When selecting an ambisonic microphone, I'll suggest a few things you
should look and listen for.
All of the older Soundfield microphones (all other than the SPS200) used
hardware to apply their calibrations. Each microphone was individually
measured and a calibration correction was built into the hardware
processor.
As the microphones aged, they fell out of calibration. To restore their
performance, you had to send the microphone and the processor back to
Soundfield for re-calibration. This was expensive, and ideally had to be
done every two or three years. When Soundfield went through multiple
changes of owndership, the knowledge to re-calibrate (and even to
initially calibrate) was lost.
With the introduction of our TetraMic in late 2006, the hardware was no
longer needed. We calibrated each and every TetraMic individually and
provided a calibration file that was applied with an A- to B-format
encoder plugin.
Soundfield later came out with their SPS200. Instead of providing
individual calibration files for each SPS200, they initially matched all
the capsules at the factory to the same standard, and then provided a
single generic calibration correction that they expected would work for
all SPS200s. It turns out that as the capsules on the SPS200s aged, a
single generic calibration file couldn't possibly work for all of them.
And they never offered a re-calibration service. Eventually, based on
what we've measured, they lost the ability to even match the capsules
correctly initially at the factory - eachone is very different than the
next.
The result of using a single generic calibration correction file is that
they can't correct for a lot of things. One of them is divergence at low
frequencies. So what they did was to cut off the SPS200's low frequency
response at around 90 Hz. The Sennheiser Ambeo takes the same approach,
and it also has a 90 Hz low frequency cutoff. The new Rode mic (as of
yet unreleased) seems to follow the same approach.
And of course, as the capsules age, even if they were well-matched from
the factory (which SPS200s are not now, but Ambeos seem to be), after
two or three years, they will not be.
Neither Soundfield (now owned by Rode) nor Sennheiser offers
re-calibration services.
SPS200 and Ambeo are first-order tetrahedral array microphones.
TetraMic is Core Sound's first-order microphone. It is probably the
best-selling first-order microphone in the world. Each one is
individually calibrated. Re-calibration is recommended every two to
three years. Low frequency response goes down to below 30 Hz. (We've
calibrated them on special order down to 10 Hz.) It's calibration is
tuned to sound like a DPA 4003.
First-order tetrahedral arrays are good for some things, but they're
weak at others. If properly calibrated, its pickup patterns are better
than pretty much any mono mic. But its listening sweet spot is only
about the size of a human head. Outside of that you'll start to hear
some loss of localization cues. Its localization cues are not
particularly strong, which is why the VR industry tends to use them for
ambience, but supplements them with spot mics for stronger localization
cues.
Second-order ambisonic mics have a much, much larger sweet spot, and
much, much stronger localization cues. They can synthesize second-order
pickup patterns that have much more directional selectivity, rejecting
much more sound from unwanted directions. That let's you get at least
twice as far from the sound source as a mono mic without losing
directional selectivity. (Mono mics are limited to first-order pickup
patterns; they can't do second-order patterns.)
Our OctoMic is the only second-order mic being offered commercially.
It's priced only a little higher than an Ambeo. We suggest that you have
a look and a listen.
Len Moskowitz (mosko...@core-sound.com)
Core Sound LLC
www.core-sound.com
Home of OctoMic and TetraMic
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