ECM8000 microphone measuring techniques and usage discussion - Page 8 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #71 of 170 Old 01-28-09, 10:20 AM
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Re: MobilePre ECM8000 oddities

Quote:
hifisponge wrote: View Post
..........can I ask a favor? Would you please retest at 3 feet (horizontal mic) to see if that induces some roll-off in your Proacs? And would you also measure on the midrange axis as well? You're Proac's use a tweeter that is similar to mine and I want to see if distance affects the HF response and also what sort effect being off-axis has.
Well, you got lucky, since I hadn't returned my ProAcs to their regular spot in my main system, I was able to do another test for you.

Anyway, I thought I would divide your test in two.

I had a couple of small graphs of my particular speaker model, that were done in an anechoic chamber at 50" (reasonable far-field measure). I thought I would try and duplicate the test conditions for those two graphs.

The first graph is the tweeter and mid-range individually tested on-axis at 50" and shown separately on the small graph.

The second is a measurement of the mic placed first between the tweeter and mid-range at 50" on-axis and then at 30 degrees off-axis in the horizontal plane. The plot is averaged across the 30 degrees horizontal window.

It seemed reasonable to carry out these two tests.

The graph below is the anechoic graph showing ~1KHz to 30KHz.
This is the on-axis, taken at 50" of the 1" SEAS tweeter and the on-axis of the 7" ScanSpeak mid-range driver. Note the rise that I experienced when I measured my tweeter at 12".

Name:  ProAc on-axis tweeter and mid anechoic.jpg
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The graph below is the anechoic graph showing the average across a 30 degrees horizontal window, measured at a point vertically equal between the tweeter and mid driver at 50".
The comments about the graphs in the article says, The tweeter has a gently rising on-axis response all the way to 30kHz, but it is very directional above 15kHz. This results in a more or less flat response when averaged across a 30 degrees horizontal window.

Name:  ProAc off-axis average of tweeter and mid anechoic.jpg
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I think my measurements below support what the anechoic responses show, in that the tweeter is certainly good past 20KHz when on-axis, but drops off at about 15KHz when off axis in the vertical or horizontal plane. This may be what you're experiencing somewhat, where you're suspecting your measurement equipment. But the way to be sure is to measure near-field on-axis, and that will prove it if the response doesn't drop prematurely.

Anyway, below is my first graph, measured at 50".
The mic is on-axis with the tweeter for the purple line, and then horizontally moved to 30 degrees for the green line (with the mic still pointing at the tweeters center).
I also shortened the gate to 3msec to remove some reflections from the stand or wires or whatever it was. I did check the gating envelope in each case on its impulse response. All these are plots with no smoothing.
You can see the drop off when it's off-axis.

ECM8000 microphone measuring techniques and usage discussion-proac-seas-axis-test.jpg

Below is simply an average plot of the above two measures (an REW feature allows averaging of multiple plots).
It shows that there is a decent response to 20KHz within a 30 degree horizontal window of the tweeter.

ECM8000 microphone measuring techniques and usage discussion-proac-seas-axis-average.jpg

Now I do the same experiment, except the mic is moved down to point at a spot equidistant between the mid-range driver and the tweeter.
Note that the vertical off axis from the tweeter creates a drop off at both on-axis (horizontally) and at 30 degrees (horizontally).

ECM8000 microphone measuring techniques and usage discussion-proac-seas-mid-axis-test.jpg

This is simply an average plot of the above two measures. It shows that there still is a fairly decent response to 20KHz off axis within a 30 degree vertical or horizontal window of the tweeter.

ECM8000 microphone measuring techniques and usage discussion-proac-seas-mid-axis-average.jpg

Quote:
My measurements show a difference of 12dB at 20KHz. What's up with that?
If you examine my two plots, I measured a 12db and 10db difference.

brucek
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post #72 of 170 Old 01-28-09, 01:36 PM
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Re: MobilePre ECM8000 oddities

Bruce -

You rock!

Thanks a million for taking the time to conduct all of those measurements. They explain a lot and put my mind at ease. I also did some digging around for graphs of the ScanSpeak tweeter used in my speakers and found that that they all showed about a 12-15dB drop above 15KHz when measured 30 degrees off axis.

The difference between the tweeter in my speakers vs the one in yours is that it does not have rising response on-axis. In fact, based on measurements I did last night, it is about 3db down at 20KHz on-axis, so the off-axis roll-off above 15K looks more pronounced.

My measurements still donít quite match those published on the ScanSpeak tweeter, but they do appear to be within a reasonable deviation.

Thanks again.

- Tim
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post #73 of 170 Old 01-28-09, 07:45 PM
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re: ECM8000 microphone measuring techniques and usage discussion

Robbo -

I was under the impression that you were using a JBL / DBX mic with a cal file that was published on this site. If that is the case, I was only suggesting that if you want accurate high-frequency readings from that mic that you should have a cal file made for your specific mic.

- Tim
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post #74 of 170 Old 01-28-09, 10:11 PM
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why it pays to have your mic professionally calibrated

Quote:
brucek wrote: View Post
That seems consistent with the graph of mic orientation versus response...



brucek
Bruce, Anthony -

I got my DBX mic back from Cross-Spectrum Labs today and the response graphs the provided explain at least part of the roll-off I was getting when I did my measurements with this mic. You see, I was doing all of my measurements with this mic in the vertical position, as I thought that was the way to get the flattest response from an omni mic. As it turns out, the DBX mic is 6dB down at 20KHz when used vertically, with a +2dB hump at 11KHz. In the horizontal position there is a 5dB peak at 11KHz and it flattens out by the time it reaches 20KHz. I would post the FR plot, but I was only given a paper graph.

For the record, the cal file I have now is significantly different that the one that was posted by another user in this forum, and neither is good to use when the mic is in the vertical position.

Cheers,

- Tim
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post #75 of 170 Old 01-29-09, 07:23 AM
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Re: MobilePre ECM8000 oddities

Quote:
and neither is good to use when the mic is in the vertical position.
It's my understanding that your mic was calibrated to be used in the vertical position, when placed in a room in a diffuse field (such as the listening position). That would mean it is accurate to rely on the results if you use the cal file in REW under those normal conditions.

The only time it would be inaccurate is when you take a close speaker measurement of the tweeter, where you need to point the tip horizontally at the speaker, and so the cal file is not accurate for that upper octave. Is that not a correct assessment?

Comments?

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post #76 of 170 Old 01-29-09, 12:54 PM
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Re: MobilePre ECM8000 oddities

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brucek wrote: View Post
It's my understanding that your mic was calibrated to be used in the vertical position, when placed in a room in a diffuse field (such as the listening position). That would mean it is accurate to rely on the results if you use the cal file in REW under those normal conditions.

The only time it would be inaccurate is when you take a close speaker measurement of the tweeter, where you need to point the tip horizontally at the speaker, and so the cal file is not accurate for that upper octave. Is that not a correct assessment?

Comments?

brucek
Herb with Cross Spectrum initially gave me just a horizontal cal file, but upon request also provided a vertical cal file.

In regards to the proper orientation, I was just discussing this with herb last night. Here's what he said:

Quote:
For in-room speaker measurements / room-correction, 90-degree (vertical mic position) is good enough - these are free-field microphones which assume that sound will be coming around from multiple directions, but there will be a some error at the top end. When doing room measurements, I always recommend people measure at various locations (in three dimensions) to minimize orientation errors as well as the effects of room modes.

When performing loudspeaker freq response measurements, using the mic at a 90-degree angle is a no-no because it's not designed for that at high-frequencies. At the highest frequencies (say, 3kHz and up), the wavelength of sound is small enough that the mic body itself will start to interfere with the wave propagation. At a 90 degree angle, part of the wave collides with the body of the mic and the rest of it grazes across the surface of the diaphragm, so the diaphragm no longer moves as a piston and what the microphone 'sees' isn't actually representative of the wave form if the mic wasn't there.
Personally, because I want more accurate HF measurements, even at the listening position, I'm inclined to use the horizontal mic position and appropraite cal file all the time. I’ve also been thinking for a while now that there is merit to the belief that we humans give priority to the direct sound in the mid and high frequencies (over the combined direct + reflected sound) so it would seem that pointing the mic in the direction of the speaker would give a bit more weight to the direct sound while still capturing the reflected. Then again, I suppose this can also be addressed with reduced gate times.

Thoughts?
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post #77 of 170 Old 01-29-09, 01:21 PM
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re: ECM8000 microphone measuring techniques and usage discussion

I totally agree Tim.
Each mic will be different since they are mass produced.
For critical measuring you will need to get it calibrated. Mine seems to be adequate for the purpose using the generic file and the tests I have done shows it is acceptable out to about 25khz.
Since my upper range these days is only about 13.5 khz I am not too worried.
I could calibrate it myself using the graphs supplied with the Ravens.
I'm glad I purchased them when I did, as two months later they doubled in price because the aussie dollar dropped. They are now $3,750 each out here.
http://www.audiomarketplace.com.au/c...art/Itemid,49/

I hope you have air-conditioning. It's going to be another scorcher over there again.
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post #78 of 170 Old 01-29-09, 05:41 PM
 
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Re: MobilePre ECM8000 oddities

Thanks for the conducting the tests and posting the results.

Anyone have any negative thoughts regarding using the Mobilepre?
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post #79 of 170 Old 01-29-09, 06:19 PM
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Re: MobilePre ECM8000 oddities

Spearmint -

Both Anthony and I have concluded that the issue we were experiencing had nothing to do with the Mobil Pre. It works as advertised.

- Tim
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post #80 of 170 Old 01-29-09, 06:41 PM Thread Starter
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Re: MobilePre ECM8000 oddities

Yeah, the MobilePre is a great portable sound card. I even use it to tie into my laptop as a mobile music station (laptop to MobilePre to stereo). Good for measurements.

It does have some flaws if you want to use it for speaker building. The line out sags its voltage under even light loads, which makes it tough to measure impedance. It is also a bit noisier than some of its counterparts (again unless it's for really serious speaker testing it's not an issue). But the portability aspect and phantom power make it worth it.
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