Couple odd problems with REW

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I calibrated my sound card then did a measurement while still in loopback and it comes out perfectly flat but it always has a large (like +10db) offset.

I expected it to come out like it does in the online help where it's near the soundcard calibration curve.

Another issue I have is when doing actual measurements software comes back saying I have something above 60db during the pre silence check.

My soundcard is pretty good, it's a USB TASCAM 144 made by Teac. The loopback test using RplusD says 66 S/N. If I remove the Mic it does pass the silence test.

The Mic is a Behringer ECM 8000. The TASCAM is supplying 48v phatom power (from the USB power).

Two other Mics (Radio Shack SPL) and a Professional Calibrated Genrad SPL meter all have this issue. I think I have something goofed up with my levels.

I went ahead with the measurement and there is lump bumpy mess just under 60db with a few bumps punching through. I did the SPL calibration etc. But I had to calibrate SPL to 95 to get it to match.

What is a reasonable noise floor in a living room of a small house not near a busy road. Nothing treated has windows etc. My Genrad says I have something at 50db in the low frequency range (somewhere between 20-250hz). The meter is good to 30db.

[brucek]

I calibrated my sound card then did a measurement while still in loopback and it comes out perfectly flat but it always has a large (like +10db) offset.

Did you Calibrate SPL and Set Target Level at 75dB?

Another issue I have is when doing actual measurements software comes back saying I have something above 60db during the pre silence check.

Can you show me the Impulse response graph.

I think I have something goofed up with my levels.

Can you show me your REW Settings/Soundcard tab.

brucek

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Yes I did calibrate SPL, it was fairly high though 96.

I went to collect the data for you and everything ran fine this time.

I'm little confused over what levels I can adjust on the soundcard (External USB with analog controls in my case) after I have calibrated the sound card. Can I change Mic input sensitivity and output level? I would assume I could because these devices have no relation to loopback levels.

What should I target the handheld SPL Meter for (i.e. Stereo system volume) during REW SPL calibration?
If I target 75, then when I measure it says signal is way too low. So I end up calibrating fairly.

I keep playing with Speaker Output and Mic sensitivty I sometimes think I'm lopsided and cause problems.

Last night when I had trouble I was calibrated at SPL 96, this morning I calibrated at 85.

I still attached what you asked in case you see anything wrong. I did an impulse with speaker connected and one with out to get a baseline of what the Mic was picking up with no signal. Last night it was much uglier than what's attached. Way less flat with big peaks punching through at 60.

Thanks for the quick response.

[brucek]

Yes I did calibrate SPL, it was fairly high though 96

You are supposed to set it to 75db.....

What should I target the handheld SPL Meter for (i.e. Stereo system volume) during REW SPL calibration?

Hopefully, you've read the REW HELP FILES......

With the mic connected to the line-in, and the line-out going to the receivers AUX in, and the receiver in stereo mode, with the mains disconnected so the sub only is playing, then when you run the Check Levels routine, set the SPL level in the room to 75dB at the listening position with the receivers volume control (after you have the REW output level set properly as seen below.

Then when you get to the part in the Check levels routine where you set the input level, set it to -12dB as shown in the pic below.

Then run the Calibrate SPL and set to 75dB..

Measure.



THis shows the interaction of the Windows mixer and the controls in REW
Windows MIXER plus REW Mixer control interaction.jpg


This shows the REW controls and the input output effects.
Check Levels soundcard_cal.jpg

brucek

[JohnM]

Looks like you are running Vista so you won't have the same mixer setup as windows. Right click the system tray volume icon when REW is running and select "Open Volume Mixer", make sure the volume control for REW is at the top.

The reason for the offset in the soundcard cal would be having the levels set higher than the reference level during cal. A screenshot of the Impulse Controls graph page after doing a loopback will verify that all is OK for that measurement. Beyond that it will be a case of getting the input level on your TASCAM set correctly. If you use the RS SPL meter then that is a direct line level input and might be an easier way to get going than having to tweak the mic input gain with the ECM.

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Correct I have very little control in Vista, but more so because I think it's a Analog Mixer box more than a sound card.

Every time I try to calibrate to 75, when I go to measure it says it's way too low a volume. and I assume if I boost Line-Out, Line-In on TASCAM or Output on Stereo to solve it then I'm no longer SPL calibrated and need to do it over (with value above 75). I can raise Output level in REW (or Vista which I have almost Maxed) but the REW starts to complain I'm out of normal range (I think, I need to retrace my steps again with this feedback). If I recall REW has recomended output range (not to exceed -6db). Sorry if I'm mixing things up, it was a late night.

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Ok, I get it now. I need to just simply set the SPL Cal to 75 (only during the loopback) so that 75 is 75 during a loopback measurement check. I now get a flat line on top of SoundCard when doing a measure during loopback.

One strange thing though is I must of run into a small bug. Because after I understood this I did and the first time it was now 3db off. So I calibrated SoundCard again. Then it jumped to like 15db offset. Strange. The only thing I did was close software, open again and then it was right on. So something wasn't quite set to what it was showing.

Also I have recalibrate SPL when I rehook the real Mic up (comparing to a handheld SPL) and it doesn't have to be exactly 75db (that's where I was confused). Also have to apply my Mic Cal file at that time (probably before I do SPL cal).

[brucek]

Also I have recalibrate SPL when I rehook the real Mic up (comparing to a handheld SPL) and it doesn't have to be exactly 75db

Well, you still need to use your SPL meter to set the 75dBSPL level in the room with your receiver (while holding it near the ECM8000)... and then you put that SPL meter away, and then set the input level into REW from the ECM8000......

bruek

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brucek wrote:

Well, you still need to use your SPL meter to set the 75dBSPL level in the room with your receiver (while holding it near the ECM8000)... and then you put that SPL meter away, and then set the input level into REW from the ECM8000......

bruek

Correct me if I'm wrong but...

I don't think it has to be 75db though (it does on loopback back test though).

You're just calibrating REW/SoundCard/Mic against a real SPL meter. As long as you match something in the vicinity of 75 it shouldn't matter. I used 84 because I used RPlusD to find the peak S/N of output and Mic sensitivity.

As long as REW knows what 84db SPL is, it should be ok. It's basically a single point calibration and you should be able to use any point (within reason).

The directions say on the Calibration dialog just put in what your SPL meter reads. If it had to be 75 then it would say adjust your audio system/soundcard to output 75db and not allow you to type in a value. You can leave it 75 and adjust Stereo/SoundCard output if you want to. Both ways should be valid.

If I calibrate at 75 I'll just end up fiddling with StereoOutput/Soundard settings to get measurements in a good full scale range, so I just leave that and get REW to understand what SPL level I'm at.

[brucek]

I don't think it has to be 75db though

Not at all. It can be anything you like.

Since we spend a lot of time evaluating and comparing responses here, it's nice to have a pseudo standard so everyone is on the same page. So we came up with a standard set of vertical and horizontal axis with a reasonable swing of 60dB that allows a +/-30dB swing around a 75dB target.

It works quite well and everyone seems game to comply.

But sure, if you like 84dB - fill your boots....

brucek

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brucek wrote:

Not at all. It can be anything you like.

Since we spend a lot of time evaluating and comparing responses here, it's nice to have a pseudo standard so everyone is on the same page. So we came up with a standard set of vertical and horizontal axis with a reasonable swing of 60dB that allows a +/-30dB swing around a 75dB target.

It works quite well and everyone seems game to comply.

But sure, if you like 84dB - fill your boots....

brucek

Ah ok, so when measuring the SPL during SPL CAL you want 75 on the Meter in hand too so we are all on the same scale. Tha tmakes sense.

But when I do that I tend to get too low a level sometimes and can't turn gain up much more on the Mic.

I'll try again now that I'm more comfortable with the software.