[brucek]

Re: MobilePre ECM8000 oddities

from zero, or from first impulse received?

In the IR window popup (after the measurement is complete), I set 1msec in the Left Pre-Window and 5msec in the Right Post-Window, and then Apply to the measurement.

brucek

 

[robbo266317]

The one from the DBX RTA-M usable thread http://www.hometheatershack.com/for...-cards/13006-dbx-rta-m-calibration-curve.html

Here is a screenshot:

 

[Anthony]

Re: MobilePre ECM8000 oddities

Okay, my results:
Red = Maggie, Vertical Mic
Green = Maggie, Horizontal Mic
Orange = Adire, Vertical Mic
Blue = Adire, Horizontal Mic

View attachment 11936

Gates were 6 ms, "Split" ECM8k cal file, 12" from tweeter on axis. 1/2 octave smoothing. Shifted to line up like curves.

You can see the trademark split between horizontal and vertical orientation of the mic in both sets of curves.

However, most notably, the Adire HE10.1 does not show the fall-off that the Magnepans show. Very interesting. I stared with a bad assumption and look where it got me

So at least now I have renewed confidence in the mic and soundcard, but less in the Magnepans

 

[Anthony]

Re: MobilePre ECM8000 oddities

So, it looks like the New ECM cal file, horizontal orientation is the best for direct measurements.

Behringer insists that vertical is better for diffuse field measurements like pink noise and such, but since 90% of my measurements are drivers and speakers directly, I will stick with horizontal.

 

[brucek]

Re: MobilePre ECM8000 oddities

Yeah, I think we can assume the ECM and the cal file are fine.

The Maggies still go fairly high, and unless your hearing is really top notch, I doubt you'd be disappointed in the sound.

The Adire measures quite good for sure....

Maybe we should be listening to music instead of swept tones..... :heehee:

brucek

 

[Anthony]

Re: MobilePre ECM8000 oddities

Bruce, to me that is music! :nerd:

 

[hifisponge]

Re: MobilePre ECM8000 oddities

Bruce & Anthony -

A big thanks for helping me troubleshoot, and for all of the effort you two have put into this series of experiements. I'm still waiting for my DBX mic and cal files to come back from Cross-Spectrum and I will post my results when I get those. In the meantime, I think I'll do another run with the ECM and split cal file.

Bruce, 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.

Thanks,

- Tim

 

[hifisponge]

Robbo -

I have the DBX mic, and I just had it calibrated. I don't think that cal file listed in the link you provided is all that accurate. It only shows about a 1.5dB rise in the treble response of the mic, when it is really more like 5dB.

- Tim

 

[robbo266317]

Hi Tim,

The close mic scan of my raven 3.2 is fairly flat from 1k to 25k. (As per it's measured graph that came with it)
I can repost just the 1k to 25k graph if you like.

 

[hifisponge]

Re: MobilePre ECM8000 oddities

Looking a little deeper at your guys' graphs vs. mine, both of you are showing a difference of roughly 6dB at 20KHz in your horizontal vs. vertical mic positions. This difference coincides with the differences shown in the FR plot of the ECM mic. My measurements show a difference of 12dB at 20KHz. What's up with that? :nerd:

 

[brucek]

Re: MobilePre ECM8000 oddities

..........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".

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.

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.

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.

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).

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.

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

 

[hifisponge]

Re: MobilePre ECM8000 oddities

Bruce -

You rock! :bigsmile:

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 don’t quite match those published on the ScanSpeak tweeter, but they do appear to be within a reasonable deviation.

Thanks again.

- Tim

 

[hifisponge]

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

 

[hifisponge]

why it pays to have your mic professionally calibrated

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

 

[brucek]

Re: MobilePre ECM8000 oddities

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?

brucek

 

[hifisponge]

Re: MobilePre ECM8000 oddities

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:

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?

 

[robbo266317]

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. :bigsmile:
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/...oduct_id,118/option,com_virtuemart/Itemid,49/

I hope you have air-conditioning. It's going to be another scorcher over there again.

 

[spearmint]

Re: MobilePre ECM8000 oddities

Thanks for the conducting the tests and posting the results.

Anyone have any negative thoughts regarding using the Mobilepre?

 

[hifisponge]

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. :yes:

- Tim

 

[Anthony]

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.