The SoundField Microphone goes back for over thirty years, starting with its initial development by Calrec, ‘up in Yorkshire’, in the UK. Its use extended beyond it being a variable polar pattern Mono Mic', or a variable polar pattern Stereo mic' with width, to its use for Ambisonics and Periphony.

Ambisonics involved the use of four loudspeakers in a ‘quad’ around the listener. Periphony took this even further, with additional recorded sources from above and below the listener, apart from the horizontal Ambisonic sources from around the listener due to the 360 degree coverage of the SoundField’s regular Tetrahedral array of four cardioid capsules. Two sets of four extra loudspeakers, in vertical quad arrangements, were arranged front to back and side to side of the listening position, for the reproduction of the sound from the full 360 degree real world.

Now, the SoundField range of mic's from SoundField Ltd, and still in Yorkshire, moves on to cover 5.1 (and 5.0) Surround and beyond. However, a Height addition to Surround has been proven to work with just four ceiling mounted speakers, added to a properly setup 5.0 Surround playback system. (Paper Four)

The writer of this paper is investigating the idea of whether the use of six cardioid capsules, in a Hexagonal array, will offer any enhancement to the way the use of four capsules in a regular Tetrahedron have been used, with all the many SoundField mic's over the years.

THE BASIC PRINCIPLE

When you use and study the full range of uses of the four Sub Cardioid capsules used in the Tetrahedral approach, you recognise that you do actually have made use of Six Cardioids. These become available from the B Format’s W, X, Y & Z signals, which are the W (omni), and three figure of eights, X front to back, Y side to side and the Z up and down.

The combining of the omni, with a closely associated figure of eight, creates a cardioid polar pattern. In fact, by changing the phase relationship of the figure of eight to the omni, you are able to separately create two ‘back to back’ facing cardioids.

The cardioids forward and back use the X figure of eight, the cardioids side to side use the Y figure of eight, and the cardioids up and down use the Z figure of eight, resulting in a total of six, With M&S decoding you go onwards to the final recording uses.

So, why not initially have a physical collection Six Cardioids, in as coincident as possible Hexagonal rig. Two front to back, two side to side, and two up and down. Will it not then be likely that there will be improvements in the system’s many uses, as the three vectors of the real acoustic sound field, are directly covered by appropriately positioned individual capsules, with immediate M&S decoding?

This is based on thinking that with the current set of four capsules in a Tetrahedron, angled to evenly cover 360 degrees, you are then have their outputs ‘shared’ amongst the three vectors captured from the real world acoustic, when the B Format W, X, Y and Z signals are created?

The Hexagonal approach can still provide the four B Format signals, by summing all the outputs together for the W omni, and then use the appropriately phased pairs to get the three X, Y & Z figure of eights. Then all the current many uses of the B Format will still be able to be used.

THE PRACTICAL RESULTS SO FAR

The nearest the writer has got to producing a useable system has been with the use of a Schoeps ‘Cluster’ of five of their cardioids, two front to back and two side to side, with a fifth one upwards. This was outlined in Paper 4, dealing with Height missing in our usual Horizontal Surround. (Pic. One)

Four Schoeps CCM4v side-address cardioids have a single CCM4 end-address cardioid neatly between them as the upwards mic'. Used only directly, you have the variable polar pattern Mono use, the variable polar pattern and width Stereo use, along with horizontal 5.0 Surround and the Height version. All these uses came over as working as successfully as other Surround mic collections have done.

One useful facility was tried out when making Stereo recordings of the organs in St. Alban’s Cathedral and also at the Brentford Musical Instrument Museum. For the Surround with Height use, the rig had to be pointed straight forward, so the high positioning of the organs, and the acoustics, could be appropriately positioned in the Surround plus Height playback.

When the Stereo mix was later post produced, a ‘tilting’ upwards of the rig was available by mixing together, as the Mid mic', the feeds from the forward and upward capsules. This has always been possible, of course, with the SoundField microphone system.

It is shortly expected that a set of six cardioid capsules, the same as used in the new SoundField SPS200 Tetrahedral arrangement, will be available to complete the prototype Hexagonal arrangement illustrated in (Pic. Two). Then many comparison recording will be possible and the playback results judged and reported on this Web Site.

ANTICIPATED AFTER THOUGHTS

Apart from the expectation of a more definable directionality being available, due to having the six capsules pointing more ‘user wise’, there is also an option when the B Format route is taken. The array could be angled to have pairs of the horizontal four, facing forward and backwards, for the X and Y figure of eights. In a way, this is like the original Tetrahedral approach, except it avoids the initial ‘sharing’ of the directional characteristics, with the Z part of the B Format.

Possibly this could create ‘wider cardioids’ for the final front, and also for the rear M&S decoding, when the Surround uses are involved. The onward processing with the physical up and down cardioids in the Hexagonal rig, are not affected by this alternative approach.

Considering the usual methods of combining two microphones for basic Stereo use, where the figure of eight will either be a single diaphragm, or a very close use of two, there must be thoughts about the use of further apart cardioids to create the figure of eight polar patterns. Relating back to the writer’s uses of many Stereo M&S mic' pairs, there is also the fact that they were always arranged to be vertically coincident to each other.

Variations in the imaging accuracy must be main consideration albeit that, compared to the large acoustic world we are recording in, even the 40mm spacing in this Hexagonal proposal, must be surely considered to be proportionally quite small?