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post #1 of 15 Old 07-26-10, 03:05 AM Thread Starter
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Etienne Dechamps
 
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Question ECM8000: phase response?

Hi,

I'm currently using an ECM8000 microphone calibrated by Cross Spectrum.

My problem is, I have the magnitude response calibrated but not the phase response (the phase column is all zeroes in the calibration file). I would prefer to have a linear phase response: indeed, I'm using DRC to calibrate my system, which is capable of correcting phase.

Question is: can I trust the "natural" phase response of ECM8000 microphones to be linear (or almost linear)? Searches I done gave me contradictory results: one shows a maximum phase difference of 15, another shows a phase response which is all over the place.

Did someone actually measure the phase response of a fairly recent (post-WM60) ECM8000 microphone?
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post #2 of 15 Old 07-26-10, 09:16 AM
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Re: ECM8000: phase response?

I have to admit I don't know. But, one of our members is Cross Spectrum, I have alerted him to your question and I expect he'll give his thoughts.

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post #3 of 15 Old 07-26-10, 10:50 AM
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Re: ECM8000: phase response?

Hi, this is Herb from Cross-Spectrum Labs.

I don't measure phase response because, frankly, there's no good way to measure the absolute phase response of microphones. Condenser microphones aren't minimum phase devices, so the Hilbert transform isn't applicable (which, AFAICT, if how most of the ECM8000 phase info on the web was calculated).

The only realistic way would be to compare the phase of one mic with another - the problem is that I haven't been able to find reliable phase information for any of my ANSI-rated reference mics. Even Bruel & Kjaer, who supplies a lot of technical info with their mics, doesn't provide that info.

About the best you can do is to assume the phase response is flat - that will certainly be fine for the mid-frequencies between 100Hz to a few kHz. Condenser mics do have a 90-degree phase shift at its resonance frequency... however, we don't know what the resonance frequency of the capsule is (for pro mics the resonance frequency might be anywhere from 10 kHz to 30 kHz). There will also be a phase shift at the lower end of the spectrum because of the capacitor used for the high-pass filter, but again I don't know what the Fc is.

Sorry.
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post #4 of 15 Old 07-26-10, 11:05 AM Thread Starter
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Etienne Dechamps
 
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Re: ECM8000: phase response?

Many thanks for your quick answer.

Quote:
About the best you can do is to assume the phase response is flat - that will certainly be fine for the mid-frequencies between 100Hz to a few kHz.
If I understand this correctly, this means that correcting a system's phase response with an uncalibrated mic is not ideal, but is probably better than not correcting the phase at all. Guess that will have to do then.
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post #5 of 15 Old 07-26-10, 12:13 PM Thread Starter
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Re: ECM8000: phase response?

I contacted iSEMcon GmbH, which also offers ECM8000 calibration services:

content.ibf-acoustic.com/catalog/product_info. php?cPath=21_23&products_id=76 (sorry for the space in the middle, the forum forced me to do that)

(see the sample data: these include phase response!)

I asked them what was their methodology for measuring phase response. Here is their (short) answer:

Quote:
The phase response above 500Hz is calculated from frequency response (inverse fourier transform calculated from my measurement software). In the range 10Hz to 500Hz it is relative to my B&K reference mic (measured in a small pressure chamber).
If their measurements are correct, this means the sample ECM8000 has a quite linear phase response throughout the whole spectrum, which contradicts Herb's affirmations. Now I'm confused...
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post #6 of 15 Old 07-26-10, 12:59 PM
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Re: ECM8000: phase response?

Quote:

The phase response above 500Hz is calculated from frequency response (inverse fourier transform calculated from my measurement software). In the range 10Hz to 500Hz it is relative to my B&K reference mic (measured in a small pressure chamber).
The B&K 4133 is likely to have a flat phase response down to 3 Hz or lower, so the low frequency comparison test is kosher. For the upper freq phase test, calculating the phase from the freq response is only valid if you know that the source has a flat phase response, or the source has a known phase response (which means measured with a mic with a known absolute phase response)

Quote:
e-t172 wrote: View Post
If their measurements are correct, this means the sample ECM8000 has a quite linear phase response throughout the whole spectrum, which contradicts Herb's affirmations. Now I'm confused...
No contradiction, I was talking about the freq range that I new to be flat, nothing precluded the ECM8000 being flatter outside that range (assuming iSEMcon's protocol is valid).
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post #7 of 15 Old 07-27-10, 01:38 AM Thread Starter
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Re: ECM8000: phase response?

I also contacted Denis Sbragion, the author of DRC. Denis stated, in section 6.2.6 of the DRC documentation (about the microphone compensation stage) that "most microphones suited for measurement tasks are minimum phase systems". I asked him the source of this information. Here is his answer:

Quote:
I've seen it stated in many academic papers, so I took it for granted. Considering the simple nature of a microphone it looked reasonable to me, but I admit that I have yet to see a clear proof of it.

I've seen many mic phase responses, and they were all smooth and almost linear, which is what's expected from a minimum phase system having an almost flat magnitude response. BTW, never checked it directly, neither I verified that the supplied phase response wasn't simply derived from the magnitude response assuming that the system was minimum phase, i.e. a chicken and egg problem. Apart from this, minimum phase or not, I don't expect the phase anomalies of any decent measurement mic to be audible.

May be you're not correcting [phase] as expected but, for sure, you're not messing with it. The phase response of a measurement mic, even a cheap one like the Behringer, is not the biggest source of phase anomalies, by a wide margin.
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post #8 of 15 Old 07-27-10, 12:39 PM
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Re: ECM8000: phase response?

I've seen assumptions made of minimum phase/not minimum phase in several contexts. When I have seen assumptions of minimum phase it's typically a simplifying assumption.

(and just to be clear, when I'm using the term "minimum phase" I'm using it as defined here to mean "there exists a unique relationship between amplitude and phase which allows a complete determination of phase from amplitude.")

There were a variety of things that convinced me that one could not (simply) calculate the phase of a condenser mic based only the frequency response, but what finally convinced me was an analysis I performed based on a figure in the book "Acoustical Measurements" by Leo Beranek. The book shows a plot of the (pressure) frequency response and phase response of the B&K 4145 microphone:



Using GraphClick, I scanned the points off the 4145 curves and imported the data in Octave with the intention of using the Hilbert transform to calculate the phase from the pressure response data. The B&K website also has the free-field data for the 4145 available, so I threw that in the mix as well.

The result:



Assuming I didn't screw anything up, the results I get via Hilbert are not the same as the published phase response. That's the point where I gave up on microphone phase.

With respect to:

Quote:
Apart from this, minimum phase or not, I don't expect the phase anomalies of any decent measurement mic to be audible.
and

Quote:
The phase response of a measurement mic, even a cheap one like the Behringer, is not the biggest source of phase anomalies, by a wide margin.
I would agree with those statements wholeheartedly.

Last edited by Anechoic; 07-27-10 at 02:49 PM.
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post #9 of 15 Old 08-26-10, 03:42 AM
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Re: ECM8000: phase response?

Is it correct to say that the phase response of a dynamic mic is linear? I have an old E-V RE55 which is just itching to become a calibration mic. The RE55 is the descendant of the 655C which has an output transformer which probably introduces phase non-linearities.
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post #10 of 15 Old 08-26-10, 05:37 PM
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Re: ECM8000: phase response?

Quote:
chris319 wrote: View Post
Is it correct to say that the phase response of a dynamic mic is linear? I have an old E-V RE55 which is just itching to become a calibration mic. The RE55 is the descendant of the 655C which has an output transformer which probably introduces phase non-linearities.
Dynamic mics are basically loudspeakers run in reverse, so theoretically the minimum-phase methods for determining phase (Hilbert transform, etc) should work.
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