Questions about SoundEasy

[bikinpunk]

I've posted these questions in various places and not had much feedback. Thought I'd give it a go here, too. Yes, I've RTFM and yes, I purchased John K's guide. I'm still having trouble with a few things, though.

My current setup is a very large open baffle (about 66x81") with a 10" window to allow for swappable 'blanks to test drivers on.
I'm using my PC, soundeasy v17, a behringer ECM8000 calibrated by Herb @ Cross Spectrum Labs, and a behringer ep2500 amp.
My PC mobo soundcard sucks, so I am using an ART dual XLR pre/card. It has (2) inputs - XLR/TRS combined and (2) TRS outputs. I'm using the left input for the probe for SPL measurements and the right input for the mic, which gets phantom power here. I'm using the left output to go to the amp. Those are the only 3 connections I have on the soundcard/preamp.

Here are my questions:

1. For distortion measurements, I'm seeing people say that you have to split the signal off the mic preamp via a y-splitter to your souncard's inputs; that simply using one input on the soundcard will result in erroneous data. Well, in my case, the soundcard and the pre-amp are one in the same. Can I simply split the mic input (which provides phantom power as well) so that I have the mic signal going in to both the L/R input? Or do I need to buy some sort of preamp that will allow for a left/right output and run that out to the dual pre (or some other soundcard)?
   
   I'm beginning to wonder if I should just buy a nice usb soundcard like the m-audio transit and use the ART XLR for phantom power only, and run it's outputs back in to the M-audio piece.
   
2. I want to merge NF and FF plots in a fashion like zaph has for his off-axis results. My baffle/window allows me resolution down to 200hz. So, I need NF to cover this lower end. The problem I have is that when I try to merge NF to FF I'm left with one color. I want each off-axis measurement to be a different color for ease of readability. I'm not seeing a way to do this. Any tips?

I'm sure I'll have more as I go. I appreciate the help,
Erin

[Z transform]

I have the same setup using the USB Dual Pre. I baked it off against a couple of the M-Audio fast track products. But as a cautionary note, I am just starting on my first project with SE and all of my testing has been done with a single driver on a bench, so take my input as inexperienced. I have not yet attempted distortion measurements.

I don't see an issue with splitting the signal from the microphone. The phantom power will be the same on both sides, so there will be no conflict with two channels connected together. The differential input impedance will drop in half, but the mic's buffer should still be able to drive this easily, at worst you will lose a couple of dB of input; the max you could lose if the output impedance of the mic is very high is 6dB.

FYI- I experimented with the M-Audio Fastrack Pro. The Fastrack Pro gave very inconsistent results in the MLS measurements. So I would be warry of thier gear, especially with mic inputs without a full trial.

Did you actually see odd distortion results using only one channel. Or is that simply from advice you have read? I ask because this does not make sense to me. The only need I can see for two inputs is to subtract a reference signal, say the output of the amp, to allow the removal of the amp's distortion from the speaker under test. But if that were true, when you hook them both together you would get near zero distortion readings, just the distortion difference between the two input channels. Perhaps someone did not like how much distortion they saw and noticed how much better it got when they hooked both inputs together? There is probably a checkbox in one of the SoundEasy screens to turn "differential distortion" on and off??? Or perhaps you do have to hook the reference input to the output of your amp. But both from the same mic does not make sense.

I will try some distortion measurements later, over the next couple of months. But if I were doing this now I would take the output of the Dual Pre and feed it to the input to see what distortion was measured (hopefully very low) and then insert a non-linear network, probably an RC filter with a diode to add some distortion. You could do this into one or both chanels to see if it makes any difference. Allways better to try your gear on reference stuff before trusting measurements.

Hope this helps, Tom