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Discussion Starter · #1 ·
Hello everyone, I just made my first plot using REW. I am kind of thinking that something must be wrong with the way I am measuring, and maybe some of the more experienced members of this forum can help me out. I just wanted to get familiar with the software, before I started measuring my home theater. I have a set of M-Audio studio monitor's that are connected to my PC that I measured. This plot just looked a little to good to me, but I have always loved these speakers. Just to make sure that everything was working right, I cut the mid's on my soundcard EQ, and sure enough the plot showed exactly what I did. So anyway here is my first plot, could anyone tell me why this might look just a little too good to be true?
 

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Welcome to the Forum, baker!

Well, I doubt they really get response down to 2 Hz! LOL For one thing, your graph scaling is really off re-set the vertical axis for 45-105 dB, and the horizontal for 15 Hz – 25 kHz. Also, it looks like you’ve engaged maximum smoothing. Try smoothing for 1/3-octave, which is acceptable for a full-range graph.

Regards,
Wayne
 

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You are probably still measuring with a loopback connection on your soundcard, the plot is just showing the effect of the meter cal file on a loopback measurement.
 

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Discussion Starter · #4 ·
You are probably still measuring with a loopback connection on your soundcard, the plot is just showing the effect of the meter cal file on a loopback measurement.
Yes, I did use a loopback connection as shown in the REW help file. I am a bit confused though as to what you are saying the plot is showing, could you elaborate a bit. Is the plot showing only the differential of the feedback loop?

You have written a really nice piece of software here John. I did a lot of lab work at Kansas State University about 7 years ago when I was studying electrical engineering. We were using Texas Instruments Labview a lot at the time. I remember writing C files for differential equations, been a while, but boy you had a lot of hard work in this. Great job, and the only way I can thank you is to learn how to use this software to its full potential.
 

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Discussion Starter · #5 ·
Welcome to the Forum, baker!

Well, I doubt they really get response down to 2 Hz! LOL For one thing, your graph scaling is really off re-set the vertical axis for 45-105 dB, and the horizontal for 15 Hz – 25 kHz. Also, it looks like you’ve engaged maximum smoothing. Try smoothing for 1/3-octave, which is acceptable for a full-range graph.

Regards,
Wayne
Wayne,

Thanks for the help, I'll take your advice and post up the results.

Thanks.
 

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I suspect John had it right, that you are using the cable looping your output back to your input so you are measuring your soundcard response with a mic calibration file loaded. What microphone were you using, and how was it connected?

Below the Filter graph, there are boxes you can check to display the the Mic/Meter Cal and the Soundcard Cal along with the measured data. If you include these, any relationship with the data will become clearer.

Bill
 

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Discussion Starter · #7 ·
I suspect John had it right, that you are using the cable looping your output back to your input so you are measuring your soundcard response with a mic calibration file loaded. What microphone were you using, and how was it connected?

Below the Filter graph, there are boxes you can check to display the the Mic/Meter Cal and the Soundcard Cal along with the measured data. If you include these, any relationship with the data will become clearer.

Bill
Yes, went back and read the instructions again. I think this is a little more acceptable. I'm basically just trying to get a good understanding of the software at this point using the M-Audio's, so if you see anything that looks odd let me know. I really like this software so far! I fixed the graphs, and attached the requested plots.
 

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If you're using any RS SPL, limit your measurements to below 3kHz or so. That mic, even with the cal file, cannot be trusted any higher.
Often people will look at another scan, just up to 200Hz with no smoothing to get a real clear picture of the bass region.
 

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Discussion Starter · #10 ·
If you're using any RS SPL, limit your measurements to below 3kHz or so. That mic, even with the cal file, cannot be trusted any higher.
Often people will look at another scan, just up to 200Hz with no smoothing to get a real clear picture of the bass region.
Sounds good. If I wanted to do a full frequency plot of the system, what mic/pre-amp pair should I be looking at? The other question that this brought up is how is Audyssey EQ making appropriate changes to the system frequency response with a $.50 mic? A little off the topic, but just curious.

Thanks
 

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If I wanted to do a full frequency plot of the system, what mic/pre-amp pair should I be looking at? ...
The RS meter works fine, but you have to know its limitations. Herb at Cross-Spectrum Labs offers calibrated ECM8000 and EMM-6 mics with a discount for forum members.

... The other question that this brought up is how is Audyssey EQ making appropriate changes to the system frequency response with a $.50 mic? A little off the topic, but just curious. ...
For them, the issue is the consistency of the microphones more than the accuracy, so as to control the variation; they deal with accuracy by building the calibration file into the receivers.
 

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The other question that this brought up is how is Audyssey EQ making appropriate changes to the system frequency response with a $.50 mic?
The same way we can take an accurate <3 kHz response reading with a cheap SPL meter: a calibration file that compensates for the mic’s deviation from response. In the case of Audyssey, the calibration is built into the electronics of the hardware.

Regards,
Wayne
 

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Sounds good. If I wanted to do a full frequency plot of the system, what mic/pre-amp pair should I be looking at?
As bill mentioned, you can buy a calibrated mic from CrossSpectrum for a very reasonable price.

As far as the pre amp, since it appears that you already have a viable sound card, you might add a small project mixer like the Behringer Xenyx 502. If you want a “cleaner” solution that requires fewer cables and connections, a USB audio interface with a built-in pre amp and phantom power is an option, such as the Tascam US122L or M-Audio Mobile Pre. With the interfaces, its best to Google some reviews to make sure there are no know issues between the one you’re considering and your OS.

Regards,
Wayne
 

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The sound card cal trace looks odd, something is not quite right there. Make sure there are no sound card effects active and the none of the input is being mixed back into the output.
 

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Discussion Starter · #17 ·
Well after a bit of thought I am interested in measuring frequency response over the whole listening spectrum, so I purchased a calibrated ECM8000 that was refurbished from Cross Spectrum. I am still a little bit confused with the choice of pre-amp USB interfaces though. I looked at the Tascam US-122MKII and the M-Audio Mobile Pre, but the reviews are a bit mixed. I would like to use the pre-amp in my DJ gear also, so my question is does the sampling rate of the DAC in the USB-interface effect the measurements that I will be taking with the ECM8000. I'm not sure what the sampling rate of REW is, but as long as the sampling rate of the USB-interface is greater than what REW is measuring I should be good right? Additionally, I am running Windows 7 if anyone has had experience with a specific USB-Interface and Windows 7.
 

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Discussion Starter · #18 ·
I personally am leaning towards the Focusrite Saffire 6 USB Audio Interface, or the Yamaha Audiogram3. They seem to offer a little more versatility, and also look like they are built a little better. Has anyone used these products?
 

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I looked at the Tascam US-122MKII and the M-Audio Mobile Pre, but the reviews are a bit mixed.
It’s good that you’re researching before making the plunge for an USB interface. :T

The thing to keep in mind with complaints in the reviews is determining if the issue they’re having is relevant to you or not. For instance, most of the reviewers are using their interfaces for recording to a computer, which is drastically different from what you’re going to be using it for. If they complain that the connectors seem cheap, or that it doesn’t sound good, or it’s noisier than they wanted, that has no bearing on your application, which is REW.

Basically, the only thing you’re concerned with is whether or not the interface will “play nice” with your operating system. Those are the comments you want to pay attention to. If there are compatibility issues you’ll probably have a problem generating a calibration file, and that’s the thing that frustrates new REW users the most. If you find positive reviews where the users have the same operating system as you - there’s your candidate. :T

Regards,
Wayne
 

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Discussion Starter · #20 ·
Great advice Wayne, and I will take all that I can get. I am going to go with either the Focusrite Saffire 6 USB Audio Interface as it has no bad reviews that I can find, and its OS friendly, or the MBOX2 as a friend has one for sale. I'll let you all know when I get them plugged in, I'm sure I'll have some questions.

Thanks again!!
 
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