# Help with measurment levels.



## mparsons99 (Oct 9, 2008)

Hi all,

This is my first post. 

I've been watching Ethan Winer at the Realtraps site and using sine waves with a test mic to make room measurements at low frequency. My problem is I'm not sure how to calibrate the levels I'm recording.

I've calibrated my monitors so pink noise at -20dB reads 83dB SPL as per Bob Katz' book. How do I set my mic input level so I can make comparisons between the pink noise level and the level of each individual sine wave level..or does it even matter??...should I only be worried about relative and not absolute dB measurements.

Hope this makes sense ...in other words if I have my pink noise at 83dB SPL and let's say 100Hz is being recorded...how do I figure out have far up or down this frequency is relative to my pink noise?

Any suggestions would be greatly appreciated.

Thanks


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## bpape (Sep 14, 2006)

If you download REW from this site, it'll run the whole spectrum at once instead of just a few frequencies. Then it does become simply a relative thing both in frequency and time domains.

Bryan


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## mparsons99 (Oct 9, 2008)

Hi Bryan,

Thanks for the post..I'll check out your site...and the software.

Regards

Matthew

:jiggy:


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## bpape (Sep 14, 2006)

http://www.hometheatershack.com/forums/rew-forum/

There are links to the free download, tutorials, etc. in that section of Home Theater Shack

Bryan


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## marcoc1712 (Oct 21, 2009)

Hi,

i'm absolutly new of this forum and even not use in any forum, so please, please be patient with me and 'rigid' if i'll broke some role or other, I apoligize in advance for this.

Back to the question, I've downloaded REW today, set everythink and taked some mesure, butat least one think i'm not sure i've properly done :

I'm using a ECM8000 Mic + ALESIS IO2 USB soundcard + MAC Air, no SPL Meter at all, so i've 'calibrated' SPL using the ECM8000 reaching a 55 db level in REW, over that the knob of my pre was far too hight and the noise too loud.

I've was able to take the measures, but i'm wondering IF they aare accourate or not due to this.

Could anyone help me?

thanks

Marco.


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## brucek (Apr 11, 2006)

> Could anyone help me?


For those that don't have an SPL meter to determine the actual 75dBSPL at the listening position during the Check Levels routine, then you must set the listening position to a reasonable listening level.

Then run the SPL Calibrate routine and set the thumbwheel to 75dB. What you have now done is set the REW SPL meter to 75dBSPL that matches the level you set at the listening position. It assumes that the level at the listening position is 75DBSPL. If it was 70dB or 82dB - matters not.

Here are the steps from the start.
Run the soundcard calibrate routine and save the file and point REW to it.
Load the microphone calibration file and point REW to it.
Run the Check Levels routine (setting the listening position to what you feel is a reasonable (~75dB) level.
Run the SPL Calibrate routine and set to 75dB.
Run the Measure routine and set your limits (i.e. 0-200Hz for a subwoofer and 0-20000Hz for full system).
Measure.

Be sure if you post graphs that for subwoofer plots, always use the standard Vertical graph axis of (45dB - 105dB) and the Horizontal graph axis of (15Hz - 200Hz) using the Graph Limits button in the top right corner of REW. Use a measurement start frequency of 0Hz and an end frequency of 200Hz. Use the appropriate target line that matches your crossover.

For full range plots, use the standard Vertical graph axis of (45dB - 105dB) and the Horizontal graph axis of (15Hz - upper limits you desire, i.e. 15KHz -20KHz. Use a full range target line.

For full range only, enable smoothing (1/3 octave) to eliminate the comb filtering. 

For waterfalls, best to use the LOG mode rather than the LIN mode using the FreqAxis button in the top right corner of REW.

brucek


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## marcoc1712 (Oct 21, 2009)

Thanks Brucek,

The procedure You explain is exactly wat I did, the only difference is that i've 'fixed' the 55 DB level instead of the 75, i think is ok is'nt it? 

Anyway, i'll run a completly new set of measurement tomorrow.

I'll surely post the REW plots, but now I cannot, i've to post some more thread before...

Thanks and I hope You'll look at my plots when I colud post it.

Ciao,

Marco


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## brucek (Apr 11, 2006)

> i think is ok is'nt it?


Well, best to use 75dB as your standard, then the plots will sit at the mid point of the standard vertical axis of 45dB-105dB.
If you set the level by ear to a reasonable listening level, you'd be surprised how close it is to 75dB.



> but now I cannot, i've to post some more thread before


That's why we have the post padding thread to pad your 5 post requirement...

brucek


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## marcoc1712 (Oct 21, 2009)

Hi again,

sorry, i'm looking for how to post an image, I Know probably you wrote down this so many times, but I'm new, so I've to look around and figure how...

As soon as i'll be able to, I'll post the plot.

Marco


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## bpape (Sep 14, 2006)

If possible, could you please start a new thread for your room so we can keep the OP's thread on track?

To post a pic, just use the "GO ADVANCED" button to get to the additional functions. Type your text and to insert the image, use the button next to the link for "MY PHOTOS" that's yellow and square and looks kind of like a mountain.

Bryan


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## brucek (Apr 11, 2006)

> i'm looking for how to post an image


Look here.

brucek


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## marcoc1712 (Oct 21, 2009)

Hi again,

I've found how to upload an image... easy when you Know!

Hope this is the time...

OK

here the plot:

RED is RIGTH CHANNEL, BLU is LEFT, FULL RANGE SPEAKER (Monitor Audio 20 SE).








I'm worried for the R. Speaker profile, to many deep i belive.

Thanks to any one could help me analyze the plot.

Marco

Marco


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## marcoc1712 (Oct 21, 2009)

Hi Bryan and Brucek, unless i'll undestend how, i'll open the new thread as you request.

Let me try...

MOVED TO "New Project for a lissening room" Thread.

See you there!

Marco


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