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-   -   ECM8000 microphone measuring techniques and usage discussion (https://www.hometheatershack.com/forums/spl-meters-mic-s-calibration-sound-cards/10224-ecm8000-microphone-measuring-techniques-usage-discussion.html)

Anthony 03-24-08 06:52 PM

ECM8000 microphone measuring techniques and usage discussion
 

2 Attachment(s)
Okay, so BoomieMCT and I did a battery of tests on our ECM8000 mics.

The setup was my M-Audio MobilePre preamp, test amp, Magnepan MG10.1. Mic stand was set 3 feet away, 2' off the ground aligned with the ribbon perpendicular to the panel.

Nothing on the speaker changed during all the tests, the mic stand was kept horizontal for all the horizontal tests and vertical for those tests (all done at once to minimize any changes).

Measurements were full sweeps in roomEQ wizard, 4 repeat average. 1/3 Octave smoothing.

We'll start with the comparing his and my mics in horizontal and vertical. The Top trace is comparing his and mine horizontal. The Bottom traces are the vertical ones.

In the second graphic, the top traces are my mic horizontal versus vertical and the bottom ones are his mic horizontal versus vertical. Horizontal is purple, green is vertical for mine.

I'll post more later, but you can see the mics agree very well across mics and horizontal versus vertical up to about 1kHz. Above that all bets are off, although some patterns emerge. More info to follow.

Anthony 03-24-08 07:48 PM

re: ECM8000 microphone measuring techniques and usage discussion
 

So it looks like the vertical response is smoother than the horizontal (not surprising), and there is significant variation between the two mics in the upper octaves.

So the first conclusion I would draw is that for bass response only, these mics are probably okay. The two tested only started varying at 800 Hz, so not bad for subwoofer testing.

Another thing that struck me is the top octave roll-off that is in every measurement. I don't see this in any of the correction files for this mic, so I'm wondering if this is more of a soundcard issue.

It shows up even when using the left channel as calibration and when I use the amp output as calibration (through a voltage divider), so the only explanation was that the phantom power circuitry and preamp in the MobilePre might be adding that. Boomie and I have plans to text using his mixer to see if that still shows up.

But it's clear that this mic needs independent calibration for tweeter testing. There's just too much variance and uncertainty.

So first I'm going to confirm that my sound card is not introducing too much error in the FR (if it is I have bigger problems :sad: ), after that I'll have to see what my options are.

A $120 calibration fee seems reasonable, but lately I've been hearing about better mics that are cheaper and slightly more expensive mics that are ruler flat. So I'm not sure what I'll do, but it's always nice to have more information. :)

Anthony 03-24-08 07:54 PM

re: ECM8000 microphone measuring techniques and usage discussion
 

1 Attachment(s)
Here's the same horizontal versus vertical with the amp calibration in the loop.

Not much change, although there is a slight correction for the amp that seems to roll off about 1dB at 20k (but that does not explain the rest of the rolloff completely).

Horizontal is Purple and vertical is Orange.

For those of you who are so inclined, I like the voltage divider setup. It allows the calibration to occur after the amplifier so your reference measurement includes amp response. This way, you've eliminated all but the mic, speaker, and room (sadly those are the biggest contributors, though). Do NOT use the amp output as calibration unless you know what you're doing, though. You can easily fry your soundcard. It took me a lot of research to get the input impedance of the card, and I measured the output of the amp at different setting and sized the jig/divider accordingly.

brucek 03-25-08 08:55 AM

re: ECM8000 microphone measuring techniques and usage discussion
 

Quote:

So it looks like the vertical response is smoother than the horizontal (not surprising), and there is significant variation between the two mics in the upper octaves.

So the first conclusion I would draw is that for bass response only, these mics are probably okay. The two tested only started varying at 800 Hz, so not bad for subwoofer testing.
hehe, we're gonna have to agree to disagree on this issue. :)

Actually, I feel the mics show very good consistency between units (as we've always claimed).

It's very difficult to draw too many conclusions when comparing microphones this way. Small microphone element positional differences can account for very large changes in the measurements, particularily as the wavelength gets shorter and in areas where the response is changing rapidly. You could obtain the same differences using exactly the same mic under those test conditions. Better conditions would be to measure a near flat response in a very damped room ensuring the mic elements were perfectly at the same spot. Hard to do.

Even under the conditions used here, the different mics and different orientations showed remarkable closeness (although I'm not too fussed about the way you changed scales on every graph). Worst case under the worst condtions at high frequencies, the two mics still only show up to 4dB difference. You could get this by moving the same mic a fraction of an inch.....

My conclusion would be that these are quite good mics for the price and full range measures can be taken with some confidence in the home environment.

brucek

Anthony 03-25-08 09:42 AM

re: ECM8000 microphone measuring techniques and usage discussion
 

Hey Bruce, I think the only real disagreement comes in the top octave.

First off, the scale changed because I was constantly moving curves around in order to separate them and group them. In fact I couldn't even find the original scale on some of the measurements.

Overall, you are right, these results are close for a majority of the measurements.

In my case, I'm most interested in the tweeter section, though, and that's where things start to become different. when you are trying to set tweeter levels or design a notch filter, the response at that end is very important. 4dB is a lot in crossover design! Most of the effects, though can be explained. For instance, the waviness in the horizontal measurements coincide with a baffle effects for a very small baffle (the diameter of the mic). Keeping the mic vertical avoids this.

As to methodology, I can say with certainty that the center of the mic capsule was within a mic capsule diameter for the measurements. I was very careful to measure distance off the floor and use an index mark to set mic location (verified via crude plumb bob). Levels were never adjusted past the initial setup.

So I'm going to recommend people use this mic vertical whenever possible for full range stuff. It seems to be how it was designed (based on other research, not just the results posted here)

The good news is that for bass, the mic-to-mic variation is very small and seem to agree with the calibration files (not pictured here). Good news indeed.

So to summarize:
For REW -- great mic, cheap, available cal files good enough, use whatever orientation you want

For two-way crossover design -- good mic, available cal files good enough up to 3kHz or so. Orientation still probably does not matter (within +/- 1dB).

For three-way, or full range work -- okay mic. Good value, but the response gets peaky. Definitely get calibration done if you want accurate results. Pay attention to orientation of your cal file and beware the "baffle" effects in the 4k to 8kHz region. Use vertical where feasible (the peak is not as great and the response is a tad smoother).

Overall a good value, unless you want to bother building your own. Probably still the best choice for REW/BFD work.

BoomieMCT 03-25-08 12:40 PM

re: ECM8000 microphone measuring techniques and usage discussion
 

Quote:

brucek wrote: (Post 89413)
hehe, we're gonna have to agree to disagree on this issue. :)

. . .

You could obtain the same differences using exactly the same mic under those test conditions. Better conditions would be to measure a near flat response in a very damped room ensuring the mic elements were perfectly at the same spot. Hard to do.

I don't think Anthony is trying to say that what the plots show are the anechoic responses of the mikes. The things he's trying to point out is a) the difference in the ECM-8000's response in the vertical vs. horizontal mounting and b) differences between different units.

Given the graphs shown I'd say there is compelling evidence to show that there is a consistant difference between both mounting orientations and a consistant and repeatable difference between the two mikes in the top octave.

I don't think this was a slam on the ECM-8000 but rather some thoughts on better understanding how to use the mike as well as its limitations.

brucek 03-25-08 01:21 PM

re: ECM8000 microphone measuring techniques and usage discussion
 

Quote:

I don't think Anthony is trying to say that what the plots show are the anechoic responses of the mikes. The things he's trying to point out is a) the difference in the ECM-8000's response in the vertical vs. horizontal mounting and b) differences between different units.
Yes, of course I realize that.

I just don't happen to agree with the conclusions (and gave my supporting arguments why I feel that way).

brucek

<^..^>Smokey Joe 03-26-08 07:10 PM

re: ECM8000 microphone measuring techniques and usage discussion
 

I have to agree with Bruce, dispite your best efforts to align the mics to the same spot a very small difference could result in different reports. Try placing the same mic in and out several times with repeated runs(atleast 4 times) then take the average. Do the same for each mic.
This may help average out positional differences.

I used to work in the sound recording business a few years ago, so spent alot of time with mics. Moving a mic a fraction could make all the difference between good and bad no matter how good or bad a mic is.

Just imagion the room with lots of square boxes(interference patterns), each frequency representing a size, those that fit perfectly to the room shape will do different things to the ones that don't. If you are placing that mic right on the edge of those interference patterns you will get varying results that can be quite dramatic. ie a frequency can cancel completely with a pure tone creating areas with no sound and tone in zones that are shaped like a box.

Scuse the pun but you could be just looking at noise.

Anthony 03-26-08 09:23 PM

re: ECM8000 microphone measuring techniques and usage discussion
 

3 Attachment(s)
I was one step ahead of you. I just did tests where I moved the mic a random bit. I also measured much closer to the speaker to eliminate as much of the room effects as possible.

The mic tip was around 16" from the speaker. Maximum move was about 3/4" in or out, side to side.

You can see some things that jump out as positional (green trace in both graphics), but some general things (humps, dips, rolloffs) are evident.

First one is horizontal, second is vertical, last is horizontal versus vertical (I picked one of each that was in the middle).

So, I stand by my assertion that there is a difference between horizontal versus vertical (it may just be the reflection/baffle effect of the mounting body of the mic). It might not mean anything to an RTA measurement or general wide-band EQ -- but it is significant enough for tweeter crossover/notch filter design (which is why I went down this road).

However, this mic is pretty flat up until the "unpleasantness" in the top two octaves. Good news if someone is just getting it to EQ a sub or see the effects of some room panels.

But for any serious tweeter work, calibration is definitely necessary.

Anthony 03-26-08 09:48 PM

re: ECM8000 microphone measuring techniques and usage discussion
 

I also made a cal file using some of these measurements. It might not calibrate to flat -- but it will calibrate to my existing Magnepans (not exactly a bad speaker to design to :D )

That'll work for now. I'll probably send the mic off to Caldwell next month. I'm interested to see if my "fake" calibration file is anywhere close :)

Thanks for the input. Disagreements or no, I just want to understand and get the bottom of this -- no matter how complicated. I guess that's part of the fun :nerd:


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