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.