It's love at first sound when our reviewer tries out this cost-effective range of microphones.

Microphones are a way of life to any musician irrespective of whether or not he is a devout MIDI acolyte. A decent vocal mic is an obvious necessity and the ability to record acoustic instruments as well as mike up amplifiers is usually required within the context of any studio. Also, the type of microphone in relation to the application is important. The ability to pick up signal from all directions - omnidirectional - is of limited use in a general sense due to feedback problems and the pick-up of sound from unwanted areas, although in the recording of room ambience or multi-instrument setups, the use of PZM types can give excellent results. This leads to the unidirectional, or cardioid type which is far more sensitive to signal on the axis of the mic but less so to sounds at the side or rear leading to greater gain availability before feedback occurs.

Hypercardioid and supercardioid are of a similar nature but with even tighter pickup patterns. Directional mics have what is termed a 'proximity' effect, which is a bass boost when used at very close distances, and some also have a presence peak somewhere between 3 and 8kHz causing the mic to sound particularly clear in the upper mid to low top region.

The factor which has the greatest effect on the overall frequency response of a microphone is whether it is a dynamic, moving coil type or a condenser, the latter of which is powered either by a separate power supply or through phantom powering on the mixing desk. Their superior top end response is invariably related to the cost and condensers are, by virtue of their much lower moving mass, superior in this area but generally expensive.

The manner in which a dynamic mic works is probably known to many readers already - a thin, circular disk called a diaphragm is connected to a voice coil which is a cylindrical former with thin wire wound onto it. This assembly vibrates in response to sound and generates a proportional electrical current. The extra mass (and consequently inertia) caused by the copper coil restricts the frequency response at the top end and most dynamics tend to roll off quite steeply above 15 or 16kHz.

There are two obvious ways of tackling this problem; either use a very powerful magnet, so reducing the distance that the coil needs to travel (as used by Electro-Voice in their N/Dym range), or else make the combined diaphragm/voice coil assembly so light that minimal energy is expended in moving it. This latter idea is the theory behind the Audix VLM (very low mass) selection of mics which consists of four types - OM-1, OM-2, UD-300/360 and UD-50/50D with RRPs of between £55.61 and £140.37.

Preliminary Tests

OM-1

Picking this up leaves one in no doubt as to its construction - heavy! (370g.) It comes in two versions, both low (200Ω) impedance, and differing only by the interchangeable grill cap. The vocal version sports a ball-shaped grill a la Shure SM58 and a close look shows up the quality of the product - woven steel mesh with an integral pop shield. All other parts are brass and the mesh appears to be soldered to the central circular ring.

The instrument version is called the 'Probe' and the grill looks like an enlarged version of a cardioid capsule on a condenser mic and is intended for use in situations where the sound pressure level (SPL) is going to be high. The polar pattern is that of a tight hypercardioid which should mean that off-axis sounds should be rejected with a high degree of success and the manufacturers claim over 30dB of reduction in these signals as well as an audio frequency response of 50Hz to 18kHz - although without giving a range this is of limited use.

Testing vocals is always a good guide to the quality of a microphone due to the complex waveform of the human voice.

My first reaction was, 'Is this really a dynamic mic?' because the top end was as clear as I have come to expect from my condensers, with a gentle roll off under 200Hz and a continuous rise from 2kHz up to 8kHz giving excellent clarity to a singing voice. Close range testing showed a proximity lift starting at around 12cm although not as pronounced as I would have expected leaving the vocal sounding clinical in detail but a little lacking in warmth.

A snare drum miked up with the OM-1 Probe gave similar results to the above - clean, extremely crisp and showing up the snare wires nicely. When recording instruments such as saxophone and the like, the positioning of microphones is of paramount importance with the fundamental sound coming from the bell and the breathiness being taken from near the mouthpiece and along the body. To accurately extract the latter requires a mic with a smooth extended top end which usually points to a condenser, but when trying the OM-1 Probe, I achieved results comparable with low to mid quality condensers costing double the price. OK, it's not in the range of a Neumann or a Bruel and Kjaer but then again neither is the price and it is certainly amongst the very top dynamic microphones in this respect.

OM-2

Apart from being a different shape and not having the facility of interchangeable grill caps, the OM-2 is practically identical to the OM-1 with the same VLM capsule. It again utilises a hypercardioid polar pattern and has the same quoted impedance, frequency and off-axis responses. However, unless my ears deceive me, this is not quite the same animal. Vocal breathiness seemed a little more pronounced and the overall feel was that of a 'warmer' sound leading to the conclusion that the top end is, perhaps, slightly more extended but that below 1kHz the frequency response has a marginally different taper to the OM-1 which may well be due to the capsule mounting or the different grill design of the latter. Whatever the cause of the difference is, I firmly believe that the basic vocal sound of the OM-2 is superior to that of its slightly more expensive brother and sounds far more like an expensive condenser mic in terms of clarity, top end and warmth.

"...when trying the OM-1 Probe, I achieved results comparable with low to mid quality condensers costing double the price."

Instrument miking was a different kettle of fish altogether with the OM-1 Probe scoring points in this area. It is quite amazing what difference a grill can make and the lack of choice on the OM-2 limits it with regards to the top end clarity although we are talking in degrees here, because the sound is still very good - just not quite on a par with the previous mic. Still, sticking either of these into a bass drum where SPLs can exceed 135dB and getting no audible distortion is no mean feat especially as the 'click' of the beater was nicely emphasised and the 'kick' of the drum was distinctly evident.

UD-300/360

These two models are identical except that the 360 incorporates a low noise on/off switch and when compared with the OM-1/2 they have a die-cast zinc alloy rather than a brass body, and stuck-on labels instead of an engraved Audix on the shell. Same impedance and manufacturer's frequency response but the polar pattern is now cardioid giving an off-axis gain reduction of about 20dB. This lessening of tightness off-axis shows up in that feedback is a little easier to induce at a lower gain level than before and unwanted noise is a little more apparent.

One characteristic which is in keeping with the previous two mics is the ability of the internal capsule mounting to absorb vibration especially when hand-held or when mounted on a stand. The top end is still nicely extended, especially in a mic of this price, but not quite as clear as before and without the sparkling presence. Still, the high frequency response makes this a good choice for use with four-track systems which are often lacking in the kind of EQ facilities necessary to bring out the best in a poor mic and the overall smoothness is to be admired.

UD-50/50D

The UD-50 is the low impedance (500Ω) version while the 50D has dual impedance (high is 50kΩ). Construction is very similar to the 300/360 but the capsule is different resulting in a quoted frequency response of 80Hz to 15kHz. Both mics have low noise on/off switches. From a performance point of view, these cannot be mentioned in the same breath as the rest of the family but bearing in mind their price, they do deliver good value for money. A mic in their class will all too often only reach to about 13kHz and while the UD-50 could be used in a PA application, it would certainly be equally at home if used in a bedroom or small semi-pro cassette multi-tracker setup.

Comparisons

It is always difficult to objectively compare items of a similar nature and microphones are no exception. Scopes and spectrum analysers will only show the clinical details and will miss out on their performance capabilities when driven by instruments creating complex waveforms, including the human voice. Having begged, stolen and borrowed a range of mics, I have made subjective comparisons. It is worth mentioning that Audix also sent their microphone 'Turtle' along which allows up to eight mics to be connected on a rotating assembly, so permitting easy selection of the mic on test.

For vocal mics, priced between £125 and £150, the OM-1/2 showed a superb top end response for vocals and was as usable in a studio as on stage. Definition was accurate although the OM-1 lacked warmth and both mics showed a distinct willingness to suppress off-axis sound. Compared with these the Shure SM58, Beyer M400 and Audio Technica ATM41a all sounded rather less transparent with a distinct lack of top leading to poorer definition. The only mic which compared favourably was the Electro-Voice ND257 which certainly matched the top end but which I felt to be a little shy smoothness-wise. For instrument miking, in the price range £120 to £150, the OM-1 Probe delivered a very crisp top end and certainly accentuated any instrument which required the 'breath' to be brought out. It is also very difficult not to be impressed with the lack of spill when close miking. The Shure SM57 again didn't equal the top end response and necessary clarity as well as being susceptible to side noise while the ND 257 had plenty of top but sounded rather thinner when tested with flute and saxophone. The Beyer M700, while not having the same degree of bite in the high frequencies certainly came across as an extremely smooth mic in comparison, but if I had to mic up a tight brass section, for instance, where the separation was important, it would be difficult to better the the OM-1 Probe without resorting to a quality condenser type.

Looking at general purpose mics, priced between £85 and £110, the UD 300/360 has very few real opponents in this very competitive price range. It has an excellent top end again, good sound definition and a smooth response. The Shure SM48 had a little more body but nowhere near the same top, a comment that would also hold for the Beyer M300. Audio Technica's ATM 11, a cheap condenser mic, certainly had the clean top but had to be used at less than 5cm to get any amount of warmth with a high tonal disparity if moved away from the source. Finally, the AKG D310 also sounded good high up but seemed to leave something to be desired in the upper mid range which affected vocal clarity.

Looking at budget mics priced between £50 and £70, the UD-50 had a smoothness and high end response which belied its price. The evergreen Shure 588SD and Audio Technica Pro 3 were both less bright in comparison.

Conclusions

I am well aware of the fact that this review has practically read like a press release for Audix but I can honestly say that it is a long time since I can remember being as impressed as this about a product that I have reviewed. The VLM design obviously works well, leading to a smooth extended top end similar to the EV N/DYM's but without the lack of warmth that I usually associate with that range. Add to this the excellent off-axis rejection of signal and the overall quality of manufacture and we are certainly talking about a serious range of mics which have a future. In their relative classes, these microphones either more than hold their own or are head and shoulders above most of the rest of the field with pride of place going to the OM-2 - in fact when the time came to return the products, I insisted on purchasing it. I impress upon you to try these mics and make up your own mind - I don't think you'll be disappointed.

The Audix Dynamic Microphones cost: OM-1 £140.37, OM-2 £136.25, UD300/360 £99.25, UD-50D £64.70 and UD-50 £55.61.

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