# Mic/meter calibration problem



## SomeGuy (Feb 25, 2010)

I've tried to find this issue in other threads, so sorry if this has been brought up before.

I followed the instructions (several times) and everything seems to go well but my mic/meter calibration graph (black line) starts falling off at 100 hz, so my room measurements end up with a huge low frequency hump.

The only things that seem odd are that I have to set the output volume on the 'settings/soundcard' tab to 0.156 to avoid clipping and the 'level' on the 'make a measurement' has to be set at -38 to avoid clipping. The resulting test tone plays at a lower volume than the slight ground loop hum I get from connecting my PC to my receiver, and the receiver's volume has to be set very high to reach 75db when I set the level.

OS is WinXP, soundcard is IDT built-in on my Intel motherboard, so I'm not sure of the model, but it does have an optical output and support for 7.1 Dolby surround, so I assume it's not an el-cheapo piece of garbage. It does have a line-in.

Does anybody know what I'm doing wrong? Thanx in advance.


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## SomeGuy (Feb 25, 2010)

PS. I'm using the Radio Shack analog meter.


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## SomeGuy (Feb 25, 2010)

PPS. When I calibrate the sound card, I get a flat result, but the level is at 68.15, not 75.


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## Wayne A. Pflughaupt (Apr 13, 2006)

It’s hard to determine anything meaningful because of the graph scaling you’re using. Please re-post them with a vertical axis of 45-105 dB.

Regards,
Wayne


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## SomeGuy (Feb 25, 2010)

No prob. I posted them with a larger scale because a lot of info is cut off with the 45-105 scale.


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## laser188139 (Sep 19, 2009)

In your first graph, all the behavior below 10Hz has the appearance of an artifact of the mic calibration file, and differences between your meter and the generic file, not of any real response. When using the RS meter, there is no reason to post any curve below 10Hz. There is a good chance that some of what you see below 20Hz is the difference between your meter and the generic file. If you follow the advice in the readme to use 15Hz as the lower frequency limit, you will have no problem following Wayne's advice to use 45Hz-105Hz for the vertical limits. 20kHz for the upper frequency limit is fine, the results might be interesting, although the RS meters vary and are not consistent above 3kHz. 

Bill


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## SomeGuy (Feb 25, 2010)

laser188139 said:


> In your first graph, all the behavior below 10Hz has the appearance of an artifact of the mic calibration file, and differences between your meter and the generic file, not of any real response. When using the RS meter, there is no reason to post any curve below 10Hz. There is a good chance that some of what you see below 20Hz is the difference between your meter and the generic file. If you follow the advice in the readme to use 15Hz as the lower frequency limit, you will have no problem following Wayne's advice to use 45Hz-105Hz for the vertical limits. 20kHz for the upper frequency limit is fine, the results might be interesting, although the RS meters vary and are not consistent above 3kHz.
> 
> Bill


My first thought was that my meter didn't fit with the calibration file. I even re-downloaded the file in case I'd done something to corrupt it. The meter's about 2 years old. I used it to set up my system and it's been sitting in a drawer (with the battery removed) since then.


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## laser188139 (Sep 19, 2009)

It's not so much that it doesn't fit, but that there is manufacturing variation. So the generic file does a pretty good job of representing the behavior common across RS meters. But as you get to the ends of the freq spectrum, they vary too much for one file to fit all. 

For most people, this variation does not matter. After all, even if your sub reaches below 15Hz, your ears don't. If your goal is to play with things at the high end, e.g., replace tweeters and crossover filters in a front speaker, then you may need to invest in a calibrated microphone. 

Bill


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## laser188139 (Sep 19, 2009)

I meant to ask, too, were you using the analog output to drive your AVR? You mentioned the optical out on your soundcard, so I was not sure. It is not a good assumption, after calibrating the soundcard using the analog loopback, that the digital output behaves identically.

Bill


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## SomeGuy (Feb 25, 2010)

laser188139 said:


> I meant to ask, too, were you using the analog output to drive your AVR? You mentioned the optical out on your soundcard, so I was not sure. It is not a good assumption, after calibrating the soundcard using the analog loopback, that the digital output behaves identically.
> 
> Bill


Yes, I'm using the analog output. Although the ultra-low frequencies aren't a concern, the up-slope looks like it starts around 40hz. 

Is the test sweep supposed to be at a very low volume? When I check the level, I have to crank the receiver's volume up so high to hit 75db that the ground loop hum from connecting my PC to my receiver is louder than the sweep tone, which isn't very loud.


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## laser188139 (Sep 19, 2009)

That is a different question, and I was hoping someone else would chime in. When you indicated that any higher level on the output level caused clipping during the soundcard calibration, was that clipping at the output stage? Or on the input? 

Yes, I found your story strange, that you seemed to say you had clipping on the output stage, and had to drop its level so low, but that by just avoiding clipping in the output stage you had to turn the AVR volume up so high. 

It can happen that you need to set different levels during the soundcard calibration than during normal use. After building the soundcard calibration file, saving it, and loading the mic calibration file, you need to perform the Check Levels process again and verify these. As described in the helpfile, you want the output stage level set high enough. Then you adjust the volume on the AVR to get around 75dB and adjust the input stage level to ensure it is high enough for REW to obtain reasonable results. Then you can perform the Calibrate step to adjust the AVR volume to precisely 75dB and set this as its SPL level, or Calibrate the SPL level to what you are actually reading.

Bill


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## glaufman (Nov 25, 2007)

I'm just guessing at the moment, because I haven't read everything here, but ...
If you have to turn the level down that low to avoid clipping, and it sounds as low as your ground loop hum, etc etc etc I'd say you need to backtrack a bit.
For starters, I believe your soundcard cal (dotted black line) is falling off so early, in addition to any problems with the mic cal. 
Does that soundcard have a line out? Some optical outputs don't work very well. does it have choices for bitrate? Make sure it agrees with that in REW.
Go into soundcard settings and make sure you're not monitoring playback.


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## laser188139 (Sep 19, 2009)

Here's another idea, Eric. I've noticed on my wife's Windows XP desktop system with its Soundblaster soundcard that the REW output volume controls just the Wave volume. I need to also go into the Control Panel -> Sound -> Playback Control to ensure that the output volume is high enough (or low enough). So, if you are needing to adjust the volume down in REW to avoid clipping, but you are seeing too little output at the receiver, it could be that the Playback Control volume is set too low. 

Bill


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## SomeGuy (Feb 25, 2010)

laser188139 said:


> That is a different question, and I was hoping someone else would chime in. When you indicated that any higher level on the output level caused clipping during the soundcard calibration, was that clipping at the output stage? Or on the input?
> 
> Yes, I found your story strange, that you seemed to say you had clipping on the output stage, and had to drop its level so low, but that by just avoiding clipping in the output stage you had to turn the AVR volume up so high.
> 
> ...


The clipping was on the input, when I was checking that my sound card calibration was successful (step 7 of "calibrating the soundcard" in the help file). 

Also, when I actually measure the room, I would run "check levels" first, and have to lower the level to -25 to avoid the "level is high" warning (the meter on the "make a measurement" panel shows the right input hitting 0 unless the level is set to -25 or less), then when I hit "start measuring" I would get the clipping message (on the input) and have to lower the level to -35 to avoid that. At that point, the sound coming out of the speakers is pretty low, and the needle on the Radio Shack meter doesn't move at all during the measurement (set to the 80db setting).


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## SomeGuy (Feb 25, 2010)

glaufman said:


> I'm just guessing at the moment, because I haven't read everything here, but ...
> If you have to turn the level down that low to avoid clipping, and it sounds as low as your ground loop hum, etc etc etc I'd say you need to backtrack a bit.
> For starters, I believe your soundcard cal (dotted black line) is falling off so early, in addition to any problems with the mic cal.
> Does that soundcard have a line out? Some optical outputs don't work very well. does it have choices for bitrate? Make sure it agrees with that in REW.
> Go into soundcard settings and make sure you're not monitoring playback.


The soundcard has a line in but no line out (only speaker out). I'm using the analog speaker out. It does have a real line in. I've checked to make sure that monitoring playback is off. I also went thru every option of the soundcard program and turned off bass management, Dolby functions, etc. The soundcard's driver is up to date. I checked all the options in the "sounds and audio properties" section of Control Panel. All processing in the receiver is off and tone controls set to flat. I'm using the main speakers for testing (subwoofer is in storage till my house is finished next month... no room for it in my apartment and I'm sure the neighbors would compain). I followed the help file step by step and feel fairly confident that I'm doing everything right.


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## SomeGuy (Feb 25, 2010)

laser188139 said:


> Here's another idea, Eric. I've noticed on my wife's Windows XP desktop system with its Soundblaster soundcard that the REW output volume controls just the Wave volume. I need to also go into the Control Panel -> Sound -> Playback Control to ensure that the output volume is high enough (or low enough). So, if you are needing to adjust the volume down in REW to avoid clipping, but you are seeing too little output at the receiver, it could be that the Playback Control volume is set too low.
> 
> Bill


I just checked that and the controls in REW are changing the correct volume levels as shown in the Control Panel sound settings.


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## SomeGuy (Feb 25, 2010)

I just started over from the beginning. A couple of things I noticed. At the "SPL Calibration Procedure" step, I'm getting clipping on the input, but the help file doesn't mention what to do in that case, it just says to input the meter reading and hit "finished" (which is what I did).









When I got to making the room measurement, I had to adjust the level down to its minimum of -40 and also turn down the volume on my receiver to avoid the clipping warning.









When I took the room measurement, tho, it looks much better. The rise at low frequencies is gone.


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## JohnM (Apr 11, 2006)

The original soundcard cal indicated that the soundcard was set up to expect a sub somewhere, which is why it rolled off so early, but your latest soundcard cal looks fine so whatever bass management was running before seems to be turned off now. The next problem is your input levels, which could be due to your SPL meter being set to a low range (it should be on the 80dB setting) or the input being a mic/line type which has switched itself to the mic setting for some reason introducing a lot of additional gain - you could try the other input channel (remembering to switch the channel in REW also) to see if that is the same.


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## SomeGuy (Feb 25, 2010)

JohnM said:


> The original soundcard cal indicated that the soundcard was set up to expect a sub somewhere, which is why it rolled off so early, but your latest soundcard cal looks fine so whatever bass management was running before seems to be turned off now. The next problem is your input levels, which could be due to your SPL meter being set to a low range (it should be on the 80dB setting) or the input being a mic/line type which has switched itself to the mic setting for some reason introducing a lot of additional gain - you could try the other input channel (remembering to switch the channel in REW also) to see if that is the same.


I did go back thru all the sound card settings, and made a couple of changes (turned off the SPID output, turned off "enable multi-stream retasking" whatever that is). The line in doubles as the surround back speaker output (using a mini-stereo headphone type jack - the mic in is a separate input [I have no mic attached]), and whenever I plug anything into it, the sound card program pops up and asks which of the two functions to use, and I always pick "line in." I'll try the other channel tomoro... off to work now.

Thanx to everybody who posted.


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## laser188139 (Sep 19, 2009)

John was right that your initial soundcard calibration rolled off very early, but the new one looks much better. 



SomeGuy said:


> ... When I got to making the room measurement, I had to adjust the level down to its minimum of -40 and also turn down the volume on my receiver to avoid the clipping warning. ...


You wrote that you had to adjust "the" level down to its minimum of -40. Which level? It appears as if you adjusted the output level downward, when it was in a reasonable range. If the output is in the green range, as it appears in your first picture, you should be able to now leave it alone. 

If the clipping is happening on input, one would want to adjust the gain on the input/mic downward to just eliminate the clipping, and only if that is not sufficient should you need reduce the receiver volume so the test tone is softer than 75dB. 

Of course, after adjusting these you need to run the Calibrate SPL again. I'm guessing that you did re-calibrate the SPL levels, but that the levels in your graph appear low because you reduced the receiver volume so your actual sound levels were no longer around 75dB. 

Bill


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