questions getting started

I am working with a guy at GIK Acoustics to treat my room. He referred me to the Room EQ Wizard. So far, I am having trouble getting him a graph that meets his expectations. In particular, he says my CAL level is too low, and should be more like 80db. I am using an RME Fireface 800 to produce the CAL. Here is a typical measurement that this produces. Any suggestions for how to raise the CAL level?

[brucek]

There's more problems than that.

Have you done a soundcard calibration? Let's see the graph of that.

brucek

Here is my soundcard calibration. Again, I did this with an RME Fireface 800, by just plugging the first two inputs into the first two outputs.

[brucek]

OK, that looks fine.

You will have to choose the input and an output that you did the calibration on and use those.

Connect the mic to the line-in and connect the line-out to your receiver and be sure it's in stereo mode with all the soundfields and effects off. If you're doing sub only, then turn off the mains.

Put the mic at the listening position and run the Check Levels routine. Set the listening position to 75dB on your SPL meter using the volume control of the receiver, then set the input level of REW.

Then run the Calibrate SPL routine and it should match around 75dB.

Run the Measure routine and it will be at ~75dB...

brucek

OK, here's what I have done. I kept the same output as I used to do the calibration. It is now feeding into my mixer, which then feeds into my two studio monitors. (I do not have a subwoofer.) I then plugged a mic cable into an input on my Fireface, and selected that input in the settings tab of REW. (This is something I have failed to do before.) I then took the attached measurement. Bear in mind, I am doing this with a mic & preamp, not with an SPL meter. Does this look any better? (I'm thinking not, since a lot of it falls outside the typical range.)

[JohnM]

Looks like it is probably a valid measurement, you just need to run through the SPL calibration steps to get the SPL figures shown on REW to correspond to actual SPL in your room. Click the "Calibrate" button on the SPL meter in the toolbar and make sure to select "Use REW speaker cal signal" as the test signal in the dialog that pops up.

[brucek]

Bear in mind, I am doing this with a mic & preamp, not with an SPL meter. Does this look any better?

Yeah, no problem. When a microphone is used rather than an SPL meter, you have to cheat a bit.

During the Check Levels routine, you'll set the listening position level to 75dBSPL. In your case simply set it in the ball park (whatever level seems reasonable). Then when the Check Levels routine is complete, run the Calibrate SPL level routine and set the level to 75dB. Now your plots will be properly placed on the REW graph. Your plot is there, it's simply set too high. You must run the Calibrate SPL routine (that everyone is telling you to run )..

brucek

Thanks for your suggestions. I will try them out later tonight when I get home from work. In the meantime, I have a couple questions about my mic input:

1. I am plugging the mic into an XLR in on my Fireface. Is this a problem? I also tried plugging the mic into a line level input on the back of my Fireface, but then I couldn't generate enough of a signal to do a measurement.

2. Plugging my mic into one input on my Fireface just produces a mono signal. I would have to plug in two mics into two outputs to produce a stereo signal. Do I need to worry about this, or is using just one mic sufficient?

3. I have been using a Shure SM58 mic to take measurements. This is a uni-directional cardioid mic. I have nicer mics than this. Should I be using one of them instead?

[brucek]

I am plugging the mic into an XLR in on my Fireface. Is this a problem? I also tried plugging the mic into a line level input

Microphones aren't line-level devices, so you wouldn't plug one into a line-input, you would use the internal mic-preamp that then feeds the line stage internally.

I would have to plug in two mics into two outputs to produce a stereo signal

Stereo signal? REW produces a mono output from a single line-out jack. If you want to feed a receiver with left and right, you simply use a Y-splitter before the receiver.

I have nicer mics than this. Should I be using one of them instead?

An omni-directional is best, but of course you require a calibration file to load into REW for any microphone or SPL meter.

brucek

REW produces a mono output from a single line-out jack. If you want to feed a receiver with left and right, you simply use a Y-splitter before the receiver.

But do I need to use a Y-splitter if I am just taking basic measurements with REW? Or am I fine to just have one mic plugged into one input on my Fireface?

An omni-directional is best

Well, all of my mics are cardioid. Will it make my life easier to just go out and buy an SPL meter?

[brucek]

But do I need to use a Y-splitter if I am just taking basic measurements with REW? Or am I fine to just have one mic plugged into one input on my Fireface?

I think you missed my point. The microphone is mono, the mic preamp-in is mono. The line-out is mono. Now, if you plug that mono cable into one channel of your AVR receiver, it will come out one speaker. If that's what you want to measure, then that's the hookup. If you happened to want to test both mains speakers at the same time, you would plug a Y-splitter into your left and right AUX or CD input of the receiver and plug the mono REW signal into that - now two speakers play.

all of my mics are cardioid. Will it make my life easier to just go out and buy an SPL meter?

I don't think it's necessary. As long as you have the calibration file for your microphone, it would ne fine.

brucek

As long as you have the calibration file for your microphone

Is this the SPL calibration that you've been talking about? This is in addition to the soundcard calibration, correct?

[brucek]

Is this the SPL calibration that you've been talking about? This is in addition to the soundcard calibration, correct?

Yes, it's in addition to the soundcard calibration file.

No, it's not the SPL calibration routine I've been talking about.

It's a file that compensates for the imperfect response of a microphone or an SPL meter.

Have you read the REW HELP FILES.

brucek

Yes, I've been working from the Help file, but I get a little lost in it. In particular, I'm not always sure which, if any, steps I can ignore given that I am not using an SPL meter.

Can you direct me to the section of the help file that discusses how to generate a calibration file for a microphone? (I'm thinking I need to focus on the 'Checking Levels' and 'Calibrating the SPL Reading' sections--is that right?)

[brucek]

Here's the steps. They all refer to the Right Channel only. Do not connect anything to the Left channel and don't check the Use Left Channel as Calibrate Ref box.

1. Connect loopback on one channel of the soundcard from line-out to line-in and create and save the soundcard.cal file in REW by running Soundcard Measure routine. (no other cables connected).

2. Run the Calibrate SPL routine and set to 75dB.

3. With only the loopback still connected, run a full range Measure to be sure the graph shows a perfectly flat response.

4. Disconnect the loopback cable and connect the Mic. Also connect from the line-out to the receiver.

5. Load the microphone calibration file.

6. Run Check levels and set listening position to ~75dB or reasonable level..

7. Run Calibrate SPL and set to 75dB.

8. Measure.

brucek

Thank you for spelling it out. This should help. A few more questions though:

3. With only the loopback still connected, run a full range Measure to be sure the graph shows a perfectly flat response.

I have tried to do this, but I don't get a flat response. It is flat until about 50 Hz, but then drops off. It looks similar to the downward-curving line that shows up in the other graphs that I have attached.

5. Load the microphone calibration file.

Is this something I need to generate based on the mic that I am using? How do I do this?

[brucek]

I have tried to do this, but I don't get a flat response. It is flat until about 50 Hz, but then drops off. It looks similar to the downward-curving line that shows up in the other graphs that I have attached.

If you have saved the soundcard.cal file you created in the slot provided in the Settings page, then a straight page should be perfectly flat. Proceed no further until it does.

Think about what is happening in this situation. First you have made a measure of the cable and saved the response as a file that is reverse applied to any further measures. So, if you measure the same cable and apply the reverse response of the soundcard, it has to be flat from 2Hz to 20KHz - right? If not, you've done something wrong. Your graph of your soundcard looked fine though.

Is this something I need to generate based on the mic that I am using? How do I do this?

We provide the cal files for the ECM8000 microphone and also the Galaxy and Radio Shack SPL meters on the download page. Any other mics have to have a calibration file that you create with Notepad. You require a calibration file from the manufacturer or in a pinch, a calibration graph for the microphonebefore you can create one.

brucek

I didn't want to deal with getting a calibration file specific to my microphone so I went ahead and just bought the Radio Shack SPL meter (33-2055). I followed all the steps you gave me (I think) and came up with the following graphs. The first one is calibrated at 75db. The second is at 90db.

One message that I do keep getting says "very low signal level -- the highest level is just -90.3dbFS." I believe this is referring to the right input channel, which stays that low since I have nothing plugged into it. The level on the left input channel generally reaches the -10 to -20dbFS range during the frequency sweep.

Incidentally, this is a relatively small, rectangular room I am measuring (about 9' x 13' x 8' tall), and I have not treated it yet at all, if this helps you know what to expect in a measurement.

[JohnM]

If you are using the left input you need to tell REW that on the soundcard settings page. At the moment it is listening to the right input and telling you that it isn't hearing anything.

[brucek]

I believe this is referring to the right input channel, which stays that low since I have nothing plugged into it.

I guess you missed the part where I said:

Here's the steps. They all refer to the Right Channel only. Do not connect anything to the Left channel and don't check the Use Left Channel as Calibrate Ref box.

brucek

Here's the steps. They all refer to the Right Channel only. Do not connect anything to the Left channel and don't check the Use Left Channel as Calibrate Ref box.

I was actually trying to follow this but I guess I messed up. I was using just one input and output on the soundcard, and the output was going to just my right monitor. I believe I had the right channel selected in the soundcard settings page (though I could be wrong), and I did not check the Use Left Channel as Calibrate Ref box. So now, thinking back on it more closely, the measurement error message
doesn't exactly make sense to me. When it did a sweep, the left channel was the one registering the signal, while the right channel stayed at -90db the whole time. I guess I will have to pay a little more attention to this when I try it again tonight.

If you are using the left input you need to tell REW that on the soundcard settings page.

I believe it only lets me set one input--left or right. Yet when it does a measurement, it gives me an error message because there isn't an input signal on both channels. Any suggestions?


Aside from this issue though, how do the graphs look? Can you tell anything yet, or do we need to resolve this other issue before drawing any conclusions?

[JohnM]

swo17 wrote:

I believe it only lets me set one input--left or right. Yet when it does a measurement, it gives me an error message because there isn't an input signal on both channels. Any suggestions?

No, it doesn't, it is giving you an error message because you have told it the input is on the right channel of the soundcard but you have actually plugged into the left channel.

swo17 wrote:

Aside from this issue though, how do the graphs look? Can you tell anything yet, or do we need to resolve this other issue before drawing any conclusions?

The graphs at the moment are just the noise on an unconnected input.

Now I'm confused. I did have the right channel selected this whole time. Yet when I run a measurement, it only picks up input on the left channel. (You can see this in the attached diagram of one of my measurements.) I have also attached my other settings. Do any of them look wrong?

My cable setup is as follows: a cable from the SPL meter into input channel 1 of my Fireface, a cable out from output channel 1 of the Fireface to one of the right channels on my mixer (nothing else plugged into my Fireface), and from the right XLR out of my mixer to my right monitor. Does any of this sound wrong?

I also noticed that if I change the input in the soundcard settings to the left instead of the right, then instead of saying that the highest volume is -90db, it's more like -20db (still too low, it says). The response graph then falls well above the cal lines.

[brucek]

Do any of them look wrong?

Don't set the sweep level to -3dB. It's clipping the output. See the Red in the output VU meter? Leave it at -12dB.

I can see that your input volume is set to 0. You need to turn it up to get a level of ~ -12dB.

I also noticed that if I change the input in the soundcard settings to the left instead of the right, then instead of saying that the highest volume is -90db, it's more like -20db (still too low, it says).

That should be telling you that you simply have the wrong cable connecting to right or left channel somewhere. Find the right channel and use that.

Are you not able to select your device in the Output and Input Device pulldowns?

brucek

OK, I changed the sweep level to -12db, but it wouldn't let me adjust the input volume. (Even if I type in a new value, as soon as I go somewhere else it goes back to 0.001.) I also manually selected the input and output channels.

I figured out that I need to plug my input into channel 2 instead of channel 1 on my Fireface in order to get a right channel signal. Now I get the following graph--how does it look?