There is one microphone I have always wanted but never could justify buying as the price was so high, making the Soundfield microphone an ideal candidate for our fantasy Christmas present series. 

It may be that the Soundfield microphone concept isn’t that well known to all our readers, so to start with, here is an explainer of what it is and how it works.

What Is A Soundfield Microphone

SoundField microphones consist of four sub-cardioid capsules (a polar response that is slightly more omni than cardioid) mounted on the surface of a tetrahedron, which can pick up all that is needed for a basic first-order representation of the soundfield, and the electronic compensation for their slight spacing can make them appear to be a highly coincident stereo microphone, or a point source, which is an ideal arrangement with regard to the lack of phase differences.

The output from the capsules is known as A format, but typically, this is converted to B format using sum-and-difference processing. 

• X = 0.5((LF–LB) + (RF–RB))

• Y = 0.5((LF–RB) – (RF–LB))

• Z = 0.5((LF–LB) + (RB–RF))

• W = 0.5(LF+LB+RF+RB). 

For horizontal stereo, you only need the W, X, and Y components. Once the Z component is brought into the equation, the height dimension is added, making it ideal for immersive formats like Dolby Atmos.

The A format to B format historically has taken place in an external control box, but now can also be handled in software. The control hardware and software also offer the option to use the soundfield mic to create a variety of polar patterns, which includes the option to be able to steer the mic in post-production. It’s also possible to do useful things like flipping the microphone upside down turning it from end-fire to side-fire configuration, simply by altering the way in which the signals are processed. 

As you can see, a Soundfield microphone is so much more than a simple coincident pair in one housing; instead, it is a sophisticated multi-pattern, steerable immersive audio capturing device, the output of which can be reproduced over almost any loudspeaker array with the appropriate decoding. Even if you don’t need all those features, it can still behave as an extremely flexible, steerable stereo pair with a selectable polar pattern.

Ambisonics - Way Ahead Of Its Time

The design of the Soundfield microphone is inextricably linked to the design of Ambisonics. The brainchild of Michael Gerzon, Ambisonics has always had a small band of followers, but there were several factors that held it back from being taken up more widely. At the time, it was difficult to work with using analogue electronics, and it really didn’t have many applications.

More recently both these limitations have gone away. Firstly with the introduction of digital technology, both hardware and software and the growth of surround and now immersive audio formats together with binaural delivery platforms, Ambisonics can now be much more easily implemented and has numerous applications.

The genius of Ambisonics is that it is platform and format agnostic. An Ambisonics stream contains all the necessary information about which frequencies are radiating in which directions at what levels. It is then down to each delivery system or format to translate this information into a form that suits each particular format, whether it’s stereo, binaural, 5.1, 7.1 or Dolby Atmos. The key here is that a single Ambisonics audio file can be translated into all these delivery formats and others that haven’t been developed yet. It effectively is a future-proof medium.

Ambisonics can be confusing for those new to it, and it’s important to understand what you have before you start considering how you might use it. You can learn much more about this in our article Ambisonic Formats Explained - What Is The Difference Between A Format And B Format in which we look at some terminology associated with Ambisonics without going as far as workflow or specific tools and plug-ins. 

My First Experience Of Using A Soundfield Microphone

I first came across the Soundfield microphone in 1991, whilst the technology was in the care of AMS. They were planning a demo CD to show off the ST250 location model they had produced. One of the challenges we faced was how to record the 4 channels of audio. We wanted to use digital recorders to get the best signal-to-noise ratio, and the solution was to lock together two Fostex DAT recorders to give us 4 tracks. We made a number of recordings with the help of The New English Orchestra and Singers, including both concert and studio recordings, where the singers were arranged in a circle around the microphone. At the end of the session, we got the singers to dance in a circle around the microphone to show off the immersive properties of the microphone.

At the time, they had a demo suite in the AMS factory in Burnley. At the time, it was one of the few places where you could listen to a Soundfield recording in all its immersive glory, including the height channels. Playing back that last recording was an unreal experience as it felt like the room was moving around you, even though you knew that everything was well screwed down, something I had not been able to experience until fully immersive formats came on the scene, like Dolby Atmos and Auro 3D.

All of this shows that the Soundfield microphone, originally developed by Calrec in the 1970s, was way ahead of its time and, in some respects, was a concept waiting for a solution that has only more recently found applications in 360 immersive video content and immersive audio with binaural rendering like Dolby Atmos.

The History Of Soundfield

Over its life from conception in the early 1970s to the present day, the Soundfield microphone has had many custodians, starting with Calrec.

Mathematician Peter Craven, who specialised in audio signal processing, and Michael Gerzon discovered that they could achieve a very tight degree of coincidence by positioning four small‑diaphragm, subcardioid capacitor mic capsules in a tetrahedral configuration. Although the four‑channel output from this microphone — known as the A‑format signal — was not natively an Ambisonics signal, Gerzon and Craven came up with a processing matrix that could transform this into a B‑format signal. They teamed up with Calrec’s Ken Farrar to develop a prototype SoundField Mk 1 microphone in 1975, with elements still integral to the design of SoundField microphones today. The first test took place with the Schola Cantorum of Oxford at St Mary’s Church. After three years of developing and improving the Calrec SoundField mics through Marks I, II and III, the team developed the first commercially available SoundField microphone, the Mark IIIB. It was launched at an AES show in Europe in 1978 and retailed for a hefty £3000.

By 1983, Calrec had ironed out some of the bugs of the MK III and created the Mark IV system, which at the time represented the collective research and development together with the experience of many users and included the introduction of new low-noise circuit techniques and the control unit facilities have been streamlined to allow full B-format recordings to be made with just a tape recorder and a set of headphones.

In 1986, Calrec merged with AMS, and although Calrec repurchased itself in 1989, the Soundfield stayed with AMS. The company first developed and released the SoundField Mark V before focusing on producing the ST250, the first truly portable Soundfield mic.

By now, users were appreciating the future-proofing capabilities of the Soundfield with its ability to do so much work after the event, even if the end result was mono, stereo, 5.1, or full 360-degree surround.

In 1992, Siemens acquired AMS and merged the company with Neve Electronics (which Siemens had bought from Rupert Neve in 1985). At the same time, SoundField was acquired by audio industry legend Ken Giles, at the time Head of Sales at Drawmer, and set up as an independent company, SoundField Limited. The new dedicated company developed a number of new Soundfield variants, including the SP451, which could output 5.1 surround from the Soundfield microphone. 

In 2000, Pieter Schillebeeckx (fresh from the University of Derby with a Bachelor of Science (BSc) Hons) joined the company as a digital hardware engineer and began work on the DSF1, a digital system for capturing classical music in stereo and high-quality surround.

The SoundField Ltd product range expanded through the early years of 2000, with the release of the DSF-2 mic-and-controller package offering the first digital surround and stereo from a single microphone; the DSF-3 processor, which decoded digital SoundField B-Format signals generated by the DSF-2 systems simultaneously into digital 5.1 surround sound and stereo at any incoming sample rate; and the SoundField SurroundZone post-production plug-in.

Under Pieter’s guidance as Head of Research and Development for SoundField Ltd., the products kept coming. The ST350 (released in 2005) was a lighter, more compact take on the ST250, sporting the straight cylindrical aluminium body found on the ST450.

In 2006, SoundField Ltd provided 12 complete kits for use in the stadiums during the World Cup in Germany. After the World Cup, broadcaster SKY chose SoundField for its HD coverage of Premier League Football and permanently installed SoundField systems in many of the major UK football stadiums.

SoundField Ltd also continued the quest to produce a SoundField microphone at a more accessible price. The answer was to dispense with the controller unit, and the SPS200 was born. Still with the tetrahedral capsule array (as all SF mics have required since Calrec’s first in 1978), the SP200 used four XLR outputs to plug straight into a DAW audio interface, to be manipulated via software plug-ins. 

In 2012, Ken Giles and now-business partner Pieter Schillebeeckx sold SoundField Ltd. to TSL Products (Television Systems Limited), Pieter and SoundField Head of Production Richard Cromack joined TSL’s team whilst Ken Giles retired from the business.

Over the next four years, TSL developed MKII products for the DSF 1, 2 and 3 controllers, the SurroundZone plug-in and, of course, the ST-450.

For TSL, the SoundField experience had proven less a commercial success and something of a distraction from their core business of broadcast control, monitoring and power management. In 2016, a deal was reached for The Freedman Group – home to RØDE Microphones – to acquire SoundField from TSL. Once again, Pieter Schillebeeckx moved with the brand, joining RØDE at its Sydney HQ as Head of New Product Development. 

In 2018, RØDE’s mission enabled a SoundField product to be sold below USD$1000 for the first time. SoundField By RØDE, was launched at NAB in Las Vegas. The first product is the NT-SF1, modelled on the SPS200’s simplicity. The tetrahedral array holds four TF45C capsules and was developed with classical music recording legend Tony Faulkner, who was at the Merton College Chapel experimental recording in 1971 that started the development of the Soundfield microphone many years earlier.

The new SoundField by RØDE Plugin, which partners with the NT-SF1 microphone, operates in a completely different way from traditional ambisonic processors. Instead of using the matrices and correction filters of previous generations, the plugin uses state-of-the-art frequency-domain processing to deliver improved spatial accuracy at all frequencies. Embracing beamforming technology, B-Format can, for the first time, create shotgun-type patterns. The Plugin has the standard surround-sound set-ups, including Dolby Atmos up to 7.1.4 and is supported by Pro Tools, Q-base, Nuendo, Reaper and Logic in VST, AU or AAX formats.

This latest iteration of the Soundfield microphone brings the format into the 21st century and offers all the features that have been lying dormant in this format for so long.

Fantasy Or Not

What I find interesting is that over the 30 plus years, I have been aware of the Soundfield microphone, I have never been able to justify the cost of owning one as I was able to do all I needed, working in Radio and TV here in the UK with mono and stereo microphones. However, with the SoundField By RØDE NT-SF1, the price is much more accessible, so who knows, maybe, just maybe, this fantasy Christmas present may turn into a reality?