# My first calibration session with REW



## michael123 (Jul 26, 2008)

Hi

I am very new to REW, I would like you to help me analyze the measurements I got and/or guide me to some other references I can use later to understand more in-depth

My calibration setups is Tascam 144 + Behringer ECM8000. 
I set a reference on 70db. For some reason, I got a clipping warning when I initially aimed for 75db.. Why?

I measured front speakers only, no sub.

Graph is attached

thanks in advance
Michael


----------



## brucek (Apr 11, 2006)

> I set a reference on 70db. For some reason, I got a clipping warning when I initially aimed for 75db.. Why?


I would try and use 75dB as your reference. When running the Check Levels routine, be sure you selected the pink noise "Use Main Speakers to Check/Set Levels" pull down and not the pink noise for the Sub. The noise used is a band limited pink noise with a low cut off of 500Hz and a high cutoff of 2000Hz. Once you take a measure you may have peaks outside that range that could trigger a clip. When you receive that warning, stop and lower your input level a bit and (rerun the Calibrate SPL routine of course) then retake the measure. Do this until there is no clipping.

brucek


----------



## michael123 (Jul 26, 2008)

Hi brucek

I did select "Use Main Speakers".. 
And then I lowered from 75db to 70db (once I received the warning..), is it too low?

What can I figured out from the graph?
How do I know if I suffer from reflections, for example?


----------



## brucek (Apr 11, 2006)

> is it too low?


Use 75dB, along with the method I outlined above.



> What can I figured out from the graph?


You have a peak at 45Hz and then some between 80-120Hz. I would try some positioning to see if you can reduce those.

brucek


----------



## michael123 (Jul 26, 2008)

What did you mean by


> When you receive that warning, stop and lower your input level a bit


? How do I do it?


----------



## brucek (Apr 11, 2006)

> How do I do it?


I presume you went through the Check Levels routine where you set the listening position to 75dBSPL using your receivers volume, and then you adjusted the input volume in REW so that the input level showing on the input VU meter was between -12dB and -18dB?

That's the input level.

brucek


----------



## michael123 (Jul 26, 2008)

I indeed adjusted the volume of my SSP using its volume control and then tuned it until I got the matched reading on the SPL meter.
I did not do anything to the input level before


----------



## brucek (Apr 11, 2006)

SSP?

You need to adjust all the volumes in the Check Levels routine. When that is completed you need to run the Calibrate SPL routine. Then you can Measure.

Of course, none of this can be done before you create a soundcard calibration file and load the meter calibration file.

brucek


----------



## michael123 (Jul 26, 2008)

Oops



> You need to adjust all the volumes in the Check Levels routine.


And adjusting the input volume is part of the "Check Levels"?



> Of course, none of this can be done before you create a soundcard calibration file and load the meter calibration file.


I did load the microphone calibration file. But how do I calibrate Tascam?


----------



## brucek (Apr 11, 2006)

> And adjusting the input volume is part of the "Check Levels"?


Yes, as I previously wrote above, run the Check Levels routine and adjust your receivers volume to obtain 75dBSPL at the listening position. Then adjust the REW soundcard *input level* to be about -12dB to -18dB on the input level VU meter. When complete run the Calibrate SPL routine.



> But how do I calibrate Tascam?


Install a loopback cable from line-out to line-in and run the soundcard calibration routine and create a soundcard calibration file and point REW to the file. remove the loopback and use that calibrated channel for measurements.

I encourage you to read and become familiar with the REW HELP files before continuing.

brucek


----------



## michael123 (Jul 26, 2008)

thanks, brucek

Actually, I read the help file about twice, but probably there are so many details for such a novice like me so I missed some important parts.

I installed the loopback cable (Wayne helped me to choose one), but I thought that REW calibrates the soundcard automatically once I check "use left channel for calibration"..

thanks for your help and patience
Michael


----------



## brucek (Apr 11, 2006)

> once I check "use left channel for calibration"


You don't ever check the "use left channel for calibration". That's a special process for checking system delays and not used for general measurements.

You only use the right channel. The left channel is not used. The loopback is installed temporarily on the right channel and a soundcard calibration routine is run that produces a file you save and point REW to. Then that right channel is used for measurements. Each measurement, REW uses the calibration file to correct for the cards inadequacies.

brucek


----------



## michael123 (Jul 26, 2008)

OK
But that's not what I understood from the pictures in the same help file.
It clearly display the loop on the other channels used in parallel (I thought so at least)


----------



## brucek (Apr 11, 2006)

> It clearly display the loop on the other channels used in parallel


Only when the "use left channel for calibration" is in play. It's not something you'll use for regular measurements.

See the REW Cabling and Connections Basics diagrams for normal hookups.

brucek


----------



## michael123 (Jul 26, 2008)

Hi brucek,

can you take a look if the measurements (CAL file) for my Tascam-144 are good?


----------



## michael123 (Jul 26, 2008)

..and this is how it looks

Did I do everything correct?


----------



## michael123 (Jul 26, 2008)

I would like to know if these peaks at 60 & 70Hz is something I can fix with room acoustics. 

[ BTW I already bought two large Auralex panels (will be installed on the wall opposite the main speakers next week), and will make the measurements after. ]


----------



## brucek (Apr 11, 2006)

> can you take a look if the measurements (CAL file) for my Tascam-144 are good?


Yes, the soundcard cal file is good.

On the waterfall plots, select LOG mode rather than LIN mode (icon in the top right corner of REW page). Also select the same axis you use for frequency response graphs for the waterfall for easier comparisons. Then if you select 1 slice (out of 30) you see your frequency response plot. Then you can add slices and see how the ringing progresses.



> like to know if these peaks at 60 & 70Hz is something I can fix with room acoustics.


You could correct them (with respect to the listening position) by some speaker movement, or EQ, but not likely treatment (too low a frequency for treatment to be very effective).

brucek


----------



## michael123 (Jul 26, 2008)

thanks, brucek (and Wayne - on other thread)

-- Michael


----------



## michael123 (Jul 26, 2008)

Hi brucek



> You could correct them (with respect to the listening position) by some speaker movement, or EQ, but not likely treatment (too low a frequency for treatment to be very effective).


What do you think about bass traps? I found some quite nice here: http://www.realtraps.com/p_planter.htm


----------



## brucek (Apr 11, 2006)

> What do you think about bass traps?


The guys in the acoustics section of our forum are far more qualified to discuss that type of thing. 

brucek


----------



## glaufman (Nov 25, 2007)

michael123 said:


> What do you think about bass traps?


Not entirely certain, but since your two speakers seem to have peaks and dips at various, unrelated frequencies, I would think corner traps would be of limited usefulness for your current graphs... 

I would take a few more scans of each speaker with the mic in different positions... depending on what I saw I would look at speaker placement.

Are these graphs with or without the Auralex panels?


----------



## michael123 (Jul 26, 2008)

Without panels.
I will install them within few weeks, I need a professional here..


----------



## michael123 (Jul 26, 2008)

Hi

Can I calculate THD or other distortion levels with REW?
For example, if I replace a speaker cable, or an amplifier, what can I measure?

I can put the microphone close the speaker to cancel the influence of the room acoustics.


----------



## brucek (Apr 11, 2006)

> Can I calculate THD or other distortion levels with REW?
> For example, if I replace a speaker cable, or an amplifier, what can I measure?


REW measures THD+N using the Spectrum Analyser. You would have to measure and subtract the THD+N from your test equipment first.

brucek


----------



## glaufman (Nov 25, 2007)

Would this be easier in a loopback mode, eliminating the speakers/mic/room?


----------



## brucek (Apr 11, 2006)

> Would this be easier in a loopback mode, eliminating the speakers/mic/room?


Only with line level devices. The reference was to an amplifier, which becomes a bit more of a problem. 

brucek


----------



## michael123 (Jul 26, 2008)

> subtract the THD+N from your test equipment first


Could you describe me this step please?


----------



## brucek (Apr 11, 2006)

> Could you describe me this step please?


It has to be manually carried out. If I wanted to check the distortion of a 1 KHz tone though a looped device with REW, the THD and THD+N would be a full loop figure including the soundcard. If I wanted to increase the accuracy, I would examine the THD and THD+N of the soundcard and subtract that from the full loop figure.

brucek


----------



## glaufman (Nov 25, 2007)

brucek said:


> Only with line level devices. The reference was to an amplifier, which becomes a bit more of a problem.
> 
> brucek


Ah, well, you're right again!


----------

