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.

 

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.

 

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

 

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.

 

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.

 

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

 

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.

 

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.

 

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 )

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:

 

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.

 

To add another data point: I've been experimenting with various real time analyzers running on my laptop. While measuring pink noise with my ECM8000 I can clearly see that the 10kHz+ part of the spectrum shifts up by at least 5dB when I point the mic at the speakers, and goes back down when I orient it vertically. The rest of the spectrum doesn't change when I re-orient the mic, and nothing changes much if I move it around without changing the orientation.

 

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

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 :bigsmile:

 

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.

brucek

 

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.

 

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.

 

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?

 

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 :devil:

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.

 

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
Rich

 

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.

 

this circuit is in the mic itself? not a separate mic amp?

Yep....

It gets "phantom power" from an external device?

Yep...

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

Yep...

brucek

 

OK... so how much more info are you looking for? Would I be telling you things you already know if I said the SA transistors are forming a differential amplifier that provides the balanced output, and some gain, the BC transistor is an input stage for that amp, to provide impedance matching between the mic element and the diff stage (matched impedances make for the most efficient energy transfer) and also to provide some gain...
The resistors all set the bias for the transistors, but at the same time define the input/output impedances and gain of their particular stages? The 1uF caps allow the signal to pass from stage to stage while keeping the biases separate...

 

That's good information. I always like learning more about circuits.

What I was really fishing for, was whether or not there is any sort of filter designed into this to compensate for anomalies in the frequency response. For instance, a notch filter at higher frequencies. It does not look like that is the case.

 

Well, C6,7,9,10 certainly limit the high frequency response, but I wouldn't call it a notch filter, looks more like a lowpass... where's the cutoff, you ask? I dunno... have to review some textbooks to calculate that... frankly, easier to measure it... or even better, look at the cal file...

 

That's the whole problem I was addressing, the cal files that are floating around the internet don't work for my mic in the top two octaves.

Also, even on the cal files, it is hard to differentiate between "baffle" (the enclosure and front face of the mic), capsule, and electronics effects in the mic.

At this point, this is more of a learning exercise. I am content to make my own cal files and revise them as I make more measurements. As of now, the top two octaves are based on a semi-nearfield (1') measurement of my Magnepan MG10.1 ribbon tweeter. Worst case, I have a cal file that matches its performance (and if its performance is flat, I have a good cal file )

 

I always like learning more about circuits.

What I was really fishing for, was whether or not there is any sort of filter designed into this to compensate for anomalies in the frequency response

No, there's no audio filter here. The small RC ladders on the outputs are way outside the audio range (those are pico-farad capacitors for HF noise).

The circuit is kinda interesting. It's purpose in the first single-ended stage (BC118) is mainly to provide high impedance buffering for the mic element. The second stage creates the out of phase balanced signal with a matched output impedance to reduce noise for long mic cables.

You can see the mic element and first buffer stage are biased from the simple zener circuit, while the output stage is turned on directly with the phantom voltage.

There's really no gain to speak of (it's all buffering and impedance matching). The first stage is a typical common emitter connection using feedback with no bypassing. The non-bypassed feedback provides for very high stability even if the temperature or beta of the transistor changes. There's a huge voltage/power gain loss in this configuration, but that's fine, since stability is more important here. They've used a high value for input impedance to the first stage so the element isn't loaded. My napkin calculation puts the gain of the first stage at less than 1.

The second stage is interesting. The single ended output transistor feeds an emitter follower configuration to drive the positive XLR 2 output that has been biased on by the phantom voltage. They take advantage of that stages collector lead signal (that is 180 degrees out of phase) to pass through the bottom transistor (that is also biased on by the phantom voltage), and then out the XLR 3 lead. This configuration allows them to use a matched set of pnp transistors to produce a balanced (differential signal). The matched components ensures a consistent output impedance for XLR 2 and XLR 3 to offer pretty good noise reduction.

This circuit makes the total mic itself light years better than simply using a stand alone element.

brucek

 

ECM8000 distortion

Hey guys,

I don't get over here often enough. This thread and the ECM schematic got linked over at AVS and I was a bit concerned with the schematic. Siegfried Linkwitz has done a lot of experimenting with the Panasonic capsules and has some DIY designs on his web page.

http://www.linkwitzlab.com/sys_test.htm#Mic

In part, he says:

When the cartridge is connected as described by Panasonic it produces fairly high distortion at moderate SPL's and is marginally suitable for serious recording and measurement purposes. The microphone itself is extremely linear, but the built in FET amplifier stage is not configured optimally.

Fortunately, is it possible to modify the external connection to the FET. This involves some delicate work of cutting a trace on the tiny pcb in back of the cartridge and soldering thin, flexible wires to the standard two hookup points, and making a different connection to the capsule housing.

I noticed that the ECM has the capsule wired the conventional way. Obviously they aren't going to do surgery on every capsule. I wondered if their circuit might overcome SL's objections so I emailed him a copy of the schematic and asked. I received a prompt reply (the guy is a real gentleman) and here's what he says.

Hi Dennis,

This mic will definitely suffer from distortion.

Too bad,

SL

FWIW.......

 

The ECM8000 is not alone in this phenomenon. In recording orchestras for film sessions we use the omnidirectional Neumann M150 (or if you can find one the original M50). One of it's characteristics is that the high frequency response is very non omnidirectional past 2kHz. Besides it's amazing sound quality this is one of the reasons it's used so much for the Decca tree recording technique. It is not a cheap microphone costing around $6,000 - $7,000. In the Decca configuration there are always 3 of them (That's around $20k for 3 mics). The M150 was never meant to be a measurement microphone as the frequency response chart shows, but it is also not truly omnidirectional.

If the ECM8000 exhibits this type of polar pattern as well then wouldn't it be correct to say the the horizontal orientation would be the only position that would be correct to measure HF response?

I guess the correct question to ask is what is the orientation for the flattest frequency response?
My guess would be on-axis to the intended target. Not really what you want for a measurement microphone but at this cost, beggars can't be choosers!!

Vince

 

I finally got around to checking out my ECM8000...the results are consistent with the data shown by Anthony in his first post. My mic has almost 5 db variation between horizontal and vertical and even in the best orientation still has several dB of peaking. The test results were entirely repeatable. Sadly, this mic is not suitable for full range speaker testing.

What is the least expensive *calibrated* mic available? Hopefully some resourceful person has found one that mere mortals can afford!

 

I realize this thread is old, but I think many people may arrive here looking for info about the ecm8000. I can help with the explanation of the schematic posted and something more...
In order of the signal path:
- C1 (3n3) is a low pass filter, 6dB/octave, cutoff or -3dB point at about 16KHz. Changing it for 1n or less improves the high end response without any undesired side effects. If you're going to change this, use a ceramic 330p to 1n capacitor.
- The intended function of the first transistor, BC118 or BC331 (Behringer has used both), is to make the unbalanced signal a balanced one. At the emitter and collector we have the same signal as in the base (input), but at lower impedance and one inverted with respect to the other. The gain is 1 (0dB). But there is a flaw here: to work as intended C6 should be connected to the emitter of this transistor instead of to ground. I have not opened any of my ecm8000s to see if actual circuit has this mistake or is only the schematic that has the mistake, but no doubt it's a mistake. The effect of this flaw is a 6dB loss of level and SNR when connected to balanced preamps. As is in the schematic, this transistor would do nothing except adding some noise.
- The last two transistors are emiter followers for each of the two balanced signals. Their only but very convenient function is to lower the output impedance. As emitter followers, the gain is obviously 1 (0 dB). C6, C7, C9 and C10 make low pass filtering, the cutoff frequency is 200KHz, well above the audio band. R12 and R13 improve the slope of the low pass filter and provide provide stability and protection against highly capacitive or inductive loads at the expense of raising the output impedance a bit. Still the output impedance remains at an excellent low value. So everything right at this stage.
- So the total gain of the circuit is 1 (0 dB), so it does not have nor need any feedback loop. The design is very good except the flaw described above, that most probably is only in the schematic and not in the actual circuit (I may confirm this if I open my mic sometime).
--------------
Some people modify the capsule to use its FET as a drain follower instead of a common drain amplifier to raise the maximum SPL (sound pressure level) handling. This is completely unnecesary for any purpose other than close drum micing or measurements at levels above 115dB SPL (permanent ear damaging levels). If you intend to make this modification, note that you must also add an ultra low noise amplification stage immediately at the capsule output. When using the capsule's FET as a drain follower, we loose all the amplification we had as common drain amplifier, and because the very low level provided by the electret mic, we need to correctly amplify it very close to the capsule to avoid a severe degradation of SNR. Regardless of author's claims, every amplification circuitry I have seen designed by people that does such modification is simply lame and will convert your good balanced mic into a lot noisier, much higher output impedance and unbalanced ouput one, incompatible with pro mic preamplifiers, only with a bit higher SPL handling and that only in some designs. If you really need to improve the maximum SPL handling of this mic and modify the capsule as drain follower, keep using the original balancing buffer, that's pretty good, and simply add a common emiter amplifier with some feedback between the capsule and the balancing buffer. Use a bipolar transitor, not a FET: after the capsule's FET, we have a low enough impedance so we want here the smallest equivalent input voltage better than the smallest equivalent input current, so a bipolar is better here). Some modification is also needed to the power suply scheme: the added amplifier stage and the BC118 should be powered by more voltage than 6V, but can't be powered from 48 since most small signal transistors can't handle that, ideal would be around 24v given that most small signal transistors specify a maxVce of 30v. A simple zener and a capacitor would do it well. If done this way, the result would be a very similar balanced low impedance mic, with 12dB higher maximum SPL handling and most probably (depends on the added stage) only a bit noisier than the stock ecm8000. Even further you may add a switch to bypass the added amplifier stage for really high SPL handling above 130dB, supposing that the electret itself could handle that. Note that you increase the minimum phantom power requirement to 24v, and that the mic pre you connect it to must be able to handle very high levels. The maximum output of the mic this way would be near +20dBu, while even the highest end pro Yamaha preamps clip at -14dBu without padding or -4 with padding, but many other manufacturers allow near 0dBu levels at mic inputs without padding, and some even handle more than +10dBu. Anyway, all this is only is to use this mic for purposes very different for which it has been designed.
-------------
About the frequency response of this mic, what I have found is that there have been very different production series. My mics are all from the very first series, and sure don't match any of the responses I have seen reported by other owners in the net. I have not sent them to a lab for accurate calibration but I have a high end B&K 4007 measurement mic and by comparison I'm sure my ecm8000s don't show shuch an exagerated peak near 10KHz, but something between +1.5..+2.5dB. So what I got was cheap and reasonably good measurement mics, but you can not trust the high frequency part of the actual piece you get without calibration or at least comparison with another mic you can trust. And if you have to calibrate, you can use actually any small condenser or electret. I purchaed the ECM8000 to avoid moving my B&K (I drop it once and repair cost was $1200), and I was satisfied with its performance, but because I'm using a portable computer for measurement, now I find much more convenient one I made based in a subminiature $1 capsule mounted in a thermoretractile tube of only 5mm diameter. When I calibrated against the B&K, I found this $1 capsule was excellent, even better than my ECM8000s, and nicely fits in a small pocket of the portable's suitcase.