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ECM8000 Microphone
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Old 16th August 2007, 08:25 PM   #11
Dave Bullet is offline Dave Bullet  New Zealand
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Quote:
3meters would mean ~ 6meters from driver to surface and from surface to the mic, is this actually necessary with 10ms gate? (some explanation would be appreciated )
Ok - so my calculations are a bit off, but you have to allow for the initial flight time of the signal (assuming 1m measuring distance from speaker to mic) as the "reflected" signal is already on its way.

Here goes the calcs...

Assuming speed of sound is 344m/sec:
It takes 2.9msec to travel 1m to mic
Add on 10.67msec to give Speaker Workshop a gate resulting in 512 point resolution down to 100Hz
= 13.57msec allowable reflection time (from when the signal first leaves the speaker to when it can hit the mic)

13.57msec = 4.66m distance

To find the vertical distance = assuming a = vertical distance, b = 0.5 m (halfway point between speaker and mic) and c = reflection distance) via good old a^2 + b^2 = c^2

c = 4.66m/2 = 2.33m
b = 0.5
a = ?

c^2 - b^2 = a^2
5.4289 - 0.25 = 5.1789

a = 2.27m

Therefore you need to position the speaker 2.27m above ground. A room with 4.54m ceiling height would be fine.

So my 3m wasn't too far off (but wasn't correct

Cheers,
DAvid.
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Old 16th August 2007, 09:08 PM   #12
claudio is offline claudio  Italy
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Quote:
Originally posted by Twisted85


And with 10" driver the upper limit with near field measurement would be around 450Hz. So I can then merge these two results?

How does near field measurement work with a driver that has phase plug?

Yes, around 450Hz if mounted on a large pannel, and a 1x1 meter can be considered large, in my opinion.
It's important to have a wide overlapping range between the NF and FF curves, to better splice them together. That is, the FF should be reliable from 200-250Hz, which means a 4-5 ms of gate window.

Phase plug: just measure the mic distance starting from the plug end. The phase plug acts at high frequencies, not in the NF range.
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Old 16th August 2007, 10:40 PM   #13
Dave Bullet is offline Dave Bullet  New Zealand
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Phase plug: just measure the mic distance starting from the plug end. The phase plug acts at high frequencies, not in the NF range.
Thanks Claudio. I've wondered about that. I have some Seas drivers to measure with plugs so wasn't sure what effect it would have.

Cheers,
David.
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Old 17th August 2007, 12:00 AM   #14
augerpro is offline augerpro  United States
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For you new Soundeasy users the manual under the help tab will answer alot of basic questions. Also download the manual from the website. For a clear explanation of teh basics I HIGHLY recommend John Krevkovsky's (sp?) manual. Well worth the $10.

Before building a testing baffle use the diffraction modeler in SE to develop a baffle with the smoothest response. I'll post some screenshots of my baffle modelling later.

As for the mic calibration debate, I'd suggest doing it. I'll post some results of mine later also so you can see the difference.
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Old 17th August 2007, 01:18 AM   #15
augerpro is offline augerpro  United States
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This is the test baffle I developed and then built:

Click the image to open in full size.


The response is quite smooth until lower in frequency where the gating will ignore anyway. I generally merge nearfield measurement around 400Hz anyway, so the farfield is basically unaffected by the baffle.



As for teh ECM8000 mic calibration here is screenshot of mine, SPL deviation from flat (pink) and phase deviation (green):

Click the image to open in full size.


Uncorrected there is large rise in the tweeter response that may cause someone to try to make corrections for something that isn't really there. Also high XO points for mid/tweeter could run into trouble. Or if you were using a metal cone woofer and wanted the breakup a minimum of -40s dB down, you won't really have an accurate idea of where the actual breakup really is unless you use the calibration file.

OTOH as has been pointed out, for XO work the relative phase between drivers is what is important and at least the phase is equally incorrect for all of them. For the price of calibration ($40 from Kim G for example) it's worth it for the piece of mind.
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Old 17th August 2007, 02:22 PM   #16
Twisted85 is offline Twisted85  Finland
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Thanks for all of your replies!

I think I could build something like this:

http://koti.mbnet.fi/twisted-/baffle.jpg

The dB level would be in +/-0,5dB down to 180Hz.
If the baffle is standing on ground the driver would be only 95cm from the floor surface. How long timegate should be used (1m distance) so that the measurement would be accurate down to 180Hz? I could build a rack that lifts the baffle to upper level from the floor. The ceiling and walls aren't a problem at all.

If I could measure farfield measurements down to 180Hz and nearfield measurements would be accurate to 450Hz with 10" driver and even higher with smaller drivers, this would be more than enough to merge the responses, right? Or do I even have to get to 180Hz, would 250Hz be enough?

I don't still know what to do with the microphone. The cheaper model needs a separate preamplifier and I just got rid off UB802 when I bought a new soundcard which has phantom power supply.
I could send my ECM8000 to them to calibrate, but the cost is 32euros and I guess the shipping would cost quite a lot (back and forth). And the ECM40 is 113euros plus shipping

I received packet from partsexpress to my home after 43 hours of my order, (fastest shipping ever) but I haven't heard anything about the help files which I paid
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Old 17th August 2007, 02:56 PM   #17
Twisted85 is offline Twisted85  Finland
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I just realized that I made a mistake simulating the baffle. I used 20cm in the baffle edge as I thought it was the shape of the corners, but of course it's just the shape of the baffle edge
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Old 17th August 2007, 03:38 PM   #18
claudio is offline claudio  Italy
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Quote:
Originally posted by Twisted85

If the baffle is standing on ground the driver would be only 95cm from the floor surface. How long timegate should be used (1m distance) so that the measurement would be accurate down to 180Hz? I could build a rack that lifts the baffle to upper level from the floor. The ceiling and walls aren't a problem at all.

If I could measure farfield measurements down to 180Hz and nearfield measurements would be accurate to 450Hz with 10" driver and even higher with smaller drivers, this would be more than enough to merge the responses, right? Or do I even have to get to 180Hz, would 250Hz be enough?

I don't still know what to do with the microphone. The cheaper model needs a separate preamplifier and I just got rid off UB802 when I bought a new soundcard which has phantom power supply.
I could send my ECM8000 to them to calibrate, but the cost is 32euros and I guess the shipping would cost quite a lot (back and forth). And the ECM40 is 113euros plus shipping

In my HP, in the download folder, there is an excel file that does the calculations for gating time, minimum frequency, etc.

1000/gated ms= minimum useful frequency. So if you have 5 gated ms, you get 1000/5=200 Hz, that is the measured response is valid from 200 Hz.
To get 180 Hz you need around 5.3 ms of gated window, which means a 1.35 m of free walls areas, with the mic at 1 m from the baffle.

Since you have the M-audio FW410, why not using its pre-mic with the cheaper mic model?
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Old 17th August 2007, 04:34 PM   #19
Marik is offline Marik  United States
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Quote:
Originally posted by claudio



Yes, around 450Hz if mounted on a large pannel, and a 1x1 meter can be considered large, in my opinion.
It's important to have a wide overlapping range between the NF and FF curves, to better splice them together. That is, the FF should be reliable from 200-250Hz, which means a 4-5 ms of gate window.

Phase plug: just measure the mic distance starting from the plug end. The phase plug acts at high frequencies, not in the NF range.
Hi Claudio,

Could you explain what is the purpose of the baffle?
I'd think it will create a pressure zone, which is usually used to eliminate comb filter effect, which in any case would be more pronounced on higher frequencies as a result of phase anomalies due to the floor reflections.

Besides, the baffle will create a LF boost in microphone sensitivity and can be quite unpredictable.

Also, I'd like to point out the fact that it is unclear whether HF ECM8000 response is corrected for free field or diffuse field measurements.
As such, the readings would be different for broadband measurements, as well as there would be obvious differences between near and far field measurements above 5Khz.
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Old 17th August 2007, 05:41 PM   #20
claudio is offline claudio  Italy
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Hi Marik,
the baffle holds the driver, separates the front emission from the back one someway, approaches the infinite baffle and follow the IEC standards. To compare different drivers response, all should be measured using the same enviroment, that is at least the same baffle, the IEC baffle.
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