# Check Level questions



## THX-UltraII (Nov 19, 2008)

I own a Behringer UAC-202 soundcard and calibrated this card tonight. This went well. I also own the ECM8000 mic and downloaded the .cal file for this mic.

Do I still need to do the step 'check levels' and if so. Can someone explain to me in another way than the online help file what I EXACTLY need to do here? I don t understand this at all. And when the help file says that eg. 24fs db is preffered, is this than the red horizontal line or the top of the bar?


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## JohnM (Apr 11, 2006)

The levels are the numbers shown, which correspond to the top of the bar. The red line shows the peaks. The idea of check levels is to get the gain controls for the input set so that at a typical measurement level (e.g. 75 dB) the input signal is neither too high (so leaving no headroom to cope with peaks in the measurement) nor too low (so not rising far enough above the background noise).


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## THX-UltraII (Nov 19, 2008)

thxz for your reply but I still don t understand it m8 :dontknow: (I m embarrased)

Can you tell me specific what I need to do?

Some more notes:

- I thought that for a RT60 measurement a high volume (far more than 75dB) is needed?

- I calibrated my Behringer UAC-202 soundcard first and I had to put the volume of the line in and output volume in the windows audio settings to 100% to get the following result:



This also means that the Input Volume in the Soundcard Preferences of REW5 is on 1,000 now.
Is this 1,000 Input Volume my starting position when I begin with the Check Levels procedure?

Is the goal of the Check Levels procedure to get the top of the R channel Input bar and the same level as the top of the Output bar (which is -12,1)?

Pff, sorry m8, but I still don t understand what I need to do here......


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## JohnM (Apr 11, 2006)

Instructions are shown in the Help section of the soundcard preferences panel, they are very detailed and say exactly what you need to do. Click "check Levels" and read through and follow the instructions.


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## Tufelhundin (Jan 25, 2010)

Remember also yesterday I mentioned in your thread that the only difference I saw with your settings and mine was that my input was at 1.000.


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## THX-UltraII (Nov 19, 2008)

i did more testing tonight and here are some pics i would like to be advised about. Are these ok?:

soundcard cal:


CHECK LEVELS with windows audio settings on 100% volume and all knobs on xenyx on 12oclock exept for the MAIN MIX knob that s at 75% and the BALANCE KNOB that is to the far RIGHT:


the check levels option before making the actual measurement which says level OK


the measurement itself running. Don t understand the red 0,00 headroom notification, I also get a clipping notification after the measurement:


some first results:




please advise

thxz


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## JohnM (Apr 11, 2006)

The clipping notification means that at some point during the measurement (probably that big peak about 95Hz) the input level reached the maximum the soundcard can represent - and the actual signal most likely continued on past that. Lower the sweep level setting a few dB (try -15dB) and try again, if it still clips, lower it a bit more until you get a measurement without clipping. Otherwise looks OK.


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## THX-UltraII (Nov 19, 2008)

I ll try it tonight.

When i m going to lower the sweep level, do i have to turn UP the volume of my receiver?


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## JohnM (Apr 11, 2006)

No, if you did that you would be back to square one. You can reduce the sweep level, or you can reduce the receiver volume, either will have the effect you want which is to reduce the volume your mic is picking up.


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## gperkins_1973 (Aug 25, 2008)

I can never get my levels high when using my turtle beach sound card. I usually get the spl meter to read 75db and go with it. I then calibrate the spl meter to 75db afterwards and just run a measurement. Never have any problems.


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## THX-UltraII (Nov 19, 2008)

I still don t understand. I tried to lower the volume of my receiver so it s just above the notification that the volume is too low. But when doing a measurement I still get RED NUMBERS at Head Room (1,5 or 2,5). I don t get the clipping notification anymore but I assume that the head room number has to be higher?

I also found out that when I adjust the volume knobs of my XenYX 502 it also has effect on the Check Levels before running a measurement but I don t know how to set them the correct way.


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## JohnM (Apr 11, 2006)

THX-UltraII said:


> I don t get the clipping notification anymore but I assume that the head room number has to be higher?


No, bigger than zero is OK, it is red to warn that you are close to running out.



> I also found out that when I adjust the volume knobs of my XenYX 502 it also has effect on the Check Levels before running a measurement but I don t know how to set them the correct way.


Leave them mid scale, as advised last week. You don't need to keep running check levels.


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## THX-UltraII (Nov 19, 2008)

thxz for your help`

how about these results? how do i read these and do they look good or not?


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## JohnM (Apr 11, 2006)

The measurement looks fine. You can just turn off the phase trace, you don't need to worry about it. For the SPL response, take a look at the Please Read: Posting a graph for details of how to set the plots up so they can be more easily interpreted. You have some fairly sharp peaks and dips that could probably be improved by placement (of listening position, sub and speakers) and room treatments.


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## THX-UltraII (Nov 19, 2008)

Thxz, I dont understand why there are so many dips and peaks. My room is professionally acoustically threated with a sombra D ceiling, flutter echo modules, diffusers and a lot of specific basstraps. I also have a Velodyne DD18 with a build in EQ. The dd18 eq graph tells me I have a perfect response from 0-120hz and after this I have a good response from 120-200hz. Why does rew shows a large dip at 80hz while my dd18 tells me its good?


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## JohnM (Apr 11, 2006)

THX-UltraII said:


> The dd18 eq graph tells me I have a perfect response from 0-120hz and after this I have a good response from 120-200hz. Why does rew shows a large dip at 80hz while my dd18 tells me its good?


The DD18 graph is probably quite heavily smoothed.


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## THX-UltraII (Nov 19, 2008)

The dd18 graph does not have a dip at 80hz, not even a small dip.


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## JohnM (Apr 11, 2006)

What frequency is your crossover set to, 80Hz? When the DD18 produces its plot are the main speakers running or just the sub? The 80Hz dip may be due to incorrect polarity of the sub relative to the mains, try inverting the sub polarity and making another measurement.

You can also try applying smoothing to your measurement to see how it compares to what you have seen previously, the smoothing controls are in the graph menu.


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## Tufelhundin (Jan 25, 2010)

THX-UltraII said:


> Thxz, I dont understand why there are so many dips and peaks. My room is professionally acoustically threated with a sombra D ceiling, flutter echo modules, diffusers and a lot of specific basstraps. I also have a Velodyne DD18 with a build in EQ. The dd18 eq graph tells me I have a perfect response from 0-120hz and after this I have a good response from 120-200hz. *Why does rew shows a large dip at 80hz while my dd18 tells me its good*?


Concerning the bold question above....I'll answer with a question.

When you run REW is one "or both" of your front speakers still connected?


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## THX-UltraII (Nov 19, 2008)

@JohnM
crossover is set @80Hz. When running the sweep of the DD18 both the subwoofer and mains are running the signal. The polarity and phase are already in good position (A velodyne expert did the setup (phase, polarity, volume, EQ etc) of the DD18 at my home with professional equipment)).

@Tufelhundin
I ve connected both my L and R

EDIT:
Here s the graph of the DD18


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## Tufelhundin (Jan 25, 2010)

THX-UltraII said:


> @JohnM
> crossover is set @80Hz. When running the sweep of the DD18 both the subwoofer and mains are running the signal. The polarity and phase are already in good position (A velodyne expert did the setup (phase, polarity, volume, EQ etc) of the DD18 at my home with professional equipment)).
> 
> *@Tufelhundin
> ...


This graph of the DD, is it sub only? If so, then unhook your FL/FR and run REW sub only and see if its similar. Your FL/FR speakers run with your sub may be the problem.


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## THX-UltraII (Nov 19, 2008)

@tufelhudin
No, this is sub+fronts running

@JohnM
crossover is set @80Hz. When running the sweep of the DD18 both the subwoofer and mains are running the signal. The polarity and phase are already in good position (A velodyne expert did the setup (phase, polarity, volume, EQ etc) of the DD18 at my home with professional equipment)).

edit (more questions):
here s a new graph (smooth 1/3). What I don t understand is that the dB is going down roughly all the way from 15 to 25k. This can t be good right? I assume it has to be a line as flat as possible and as horizontal as possible from 15 to 25k. If so, then I m in trouble .
One more thing i don t understand. On my velodyne dd18 graph you see a flat horizontal line from 15-30hz while rew shows a flat downgoing line.


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## JohnM (Apr 11, 2006)

THX-UltraII said:


> When running the sweep of the DD18 both the subwoofer and mains are running the signal.


When both mains run at the same time they can cause comb filtering if the mic is not exactly the same distance from each speaker. Mainly an issue at higher frequencies if the difference in distance is small, but the bigger the difference the lower the effect extends. Try measuring your sub with your main speakers off and with one main speaker.



> What I don t understand is that the dB is going down roughly all the way from 15 to 25k. This can t be good right? I assume it has to be a line as flat as possible and as horizontal as possible from 15 to 25k.


This is normal for measurements made from the listening position. The measurement picks up the sound that comes directly from the speaker and also the sound that bounces off the walls, floor and ceiling. The off-axis response of loudspeakers drops off rapidly at high frequencies, and high frequencies are also absorbed more in typical rooms. The overall effect is that the response usually slopes down as frequency increases. If you measured in an anechoic chamber, which would remove the off-axis contribution of the loudspeaker and the contribution from surfaces, you would see a much flatter response. Measuring closer to the speaker (about 1m away, say) would also give a flatter response at high frequencies.




> One more thing i don t understand. On my velodyne dd18 graph you see a flat horizontal line from 15-30hz while rew shows a flat downgoing line.


The REW response includes the effect of the mic cal file (the black line on your plot). The drop in the mic response at low frequencies is being corrected for by REW and causing a rise in the measurement. You are using the generic ECM8000 cal file, which will not exactly match the response of your particular microphone - yours may have less roll-off at low frequencies and so not need as much correction. To know for sure you would need to get your mic calibrated.


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