# First time REW Plot



## bakerandshaker

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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## Wayne A. Pflughaupt

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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## JohnM

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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## bakerandshaker

JohnM said:


> 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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## bakerandshaker

Wayne A. Pflughaupt said:


> 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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## laser188139

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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## bakerandshaker

laser188139 said:


> 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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## bakerandshaker

Forgot to mention, I'm using the standard Radio Shack analog SPL, and the cal files for it.


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## glaufman

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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## bakerandshaker

glaufman said:


> 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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## laser188139

bakerandshaker said:


> 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. 



bakerandshaker said:


> ... 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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## Wayne A. Pflughaupt

bakerandshaker said:


> 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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## Wayne A. Pflughaupt

bakerandshaker said:


> 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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## glaufman

I second all those.


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## bakerandshaker

Thanks a lot guys, I am really liking this little community over here.


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## JohnM

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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## bakerandshaker

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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## bakerandshaker

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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## Wayne A. Pflughaupt

> 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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## bakerandshaker

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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## bakerandshaker

So I got my ECM8000 mic today, and I'm a bit confused on what a calibrated microphone is. I see the frequency plots with and without smoothing, and also the off axis polar plot. So I guess my question is the following: Is the microphone measured, and then the included mini-disk will perform the necessary corrections for measurements in REW, or is the physical microphone calibrated by variable resistors in mic circuit? I'm guessing that the disk included is to offset the response of the mic. Any help would be greatly appreciated.


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## bakerandshaker

Never mind guys, sorry. Helps to put the CD in and read before you start asking questions.


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## bakerandshaker

So I hooked up the calibrated ECM8000 mic and the Focusrite USB interface, no problems there. I am receiving errors from REW that the mic signal is to low. Its saying that it should be greater than -10 dB, and I am getting somewhere around -90 dB. I have the input gain on the channel I am using within the USB interface almost maxed out. I did load the calibration files that came with the mic, and they are in .FRD file format. REW didn't seem to have a problem when I loaded those. I know the microphone is working correctly, because when I adjust the mixer knob to balance the input and the playback, the mic is very audible. The only other information I can provide is that I did set the level on the speaker to 75 dB with a SPL meter. So what am I doing wrong here?


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## JohnM

Either REW is set to the wrong channel (e.g. mic is on left but REW has been told to listen on Right) or it is not set to the input the mic is connected to (for -90dB it is probably on a line input rather than the input the mic is connected to). Also make sure that you set up your mixer so that *none* of the mic signal is fed back out to playback when you are making measurements.


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## Wayne A. Pflughaupt

Also, make sure the phantom power is turned on...

Regards,
Wayne


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## bakerandshaker

Well I need some more advise. Here are the plots, and from what I have read here on the shack forum, these look about right. I'm just measuring my M-Audio studio monitors that I have at my PC. If everything looks good, I will wait on my cables from Monoprice and measure the home theater. Thanks for all your help everyone!


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## Wayne A. Pflughaupt

Yup, everything looks good. Including your response. :T

Regards,
Wayne


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## bakerandshaker

When I am taking measurements of my home theater should I place the mic in the primary listening position with the mic pointed towards the ceiling (90 degree angle)?


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## Wayne A. Pflughaupt

> When I am taking measurements of my home theater should I place the mic in the primary listening position with the mic pointed towards the ceiling (90 degree angle)?


The short answer for subwoofer measurements is, it doesn’t matter. 

If you’re doing full-range measurements (your main-channel speakers), the short answer is, you only point the mic towards the ceiling if you’re using a 90-degree incident (vertical) calibration file.

Horizontal orientation with the mic on-axis to the sound source (0-degrees incident) has been the traditional method for full-range, _free-field_ measurement, where the room was an open space relatively free of reflections. However, that’s mainly because most stand-alone RTAs (which until several years ago was about the only thing available) came with mics that were calibrated for on-axis measurements.

Some RTA manufacturers offered the option of 90-degree orientation for _random incident_ measurements (aka diffuse field), where the sound arrives from all directions more or less simultaneously, with equal probability and level. In other words, an exceedingly reverberant environment. Random-incident measurements required a different capsule for the measurement mic (which the manufacturer made available), for reason of the specific calibration as well as a housing better suited for 90-degree orientation. 

That just refers to the mic’s position during its calibration, however. Everything I’ve seen for actual “in the field” measurements says the standard protocol for horizontal (on-axis) measuring is 20 degrees, and 70 degrees for vertical (i.e., angled slightly forwards towards the sound source). This may have something to do with compensating for interference from the mic’s housing with the sound waves, I forget exactly why. 

The ready availability these days of mics with 90-degree calibration certainly opens up more measurement options. Others have their opinions, and maybe they’ll weigh in, but mine is that you will generally get the best results with on-axis measurements. It should be a no brainer to figure out that the home theater environment, while certainly not totally free of reflections and reverberation, more closely resembles a free-field environment than a random-incident environment. 

Vertical orientation may add more upper-frequency information from ceiling reflections than you’d get with on-axis, and as such will probably influence what the RTA displays. How much so will depend on your particular room – how “live” it is, the height of the ceiling in relation to the distance between the sound source (speaker) and measurement mic (i.e. inside or outside the “first reflection” zone), etc. Even though the ECM8000 is omnidirectional, its capsule is rather large for a measurement mic. As a result, its off-axis response (compared to on-axis) starts skewing as low as 2 kHz. So differences >2 kHz are what you might see with horizontal vs. vertical readings. (Smaller-capsule omni mics typically retain uniform 0 vs. 90-degree response at least an octave higher.)

Regards,
Wayne


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## bakerandshaker

Thanks Wayne!!

One thing that I eventually wanted to measure was how much room reflections were playing into my home theater (living room). I will be adding some acoustic treatment in the next few months, and I would like to get a measurement before and after to see how much the treatments help. I haven't dug in to deep on the forum material concerning waterfall plots with REW, but I believe they can be used to show reflections through the decay time. So, from what I gathered in measuring the sub the orientation of the mic shouldn't make much difference, assuming I use the correct calibration file. When measuring the mains, I should tilt the mic to approximately 20 degrees for on-axis measurements (towards the speaker I am measuring), and use the 0 degree incidence file included with the mic. 

Thanks again for all your time!


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## bakerandshaker

Finally got an opportunity to get some measurements. Here they are with Audyssey EQ turned on:


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## bakerandshaker

And here are the measurements I toke with Audyssey EQ turned off:


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## bakerandshaker

So I would love to get some comments on these, as they are my first round of measurements in my home theater. These were taken with only one mic location, in the sweet spot. I would like to get a better understanding of some of the other plotting features of REW, so I will start with the phase. Should I post the phase of the mains and sub separate around 80 Hz, and match the phase for blending?

I can re-affirm what I had heard in regards to Audyssey EQ. It is chopping the highs quite a bit, which may be okay for movies. It seems like it is giving a little more of a "house curve" like I have read about here. But for music, no Audyssey for this guy. It sounds like someone through a towel over my speakers, no cymbals at all!! I guess its a personal preference, which I am more inclined to prefer a flat response. Also wanted wanted to add that I have a very meager sub-woofer right now. The sub is from a Onkyo HTIB I ran for about a year. I will be upgrading to a SVS PB-12 Plus in the next few months, so that should fill in the low end frequencies a little more.

Any advice would be most appreciated, and REW v.5 is a very nice piece of software. Thanks so much for giving us such a powerful tool and your dedication to constantly improve the software John.


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## laser188139

bakerandshaker said:


> ... But for music, no Audyssey for this guy. It sounds like someone through a towel over my speakers, no cymbals at all!! ...


Does your receiver offer a choice of the Audyssey Reference curve or the Audyssey Flat curve? As you mentioned, the Audyssey Reference curve is designed for movies. Some people find the Audyssey Flat curve works better in their room for music, where others find it is too bright in their environment and the Audyssey Reference curve works best for both. 

Bill


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## bakerandshaker

I measured the flat curve also when I set up for the others. It too seemed a bit to reserved for me on the high end. The "flat" curve seemed to be the curve where no EQ was applied at all (see above post), as in the most flat through 20kHz. This is also what I noticed during critical listening, that the cymbals and high end were not natural to me. The measurements I obtained do seem a bit counter intuitive, because I have a highly reflective room. Below is the FR for the Audyssey "flat" curve:


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## bakerandshaker

Could anyone help me out with what's going on at 60Hz. Is that a mode?


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## laser188139

See how 60Hz and 120Hz (I think) look flat, not decaying across time? That's an indication that this is power noise, probably from the computer, being driven through the soundcard and into the system. If it's from the computer, it's not real and you don't need to worry about it. If you are on a laptop and can successfully take measurements while running off battery, you will probably see these disappear and then you will have confirmed that they are measurement anomalies. 

Does your USB soundcard have its own power supply? If so, the battery in the laptop might not make any difference, you may be stuck with the power noise isolation, or lack thereof, from the USB device. 

Bill


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## bakerandshaker

Yeah I will try it from the laptop, but that is a little disturbing considering I bought an Antec Signature power supply when I bought my PC. The USB sound card is powered by the USB itself, not a dedicated power supply. I never thought of associating that frequency with power operating at 60Hz though, good call!


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## SIY

I think 60Hz is more likely hum pickup from a ground loop. PS noise is typically 120Hz or its harmonics. The other strong possibility is that there's something mechanically humming nearby and the mike is telling you the truth!.


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## eyleron

*Re: First time REW Plot - did cards work?*

So, did either of those cards work for you?


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