ECM8000 microphone measuring techniques and usage discussion - Page 2 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #11 of 170 Old 03-26-08, 09:49 PM
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re: ECM8000 microphone measuring techniques and usage discussion

Yeah, position errors aside, the delta between my mike and Anthony's stayed the same every time we did different runs. Also the difference between horizontal and vertical was very repeatable.


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post #12 of 170 Old 03-27-08, 07:41 AM Thread Starter
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re: ECM8000 microphone measuring techniques and usage discussion

I have some followup data (that was actually in front of me all along, I just did not have perspective for it)

Quote:
Omnidirectional microphones are capable of very flat response over the entire audio spectrum because only the front of the diaphragm is exposed to the incident wave, eliminating the phase cancellations experienced with directional microphones. However, there is one caveat. Omnidirectional microphone response to a plane wave becomes increasingly directional as the diameter of the diaphragm becomes comparable to the wavelength of the frequency being measured. Low frequencies "flow" past the diaphragm without incident. High frequencies, however, cannot bend around the microphone structure. They pile up on the diaphragm surface causing a rise in pressure that is higher than the pressure in the surrounding field (This is one manifestation of acoustic diffraction)
-- "Testing Loudspeakers", Joe D'Appolito, page 54

he goes on to say how "free field" microphones try to counter this by controlling the resonance and damping. I guess better mics do a better job at completely eliminating this phenomenon. Entry level mics like the ECM8000 do not. Which is a shame, because older cal files that I've seen for the 8k show only a small bump in the response. The newer ones (mine included) show a big peak and rapid falloff -- apparently some are worse than others.

It probably has something to do with the capsule change (as Panasonic no longer makes what was used in the older ECM8000's).

Hmm, maybe a DIY microphone is in my future
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post #13 of 170 Old 03-27-08, 09:54 AM
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re: ECM8000 microphone measuring techniques and usage discussion

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High frequencies, however, cannot bend around the microphone structure
Exactly, and this was my original disagreement with comparing these mics. At short wavelengths, they are extremely sensitive. I like your idea of the DIY. I've come across lots of exposed capsule DIY mic tests with the old Panasonic unit - and others. That would make a great tweeter test mic. I've seen some really inexpensive (couple bucks) capsules that are sold with calibration files that would work great.

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post #14 of 170 Old 03-27-08, 10:03 AM
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re: ECM8000 microphone measuring techniques and usage discussion

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Exactly, and this was my original disagreement with comparing these mics. At short wavelengths, they are extremely sensitive.
Two reasons I don't agree. First off we are mainly noticing differences in the last two octaves (5kHz - 20kHz). This represents wavelengths of 2.7 - 0.7 inches. I'd say that assuming you have to get the mikes lined up to within a half wavelength that is still well within our test stand / plumb line capabilites. Anthony shows in post #9 that the position errors were minimal.

The other point is that our results were repeatable. We could switch back and forth between Anthony's and my mike and the difference in the readings stayed the same. Similarily the differences with the same mike between vertical and horizontal were very repeatable.


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post #15 of 170 Old 03-27-08, 10:07 AM Thread Starter
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re: ECM8000 microphone measuring techniques and usage discussion

Bruce, I came across this schematic for the ECM8000 online. The words needlessly complicated come to mind. It amazes me how those capsule mics have such simple schematics and the ECM uses this one.

I wish I knew more about op-amp circuits (I'm good with filters and other LCR stuff, but transistors and diodes are out of my league) -- so I could tell what's going on there.

It may be as simple as Behringer designed a circuit for the old capsule, didn't change it for the new one, now the new ones are "compensated" for a different response -- therefore the peak that was tamed in earlier versions is now a problem.

I'll look into it, it seems like a fun side project. Also, a guy over at DIYAudio e-mailed me about an article testing a bunch of different mics. Apparently there are some offerings from Naiant and Beyerdynamic that blow away the ECM8000 (one cheaper, one more expensive -- although both are much cheaper than the EarthWorks models).

This weekend I'm going to rerun some frequency tests of my center channel baffle using these corrections. If all goes well, I may just use my "quasi correction" and be done with it.
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post #16 of 170 Old 03-27-08, 10:18 AM Thread Starter
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re: ECM8000 microphone measuring techniques and usage discussion

Boom,
We seem to all be nit-picking very small differences and disagree on what we consider important. Like any religious argument, the smaller the difference, the bigger the controversy

Anyways, I think we've shown that for the vast majority of uses, these mics behave rather wall unit to unit and are reasonably flat up to 5kHz, regardless of orientation.

It is comforting that using the same unit in different measurement batches shows almost no variation. so you can believe what you measure on Tuesday is as accurate as what you measured on Monday.

As for orientation differences, I still claim that there is a difference in both bandwidth and amplitude of the hump and the measurements show it. Now, how important this is debatable. For room equalization, bass response, , even 2-way crossover design, it would make absolutely no difference. 4dB in the top two octaves is only marginally outside the overall system accuracy (+/- 1dB).

But for more sensitive tweeter work, it's definitely important (as my trial and error notch filter attempts have shown me) to have as accurate a measurement as you can and 4dB or a couple kHz of bandwidth are very important and make the difference between a bright speaker and a dead one.

So everyone who bought an ECM8000 can relax, unless you have to put a notch filter at 13khz, your mic is pretty good.
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post #17 of 170 Old 04-02-08, 12:01 PM
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re: ECM8000 microphone measuring techniques and usage discussion

I have 9 of the ECM 8000 and access to An Earthworks TC 30K. Next week I'll post a comparison
graph .If I recall correctly they are quite similar.I'm not that concerned about 10K to 20K as
motorcycles and gunfire in my youth took care of that.

Later
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post #18 of 170 Old 04-03-08, 08:39 AM
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re: ECM8000 microphone measuring techniques and usage discussion

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I wish I knew more about op-amp circuits (I'm good with filters and other LCR stuff, but transistors and diodes are out of my league) -- so I could tell what's going on there.
I may be able to help here, depending on exactly how in depth you want to go, and how far you can take it yourself... but this schematic is not completely correct...I have to make a few assumptions or answer a few questions....

this circuit is in the mic itself? not a separate mic amp? It gets "phantom power" from an external device? that voltage comes in on BOTH XLR 2 and 3, and is referenced (or returned) to XLR1?
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post #19 of 170 Old 04-03-08, 08:52 AM Thread Starter
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re: ECM8000 microphone measuring techniques and usage discussion

I think so. I got this off of a website that shows how to mod the ECM8000, so this is the schematic of what's in the mic (not a preamp).

Phantom voltage is applied at 48V to pins 2 and 3 relative to pin 1 (chassis/earth ground).

Someone over on DIYaudio looked at this and had some comments on the design. It serves two functions: to break the signal from the capsule into two components to send balanced over the cable, and to make sure the bias voltage stays consistent on the capsule.

I was confused because similar design mics use a bias voltage across the condenser element, but just put out an unbalanced signal back to the preamp. That is obviously a much simpler circuit.
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post #20 of 170 Old 04-03-08, 09:00 AM
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re: ECM8000 microphone measuring techniques and usage discussion

Quote:
this circuit is in the mic itself? not a separate mic amp?
Yep....

Quote:
It gets "phantom power" from an external device?
Yep...

Quote:
that voltage comes in on BOTH XLR 2 and 3, and is referenced (or returned) to XLR1?
Yep...

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