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Soundfield ST250

Stereo Microphone By Paul White
Published June 1995

Paul White takes a SoundField ST250 Stereo Microphone to a live recording session to see if the reality lives up to the legend.

There can be few audio products that have built up such a dedicated following as the SoundField mic, an electronically steerable array of four capsules which, when fed through the mic's dedicated control unit, can produce virtually any mono or stereo pickup pattern imaginable.

The ability to, in effect, steer the mic remotely, plus its uncanny ability to capture the illusion of 'being there', places the SoundField in a unique position. Such perfection comes at a price, but compared to the cost of buying two discrete mics, the ST250 Stereo Microphone under review is actually very cost‑effective.

The System

The system comprises the microphone head itself, a shock mount, wind‑shield, multi‑pin connecting cable and control unit, presented in a sturdy aluminium field case. Power for the microphone and control box can be via conventional phantom powering, battery or mains, though the capsule heater (provided to help prevent condensation) doesn't operate when battery power is selected, as it would shorten the battery life to an unacceptable degree. Under normal conditions, a set of batteries will run the mic for around 10 hours. A red LED shows the controller is powered up, and the controller itself connects to the recorder or mixing console via two conventional XLR mic leads. It is advisable to power up the unit several minutes before use, as the capsules can take a while to stabilise, especially if the system hasn't been used for long periods.

The default setting for the controller is to have all of its six buttons out, and in this mode, the ST250 functions as a side‑entry mic, the side with the logo being the 'live' side. A button is included to switch the operation from side‑entry to end‑entry, and the left and right outputs may also be inverted for occasions when the mic is to be used upside down. Other buttons provide bass roll‑off (120Hz, 2‑pole) and a 20dB attenuator, and select Battery operation. The output signal may be selected as M&S (prior to matrixing) or conventional left/right format, and there is also a B‑format option, which represents the outputs from the capsules as four discrete signals based on additions and subtractions of the capsule outputs, with each capsule contributing equally. If the four B‑format signals are recorded separately, they can be matrixed after recording using a MkIV or MkV controller to, in effect, steer the microphone or change its polar pattern retrospectively.

In addition to the selector buttons already discussed, the controller features rotary knobs for 'Width' and 'Pattern'. These controls are normally flush with the panel surface, but when you push them, they pop out so that you can turn them. Pushing them again returns them to their flush position, which ensures good immunity from accidental movement. To understand these controls, you have to visualise the four‑capsule array as forming two virtual capsules, one a sideways‑firing figure‑of‑eight, and the other a variable pattern omni‑cardioid firing forwards. 'Width' controls the level of the side or figure‑of‑eight component of the mic which, in turn, allows the pattern to be varied from mono to an unnaturally wide pattern which is the equivalent of two back‑to‑back cardioid mics pointing hard left and right. 'Pattern', on the other hand, varies the pattern of the 'virtual' mid capsule from omni, through wide, regular and hypercardioid, to figure‑of‑eight. In other words, without moving from the control room, you can construct any type of M&S mic array you choose and hear the results as you make your changes.

For live use, there's a headphone socket on the control unit, but this isn't intended for serious monitoring work and, according to the manual, using it may even compromise the performance of the mics (the extra current demand may cause a loss of headroom) unless mains powering is employed. It's really included for confidence checking prior to recording, where it can be used to verify that the stereo image is as expected.

Sound In The Field

The ST250's first challenge came when a friend of mine needed to record a drum kit in his timber hallway, but because he only had eight tape tracks, he didn't want to use more mics than strictly necessary. This seemed an obvious job for the SoundField, so I set it up a few feet in front of the kit on a high stand and then added discrete kick and snare mics. With a wide cardioid Pattern chosen, and Width set to around 6, the result was an incredibly natural sound with plenty of real woody ambience.

Because the drummer in question had a selection of what I call 'heavy artillery' cymbals, the ride and crash cymbals often dominated the sound (equally so in real life as in the recording), so the close snare and kick mics were brought into the mix to offset this. We all agreed that the SoundField's contribution to the mix was incredibly accurate, and whereas some mics take ages to set up, this was all so easy. Between takes, we also had a chance to hear the mic on speech and general ambience sounds, and the experience was really second only to being there. Further tests confirmed that the mic was just as accurate regardless of the sound source — I can see why SoundField mics are so popular for direct‑to‑stereo recording of live classical events.


It's tempting to think of the SoundField mic as something designed for live orchestral or classical recording but, as many people have discovered, it's just as useful in the recording studio, and just as happy handling rock and roll as a string quartet. The ability to fine‑tune mic characteristics from the control room is not to be underestimated, and the SoundField comes as close as is probably possible to behaving as a true coincident system, which means there's no comb filtering when you sum the outputs to mono.

On the face of it, SoundField might still seem like an expensive system, but you have to remember that it is a system, and that it can often take the place of a whole array of conventional mics, saving not only on cost (the ST250 is significantly cheaper than, for example, the list price on a pair of U87s) but also on time. If you're a sound professional regularly involved in stereo recording, music, film or broadcast, then you might find the cost of not having a SoundField mic far outweighs the cost of buying one!

Field Studies: How The Soundfield Works

The ST250, like the SoundField MkV, employs four capsules configured as a regular tetrahedron, to provide a continuously‑variable range of coincident stereo pickup patterns after matrixing. In an ideal world, all four capsules would occupy the same point in space, but in reality, they have to be located a few millimetres apart. Electronic amplitude and phase compensation is applied to the capsule outputs, which has the effect of moving all four sources to a virtual point at the centre of the cluster.

Essentially, the SoundField ST250 works on the M&S (middle and side) principle, where, traditionally, one capsule captures on‑axis sound while a figure‑of‑eight 'looks' sideways to capture the left/right information. The outputs from these two capsules are then matrixed to extract discrete left/right information. The ST250 takes the M&S principle even further by providing M&S, L/R stereo or B‑format outputs from the control unit. For M&S applications, the four capsules can be considered to behave as two variable‑pattern capsules, and by controlling the characteristics of these two 'virtual' capsules via the control unit, a number of changes can be made to the stereo pattern, including the overall stereo width and the amount of 'rear' sound included in the output.


  • Very accurate sound with excellent off‑axis characteristics.
  • Pickup pattern can be adjusted remotely.
  • Simple to use.
  • Seems to work well on all sound sources.
  • Very low noise and distortion.


  • B‑format recordings can't be decoded after the event without a Mk IV or Mk V controller.


A unique, versatile and truly wonderful microphone system.