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Author Topic: miniDSP AmbiMIK-1 Ambisonic Mic  (Read 1745 times)

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Offline Len Moskowitz (Core Sound)

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Re: miniDSP AmbiMIK-1 Ambisonic Mic
« Reply #15 on: July 14, 2020, 11:15:40 AM »
"Generically calibrated" is the opposite of "individually calibrated."

Ambisonic microphones work on principles that depend on the very small differences between their capsules, plus a bunch of other factors. In this case and for the Zoom H3-VR, just because a microphone has four capsules mounted on a tetrahedral mount, that doesn't mean that it will work well.

It turns out that every microphone capsule is different. They might all meet the manufacturer's specifications, but those specifications have ranges of variation, and those variations are incredibly important for ambisonic microphones.

There are also other sources for variations in addition to the capsules.

Calibration is the process that measures the variations and provides corrections for them.

Because it takes time and equipment and manpower to test each of the capsules and then match them into sets, and because it costs money to test each assembled ambisonic microphone to develop correction files for each complete microphone, the manufacturers compromise. They make the assumption that all of their capsules are sufficiently matched, and all of their assembled microphones are sufficiently identical to use a single generic calibration correction.

From the beginning the assumptions are wrong. And as the microphones age, the assumptions get even worse. You're still stuck with the original generic calibration.

That's why each ambisonic microphone needs to be individually calibrated, and supplied with its own correction file. And as it ages, it will need to be re-calibrated.

Ideally, each set of capsules should be matched for a bunch of specifications - not just frequency response and/or sensitivity. And then when they're assembled into the microphone mount, each microphone should be measured so that its performance can be corrected with an individual calibration file.

All Core Sound OctoMics and TetraMics are individually calibrated. We recommend re-calibration every two to three years, depending on application. We offer a recalibration service for our microphones.

(We also offer an initial calibration service for other manufacturers' ambisonic microphones, including those from Rode, Sennheiser and the old Soundfield.)



« Last Edit: July 14, 2020, 12:46:03 PM by Len Moskowitz (Core Sound) »
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Offline MIQ

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Re: miniDSP AmbiMIK-1 Ambisonic Mic
« Reply #16 on: July 14, 2020, 12:34:58 PM »
Hi Len,

I appreciate you taking the time to explain that.  When they state that they worked to create a "VST plugin specifically calibrated for the ambiMIK-1 with over 1200 measurements", they are talking about taking measurements of the mics in development and applying those measurements/calibrations to all the mics they produce, regardless of the slight differences in the individual capsules actually being assembled. 

I've read a fair bit about mic calibration, even the old Soundfield design (I'm a big fan of Gerzon), and can see how the costs could quickly get out of hand for a budget minded product.  As usual, the more accurate you get, the more time and money it takes. 

Offline Len Moskowitz (Core Sound)

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Re: miniDSP AmbiMIK-1 Ambisonic Mic
« Reply #17 on: July 14, 2020, 12:39:57 PM »
When they state that they worked to create a "VST plugin specifically calibrated for the ambiMIK-1 with over 1200 measurements", they are talking about taking measurements of the mics in development and applying those measurements/calibrations to all the mics they produce, regardless of the slight differences in the individual capsules actually being assembled. 

Yes. And the variations in mounting the capsules in the capsule mount. And the effects of the cables behind the capsules on the venting of the capsules. And others.

Some of the "slight differences in the individual capsules" are not slight. Some are quite large and significant.
« Last Edit: July 14, 2020, 12:45:18 PM by Len Moskowitz (Core Sound) »
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Re: miniDSP AmbiMIK-1 Ambisonic Mic
« Reply #18 on: July 14, 2020, 12:50:56 PM »
Len, how do you know for certain whether or to what extent other manufacturers calibrate the capsules for their ambisonic mics?  Or what those manufactures' internal specs are for the capsules they use in their ambisonic mics?
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Offline Len Moskowitz (Core Sound)

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Re: miniDSP AmbiMIK-1 Ambisonic Mic
« Reply #19 on: July 14, 2020, 01:05:18 PM »
Len, how do you know for certain whether or to what extent other manufacturers calibrate the capsules for their ambisonic mics?  Or what those manufactures' internal specs are for the capsules they use in their ambisonic mics?

We buy their mics and test them. As they do for ours.

We know that all the manufacturers match their capsules. Some do a better job than others. Some clearly don't test other important specifications. None of them calibrate completely assembled microphones, except perhaps one, and we think that they don't do a very good job. Almost all use a generic calibration.

In one case a generic calibration is warranted because they use MEMS elements as capsules; they are reliably long-term stable. They're noisy though.

The rest of the manufacturers use standard electret capsules. No matter how tightly matched they are during manufacture, two years (or less) downstream they've drifted.

One microphone manufacturer provides individual calibrations that correct for sensitivity variations only. They buy their capsules (they don't manufacture them) and presumably decided that correcting errors is less expensive than sorting for tightly matched capsules. They don't provide individual calibrations for the assembled microphones.

If you really want to know how good an ambisonic microphone is, look at its decoded polar directivity patterns - for example on a tetrahedral first-order microphone, look at the B-format's omni and the figure-8 channels . If they don't publish them, ask them why.

If they publish a frequency response specification, does it provide tolerances? If not, it's not real - it's a marketing specification. (Individual calibration allows for roughly two more octaves of bass.)

« Last Edit: July 14, 2020, 07:38:47 PM by Len Moskowitz (Core Sound) »
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