ECM8000 microphone measuring techniques and usage discussion - Page 10 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #91 of 170 Old 07-01-09, 10:54 AM
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Re: ECM8000 microphone measuring techniques and usage discussion

Mostly based on my experience, the experience of my colleagues and my understanding of the science involved. I don't really have any literature I can point you to.

As for what position I place the mics - it really depends on a whole bunch of factors. For smaller rooms, I usually just hit the listening spot. For larger rooms where the listening point might be spread out, I like using spacial averaging techniques either my measuring at multiple positions and averaging the curves together, or (for systems where it's not practical to use the multiple-position method) I slowly move the microphone around in large vertical and horizontal circular motions. The "sweet spot" result getting this method isn't as good as the 1-position method, but you get an improvement at the other positions.
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post #92 of 170 Old 07-01-09, 11:48 AM
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Re: ECM8000 microphone measuring techniques and usage discussion

Are you saying that for calibration you place the mic in different positions, or for measurement?
I am really asking about the position that you place the mic in for calibration.

Thanks,
Jay
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post #93 of 170 Old 07-01-09, 11:56 AM
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Re: ECM8000 microphone measuring techniques and usage discussion

Are you asking about home theater system calibration or my microphone calibration?
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post #94 of 170 Old 07-01-09, 12:00 PM
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Re: ECM8000 microphone measuring techniques and usage discussion

I am asking about the microphone calibration. Sorry for being too vague.

Jay
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post #95 of 170 Old 07-01-09, 12:48 PM
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Re: ECM8000 microphone measuring techniques and usage discussion

Hi, I hope the following question for Herb isn't considered abusing a dead horse (that has already been discussed extensively in the thread):

Can you comment, based on your measurements, on "typical" variation of the ECM-8000 frequency response between the horizontal (0 degree, on-axis) and vertical (90 degree, grazing incidence) orientation? More specifically, is it possible to give an estimate of the frequency where the splitting between the horizontal and vertical response reaches +1 dB, or the frequency where the splitting reaches +3 dB? Or, does the horizontal-vertical splitting show a large variation between individual microphones, that makes it difficult to estimate "typical" values?

I'm asking this more for general interest in "how it works" than for practical reasons. My guess is that the directionality (horizontal-vertical splitting vs. frequency) is more consistent between individual ECM-8000 microphones than the basic frequency response, because the directionality is determined mainly by diffraction of sound waves from the microphone body, and the size and shape of the microphone body is highly consistent between units. But maybe I'm overlooking something
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post #96 of 170 Old 07-01-09, 10:41 PM
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Re: ECM8000 microphone measuring techniques and usage discussion

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JMB wrote: View Post
I am asking about the microphone calibration. Sorry for being too vague.

Jay
Hmm, I could have sworn that I explained my process here at some point, but I guess I didn't.

Above 100 Hz, I use a free field substitution method. I mount a reference microphone (ACO Pacific 7250, calibrated by Scantek) 27 inches away from a custom speaker with a full-range (coax) driver to measure a reference response. I replace the reference microphone with the ECM8000 and repeat the test. I use the reference response to correct the response of the ECM8000 and that gets me the >100 Hz response for that particular microphone. I use this procedure to measure mics at 0 degrees and at other angles. I use ARTA (swept sine) to make the measurements.

For frequencies below 100 Hz, I use the same substitution method, but rather than doing a free-field measurements, I use a small pressure chamber to obtain the pressure response of the reference mic and the ECM8000-under-test (at low frequencies, the pressure response is equal to the free field response for omni-directional microphones).

These measurements are consistent with IEC 60268 standard for microphone measurements.

For the polar measurements, I have a mic stand mounted on a lazy susan. I mount the mic in front of a speaker playing pink noise and rotate it around 360 degrees (in 10-degree increments) to measure the 1/3 octave band responses. I have the mic mounted so that the mic diaphragm is the axis of rotation - basically the mic capsule stays in the same ~ 1 sq cm of area and the cable end of the mic rotates around the diaphragm (make sense?).

For sensitivity measurements, I have an acoustic calibrator (GenRad 1986 Omnical, also regularly calibrated by Scantek) which puts out calibration tones at several frequencies and sound pressure levels. I place the calibrator over the ECM8000 capsule, hook up the ECM8000 output to my Sencore RTA, and just read the sensitivity figure from the RTA.

For noise floor measurements, I have a soundproof chamber within which the background noise (at night) is lower than 17 dBA. Since the noise floor of ECM8000's are generally higher than 30 dBA, I can just put the ECM8000 into the chamber and read the noise floor off my meter.
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post #97 of 170 Old 07-01-09, 10:57 PM
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Re: ECM8000 microphone measuring techniques and usage discussion

Quote:
Sonic Icons wrote: View Post
Hi, I hope the following question for Herb isn't considered abusing a dead horse (that has already been discussed extensively in the thread):

Can you comment, based on your measurements, on "typical" variation of the ECM-8000 frequency response between the horizontal (0 degree, on-axis) and vertical (90 degree, grazing incidence) orientation?
They say a picture is worth a thousand words, so here are plots from 6 random multi-angle measurements:



It looks like they all start to diverge around 2 kHz, which is consistent with the data you see from measurement mic vendors.

I think you're correct that the angle difference from mic-to-mic is more consistent than the frequency response, but I haven't gotten around to actually comparing difference curves to verify that. The "x" factor is the positioning of the mic element in the capsule which I noticed seems to have an effect on the upper frequency response of these mics.

Last edited by Anechoic; 07-01-09 at 11:12 PM. Reason: clarification
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post #98 of 170 Old 10-12-09, 01:01 AM
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Re: ECM8000 microphone measuring techniques and usage discussion

Heh heh, just going back to that schematic of the ECM8000, we've had a couple of interpretations on it, I thought I could really helpfully confuse things by adding another!

My feeling is that the circuit as drawn is probably correct, even if confusing. I think that first transistor (immediately after the capsule) is just there for absolute phase reversal, to make the mic conform to the "positive pressure wave hits diaphragm causes positive voltage excursion at pin 2" standard. Without it, pin 2 would go negative when the positive pressure wave arrived. Not a problem with a single mic, but a trap if you have more than one.

That then raises the question of what the lower transistor of the output pair is doing. If the circuit is wired as shown, its base connects (via a dc isolating capacitor) to earth (follow it round - it's a rather confusing bit of schematic drawing). So with a grounded base, there can be no signal on its emitter feeding pin 3. So it's not there to provide a balanced output voltage.

My guess is that it's there just to provide a guaranteed impedance balance to the real output on pin 2. So that any noise, hum, etc injected by way of cables passing by speaker amp transformers, lighting rigs, or even by way of a mic body in electrical contact with something nasty will be balanced out as well as the preamp's cmrr permits.

They could have achieved the same phase reversal without need for the extra stage by swapping XLR pins 2 and 3, but that would prevent people using the mic in unbalanced mode.

It would be easy to test my theory - there should be signal output on pin 2 only. Of course, we can't be sure that they haven't changed the circuit over time!

The treble rolloff provided by C1, the 3n3 capacitor across the capsule FET, is pretty brutal - I wonder what the the response would be like without it! Ultrasonic? If we ignore the FET for the moment, the combination of R1 and C1 would roll off at 6dB per octave from around 3KHz. If we assume the FET drain impedance is similar to R1, that would lift the start of the treble rolloff to around 6K. Presumably the capsule exhibits a serious resonance somewhere near the top of the audio spectrum and this heavy rolloff helps keep it under control. The economics of commercial electronics manufacture has always been a mystery to me - I wonder how much more you have to spend to get a capsule that doesn't need such aggressive EQ. Another dollar probably!

Looked at another way, you could probably get in there with a more tailored network of components and achieve a far better response before calibration. But hey, that would be overcapitalising (to use real estate language). And you'd probably need an electron microscope to deal with the size of the components.

Terry
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post #99 of 170 Old 10-12-09, 02:11 PM
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Re: ECM8000 microphone measuring techniques and usage discussion

Quote:
My guess is that it's there just to provide a guaranteed impedance balance to the real output on pin 2.
Perhaps, but there would have been a lot easier ways to do that.

Looking at it again, I suspect it's simply drawn wrong and that the first BC118 stage is not only acting as a high impedance buffer for the mic element, but also as a zero gain phase splitter.

I edited the diagram (below) to show where C6 is likely connected to the emitter of BC118 and not to ground. Generally a phase splitter would have equal resistors on the collector and emitter, but they're close, and it's the only solution I could come up with....

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post #100 of 170 Old 10-12-09, 04:14 PM
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Re: ECM8000 microphone measuring techniques and usage discussion

Quote:
Perhaps, but there would have been a lot easier ways to do that.
Yes, that is possibly a weakness in my hypothesis! Very thorough though - perhaps too thorough for a low price mic?

Quote:
Looking at it again, I suspect it's simply drawn wrong and that the first BC118 stage is not only acting as a high impedance buffer for the mic element, but also as a zero gain phase splitter.
It's not really acting as an impedance buffer - note the values of R4 and R10 (ignoring other components for the moment) are virtually the same. But that doesn't rule out your zero gain phase splitter interpretation.

Quote:
I edited the diagram (below) to show where C6 is likely connected to the emitter of BC118 and not to ground. Generally a phase splitter would have equal resistors on the collector and emitter, but they're close, and it's the only solution I could come up with....
Yeah, that would certainly work, and would seem to explain the carefully balanced back end better. I wouldn't be too worried about the 1k2 vs 1k discrepancy. It might have been empirically found to give better balance, or it could be another typo - for example how many 46K resistors (R3) have you come across? Presumably a 47k and a partially colour-blind technician!

On behalf of my hypothesis, we need to note that there is no real need to balance the output voltage of a microphone - it's not as if the enormous voltages concerned are going to make a nuisance of themselves in a jackfield (which is why we balance output voltages). So either of our interpretations will work fine). Yours will have 6dB more output, but if they wanted that they could have put some gain in that intermediate stage (but not if they wanted the phase shift needed in your interpretation). Mine will be less fussy about working into an unbalanced load, but most people needing 48V phantom power will be getting it from a balanced preamp.

I guess there's nothing for it, one of us is going to have to go in there to find out! Or at least check that there is output on both 2 & 3 (your interpretation) or on 2 only (my interpretation). I'll get to that and report back. I'd have to say, I suspect on the balance of probabilities that yours is more likely!

I think the most remarkable element in the circuit is the enormous HF load represented by C1 across the FET. Like sending a boxer into a ring filled to head height with marshmallows! Hmmm, suddenly suspicious, could this be another typo? Perhaps not, given that the turnover frequency would appear to be consistent with the variable top-end boost shown in the various response graphs?

Good to talk audio circuits with you, brucek!

Terry
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