PAUL WHITE tries out an intriguingly-styled US-built mic and discovers that tonal honesty can occasionally be a very good studio policy.

Omni-pattern microphones have the overwhelming advantage of a noticeably more open, natural sound than cardioid models of similar quality, not to mention a total lack of proximity effect (the tendency to an increase in bass when a mic is used very close to a sound source), but they have one major disadvantage: they're quite impossible to build -- or at least, a perfect omni mic is impossible to build, because the laws of physics conspire to make it that way. Heisenberg's uncertainty principle makes it clear that in trying to measure something, we invariably change the thing being measured, and the smaller the thing we're trying to measure, the more we change it. Microphones measure small things -- they average the sound energy carried by multitudes of air molecules -- but to measure what the air molecules are up to, some of their energy has to be used to move a diaphragm, and if we take energy from the air, we're changing the very sound we're trying to measure.

The other problem is the physical size of the microphone: a theoretically perfect omni mic would comprise a minute, infinitely-light diaphragm suspended freely in space. Stick the diaphragm in a housing of any kind and you're partially masking the sound you're trying to capture. That's why traditional studio mics with housings like flower pots have omni response plots that show significant changes in frequency response as the sound source moves off axis. If the mic were a true omni, there wouldn't be any axis! However, we need omni mics and it would be defeatist to allow a little thing like the immutable laws of physics to prevent us from achieving at least a favourable compromise.

All this preamble is by way of explaining why the Earthworks OM1 looks like a stainless steel electric toothbrush that's lost its head. You can't have a point source (or should that be point destination?) diaphragm, but you can have a very small one. Similarly, you can't just hang the capsule in space, because of local gravitational conditions, but you can put it at the end of a thin probe designed to interfere with the soundfield as little as possible. Some people have commented that the Earthworks OM1 looks rather similar to a B&K reference microphone, but this isn't because they've deliberately copied B&K. To make the most accurate omni possible, you have to take the 'probe' approach.

Earthworks currently have three mics in their range, the OM1 being the least expensive and designed to work with transformerless consoles equipped with standard phantom power. A different model is available for those using transformer-coupled consoles or mic preamps. Looking more closely at the OM1, the body appears to be machined from stainless steel, with a balanced XLR connector at the thick end, and a very fine grille over the tip of the probe end. Because the grille is so fine, it's essential not to get the mesh blocked by dirt, yet according to the designers, whenever they first show the mic to somebody, their first instinct is to touch the end. They claim to have a team of psychologists working on this problem, but I suspect this is merely an attempt at levity! This suspicion is reinforced in the manual, when the user is advised not to stir drinks with the OM1.

No pad or roll-off switches are provided on this mic, and the only support is a standard mic stand clip. For serious use, I'd be inclined to get hold of a shock mount, especially considering the extended low-frequency response of this mic. A nice touch is the box the mic comes in; for review, we had a stereo pair which came in a box machined from a solid block of wood with foam linings in the base of the cutouts. The hinged lid closes with a positive catch mechanism, and the whole package looks very expensive.

This mic has a massive tolerance to loud noises, topping out at 146dB, but correspondingly, its sensitivity and noise figures are slightly down on what you might consider to be typical figures for a top-end microphone. Its sensitivity, of 8mV/Pa, is around a third of what you might expect from a typical quality studio mic, and the noise figure, of 27dB A-weighted, is a tad on the high side, but these apparent disadvantages are offset to some extent by the fact that the mic can be used very close to the sound source with no proximity effect. Indeed, in typical studio applications, neither noise nor sensitivity presented any problems whatsoever.

Tested side by side with a large-diaphragm capacitor mic, the OM1 sounds obviously more accurate and detailed, with a much snappier transient response, but it doesn't seem to have that cold, clinical feel that many small-diaphragm mics have. Used with a pop shield, the OM1 makes a great vocal mic, though some singers will obviously prefer the flattery of a specific large-diaphragm model. If the transient response of the OM1 is too revealing, however, vocalists can always sing side-on to the mic to tame the top-end response a little.

Where this mic really excels, though, is on acoustic instruments: several users have reported excellent results on pianos and drum overheads. I found the mic superb for recording acoustic guitar and percussion, and even though omnis invariably pick up more room sound, you only have to move a little closer to the source to compensate for this. What's more, the microphone position for the OM1 seems less critical than it does using many cardioid mics.

What I haven't mentioned yet is the surprisingly low cost of this microphone, given its very high standard of performance. You could easily pay two or three times the cost of this mic without getting better results, and indeed, you can buy an Earthworks model costing twice as much as the OM1, which offers a wider frequency response and a slightly better spec. For general recording work, however, where you want to capture a sound as realistically as possible, the OM1 is an excellent performer. I'd read a lot of testimonials for this mic before conducting this review, but I have to say that they were borne out in practice.

While not everyone needs an accurate mic all the time, I feel that the excellent transient response of this model, combined with the omni's characteristic lack of proximity effect, makes it a very valuable addition to any well-stocked microphone locker. If you can run to it, get a stereo pair, because that means you can work in stereo for jobs such as drum overheads, pianos, acoustic guitars, percussion, vocal ensembles, and so. Despite its rather clinical appearance, this is a very musician-friendly mic, which looks sleek enough to impress clients. Its only shortfall is that it may be a little noisy and a little insensitive for long-distance work, but for the majority of real-world tasks, it comes about as close to the ideal as physics will allow it to!

The design brief for this mic was for accuracy: it has an on-axis response of 9Hz-30kHz (+1dB/-3dB), which, off-axis, rolls off gently above 10kHz. Even the 'probe' approach (see main text) causes some high-frequency shadowing, and any diaphragm not infinitely narrow will show a roll-off in high-frequency response off-axis, because the same sound will arrive at different sides of the diaphragm at slightly different times. Having said that, in terms of what is physically possible, the OM1 is extremely well specified.

pros
• Surprisingly affordable.
• Natural, detailed sound, but without seeming clinical.
• Very versatile.

cons
• Slightly less than optimum noise and sensitivity figures.

summary
If you don't have an accurate omni mic already, and if you don't have an infinite budget, this is definitely a model to consider very seriously. Buy two if you can afford to, because they make a great stereo pair.

info

£ OM1 £499.37; TC30K (for use with transformer-coupled consoles or mic preamps) £540.50; TC40K (with extended frequency response) £989.35. Prices inc VAT.

A Unity Audio, Upper Wheeler House, Colliers End, Herts SG11 1ET.

T 01920 822890.

F 01920 882892.

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