone to give stereo positioning. Replace the cardioid

‘M’ microphone with an omni, and adding a further two figure-8 microphones oriented front/rear and up/down and you end up with four audio components.

These four components, known as ‘B

Format’ are named W (omni), X (front/rear), Y (left/right) and Z (up/down) and can be used in several ways. For example, by combining them it’s possible to create the pickup of a virtual microphone, in both pattern and angle. And because you can use the components to derive multiple virtual microphones, creating one oriented to each position of a 5.1 speaker arrangement allows decoding to this format, or any other. This decoding can then be binaurally rendered for headphone playback, and because it’s virtual, can be constantly rotated by head tracking information — hence the appeal for VR applications.

In practice though, the microphone arrangement described above is somewhat impractical, as it’s physically impossible to locate all of those capsules coincidentally. Instead, most current ambisonic microphones employ an array of four cardioid or sub-cardioid capsules arranged in a tetrahedral fashion, such that each capsule is located on each of the four faces of a tetrahedron. This arrangement is known as ‘A Format’ and allows the corresponding ‘B Format’ components to be derived by some relatively simple matrixing. Of course, this doesn’t get around the issue of non-coincidence, but simply minimises and distributes any phase issues equally. As a result, the A-to-B format conversion process requires some complex filtering, the nature of which is somewhat dependent on the exact spacing of the capsules, so varies from manufacturer to manufacturer. Some address this by providing hardware or software convertors, others by supplying filter coefficients to plug into OEM software.

Finally, everything described so far is known as first-order ambisonics (FOA), using only four components. One criticism of FOA is that it doesn’t provide particularly good spatial resolution — in the same way that a coincident stereo pair, relying only on level differences between the two channels, may not deliver as good an image as a spaced pair. What this means in practice is a blurring of the location of sources. Higher orders (more components to the B format, so more resolution) do exist — second-order has a total of nine components, and third-order 16 components. But there’s a trade-off here: more components means more capsules, which means more noise from summing as well as more aggressive filtering requirements, all of which have an impact on overall fidelity. And higher-order microphones can’t simply use traditional polar patterns and matrixing to generate these components. Instead, they rely on larger multi-capsule arrays married to DSP.

Here’s a round-up of both some longestablished and new contenders in the field.

SoundField

The marque that started it all with the prototype SoundField microphone developed in collaboration with Calrec in the 1970s. Several iterations later, the Mark IIIB became the first commercially available Ambisonic microphone in 1978. The marque has had many changes of ownership since — from Calrec, to AMS, then to the independent SoundField Limited, and most recently to The Freedman Group, owners of RØDE Microphones where it remains to date. The SoundField brand remains, though, with a range of products that have truly benefitted from 40-plus years of continual R&D.

There are four microphones currently in the range, all first-order, together with a range of associated hardware and software decoder/ convertors. The SPS200 is the simplest, designed to operate entirely with software for conversion and decoding. The ST450 Mk II is the field recording specialist — and is supplied with its own (battery operable) hardware for powering, decoding and monitoring. The DSF-1 and DSF-2 Mk II are aimed at the concert recording and broadcast markets and are beloved by both for the compact size, ease of rigging, and the range of hardware interfacing and decoding options available. Neat touches like internal heaters in the ST450 and DSF-2, to prevent condensation on the capsules, also shows a real understanding of the needs of these markets.

Reynolds A-Type 4

Jack Reynolds’ eponymous microphone company has stayed largely in the realm of producing low volume, classically inspired tube microphones based on tried and tested design approaches. The A-Type 4, though, is none of those things. An FOA mic featuring four matched 14mm capsules in a tetrahedral array, the whole microphone weighs in at an astonishingly light 68 grams. This is due to the construction employed, which uses 3D printed nylon (using selective laser sintering) to create the entire, quite open structure, which is then coated with a fine nickel plate for electrical shielding. As well as lightness, the use of this material gives other advantages — it’s properly field-rugged and suffers less from condensation in high humidity environments. Supplied with a custom Rycote windshield and shockmount, the design was created with input from leading end users.

Sennheiser VR mic

Perhaps the first established mainstream manufacturer to produce an Ambisonic microphone, Sennheiser’s VR microphone is part of a larger immersive audio product family named Ambeo. The VR Mic itself looks quite ‘normal’ at first glance — you could mistake it for a handheld vocal mic that’s put on a bit of weight. Unscrew the head grille though, and you see the four matched KE14 capsules in a tetrahedral array. It’s a good balance between compactness and ruggedness — equally at home indoors on outdoors on location. A-to-B format conversion is performed by Sennheiser’s own Ambeo plug-in, but for further processing of B-format signals, you need to look elsewhere. However, Sennheiser’s acquisition of a majority shareholding in immersive audio software specialists Dear Reality means that there’s no shortage of tightly integrated options here.

RØDE NT-SF1

The first fruits of The Freedman Group’s acquisition of SoundField Limited, the NT-SF1 bears some superficial similarity to the SoundField SPS200. Look closer and you realise that it is a very different design, though — after all, there are only so many ways that you can arrange a tetrahedral array on a stick. The capsules themselves are true condenser ½” cardioid capsules manufactured in-house in Sydney, with their housing and the rest of the microphone constructed from solid brass. The whole assembly feels extremely solid and rigid. It’s supplied in a complete kit, with a spherical windshield and sock and dedicated shock-mount — so, despite looking quite exposed it’s more than up to location work. The supplied software for A-to-B format conversion — and then further manipulation of the B format — is extremely well thought out, and sounds excellent.

TF11 FET

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Voyage Audio Spatial Mic

The Spatial Mic is a second-order ambisonic design, featuring eight individual capsules. It’s distinctive in that it also integrates amplification, analog ue-to-digital conversion, and a degree of DSP into the microphone itself. One clear advantage of this is that the outputs of the capsules emerge digitally — in either USB or ADAT Lightpipe format — so no bulky breakout cables to individual XLRs are required. Control of gain and other settings is performed by a push-and-turn rotary encoder located on the microphone body, aided by an LED ring for visual feedback. A 3.5mm headphone socket at the base of the microphone allows both stereo and binaural confidence monitoring. The Spatial Mic is supplied with its own software for A-to-B format conversion and creating virtual microphones, and also a remote-control application for setting gains and such over USB.

Zylia

Zylia’s ZM-1 is the only third-order microphone featured here. It’s also unusual in that it doesn’t employ conventional condenser capsules. instead, it uses MEMS (micro-electromechanical-systems) digital microphones; think of these as tiny capsules etched into an integrated circuit, which also performs the functions of preamplification and the analogue-to-digital conversion process. 19 of these form a spherical array, and the outputs of the MEMS microphones are combined using DSP to create a third-order B format. Being almost entirely digital, Zylia claim benefits in ensuring that tolerances are tight and that the initial calibration settings of the 19 sensors (stored internally onboard the microphone) never need changing. Output is via USB, and there is a range of interesting software options to work with the B format, including the ability to auto-detect multiple instrument sources in the sound field and isolate them for mixing/ balancing.

Core Sound TetraMic and OctaMic

New Jersey-based Core Sound has been producing microphones for both Ambisonic and binaural capture since 1990. From the Ambisonic side of the product range, two products are offered. The TetraMic is an FOA design featuring four capsules, whilst the OctoMic is a second-order offering with eight capsules. Both designs are solid-looking, precision built but very compact affairs with closely matched 12 mm diaphragm capsules. A good range of accessories are available from Core Sound themselves (Phantom Power Adaptors and windshields) or recommended OEM accessories such as shock mounts. Somewhat curiously, the eight-capsule OctoMic, on paper, has better noise performance than the fourcapsule TetraMic — quoted at 15 dBA for the whole array versus 19 dBA per capsule for the TetraMic. A key selling point for both is that every microphone is individually calibrated, and the resulting correction files for each are provided to use with the supplied A-to-B format conversion plug-in.

Nevaton VR

The very latest Ambisonic offering comes from Russian manufacturers Nevaton. Just released, the Nevaton VR is an FOA microphone, featuring four 33mm capsules, some of the largest employed in a tetrahedral array. Weighing in at 600 grams, it’s one of the more heavyweight options here — but that is more than offset by the quoted noise performance, with equivalent noise of 4dBA. However, using larger diaphragm capsules means that the spacing between them increases, and that tends to limit the frequency at which the virtual B format patterns can be successfully derived without significant filtering — so it will be interesting to see how the mic performs here. Nevaton doesn’t provide its own conversion software, but filter sets for use with third-party software (generated by Angelo Farina from the University of Palma) are available for download.