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Avenson STO2

Omnidirectional Microphones
Published March 2014
By Paul White

Avenson STO2

Need a stereo pair of high-quality omni mics for under £500$600? Look no further!

One of the unavoidable factors imposed upon us by the laws of physics is that whenever we try to measure anything, we also change it to some degree. Maybe a radar trap measuring the speed of your car doesn’t affect the car’s motion to any significant degree, but when you get down to the scale of microphones and sound waves, mics will always interfere with the sound they are trying to capture. Where you have a large-diaphragm microphone the size of a plant pot, it is bound to reflect, deflect and diffuse the sound it encounters to a significant degree, as the wavelengths of the higher-frequency components of the sound will be shorter than the dimensions of the microphone capsule and housing. It follows, then, that the less you disturb the sound waves, the more accurate the result, especially if you’re after a true omni polar pattern where the disruption created by a large mic body can have a significant effect on the off-axis response.

In theory, an infinitely small microphone is what’s needed, and while we can’t make infinitely small microphones, sometimes simply ‘small’ is good enough. That’s the thinking behind these ‘electric toothbrush’-style microphones, which have a small mic capsule at the end of a narrow probe-like support so as to keep disturbance of the sound field to a minimum. This is a good arrangement for sampling pressure changes at something close to a single point in space, which is what an ideal omni mic strives to do. Mics of this type are often used for measurement work, such as loudspeaker performance analysis or calibrating room correction software, but they also have a place in the studio. Perhaps the most noted exponent of this type of studio mic is Earthworks, but the format is an established one and is used by many different mic companies.

To The Point

Avenson’s STO2 omni microphone utilises a small-diameter, high-performance electret capsule (just a quarter of an inch across), manufactured by Panasonic. The narrow support widens out to allow for the on-board transformerless circuitry and the balanced XLR connector, and standard 48V phantom power is required for operation. The microphone body, which has an overall length of five inches and a maximum diameter of one inch, is machined from aluminium, with a smooth transition from the narrow probe to the wider body. Matched pairs ship with plastic stand clips in a padded, velvet-lined wooden case.

Designed to be essentially ‘flat’, these mics exhibit less than 2dB or so of drop-off between 20Hz and 20kHz, and no special ‘voicing’ bumps and humps. Indeed, below 12kHz there’s barely half a decibel of deviance from a straight line. There are no pads or filters, though these mics can handle SPLs of up to 145dB, so level should never be a problem.

Using such a small capsule does have a downside, however, which is that by intercepting a smaller amount of sound energy, more amplification is needed to get a usable signal, and so the noise performance is invariably worse than for a large-diaphragm microphone of similar quality. The Equivalent Input Noise (A-weighted) is quoted as 28dB, which would be disappointingly high for a large-diaphragm mic but is quite realistic for this style of microphone. In practice, the noise floor shouldn’t present a problem when close-miking typical studio sources or for hanging over loud instruments, such as drums or brass, but will become progressively more significant with quieter or more distant sources. Also worthy of mention is that, being an omni pattern design, the STO2s can be used close to the source without exhibiting any proximity bass lift, so in many cases careful mic positioning can compensate for the higher noise floor. The sensitivity of the mic is 5.25mV/Pa, which means most sound sources can be recorded without having to the turn the mic preamp gain right up.


My usual voice and instrument tests, using a stereo pair, confirmed that, indeed, these mics have a very detailed, natural sound with little variation in quality as you move off-axis, and though the background noise is higher than I’m used to, I could live with it in most close-up applications or where the sound source is reasonably loud. Acoustic guitar came over as solid and lively and there were no problems with hand percussion or small bells, so it would seem these mics introduce little distortion to create intermodulation effects. Indeed, as these mics are designed to be neutral sounding, they can handle most sound sources without difficulty as long as they are loud enough or close enough to maintain a healthy signal-to-noise ratio. The STO2s, which capture transient detail very effectively, also make good drum overheads or percussion mics. And due to their omni response, when used in situations where spill is unavoidable, you can at least be confident that the spill will be picked up cleanly so that it can contribute to the overall mix rather than being a problem. A spaced pair also produces the expected high-quality stereo result, so where you need a stereo pair of omnis and can live with around 10dB more background noise than from a typical half-inch capsule mic of a similar price, the STO2s will fit the bill very nicely.


The most obvious alternatives come from Earthworks, though many half-inch-diameter omni mics with a more conventional stick format also produce excellent results when used on acoustic instruments in most practical situations.

Published March 2014

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