My first calibration session with REW

**michael123** · **10-28-09, 06:58 AM**

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** · **10-28-09, 07:31 AM**

Quote:

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** · **10-28-09, 07:49 AM**

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** · **10-28-09, 07:59 AM**

Quote:

is it too low?

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

Quote:

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** · **10-28-09, 09:39 AM**

What did you mean by

Quote:

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

? How do I do it?

**brucek** · **10-28-09, 11:19 AM**

Quote:

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** · **10-28-09, 01:35 PM**

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** · **10-28-09, 01:52 PM**

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** · **10-28-09, 02:09 PM**

Oops

Quote:

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

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

Quote:

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** · **10-28-09, 03:00 PM**

Quote:

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.

Quote:

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** · **10-28-09, 03:04 PM**

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** · **10-28-09, 03:38 PM**

Quote:

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** · **10-28-09, 04:36 PM**

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** · **10-28-09, 05:49 PM**

Quote:

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** · **10-29-09, 05:22 PM**

Hi brucek,

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

**michael123** · **10-29-09, 06:19 PM**

..and this is how it looks

Did I do everything correct?

**michael123** · **10-30-09, 02:28 AM**

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** · **10-31-09, 10:46 AM**

Quote:

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.

Quote:

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** · **10-31-09, 01:46 PM**

thanks, brucek (and Wayne - on other thread)

-- Michael

**michael123** · **11-01-09, 01:52 PM**

Hi brucek

Quote:

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** · **11-01-09, 02:31 PM**

Quote:

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** · **11-02-09, 09:49 AM**

Quote:

michael123 wrote:

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** · **11-02-09, 02:03 PM**

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

**michael123** · **11-06-09, 03:54 AM**

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** · **11-06-09, 09:23 AM**

Quote:

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

Advertisement