# My first measurements and how to interpret them



## WhereIsMyMac (Jan 14, 2015)

Hi,

I just did my first measurements and I need a little help from you guys to interpret them. My first question would be, how bad is it ? :sweat:
The first screenshot is my soundcard calibration and then I did 4 measurements.

I kind of worry a lot about the 1 KHz to 20 KHz range. Seems to be a complete mess ...!!!!!

There's something else i don't understand. What is the scale on the left ? Is it dB SPL ? I don't think so cause if I were doing measurements up to 160 dB SPL, I would destroy my ears and speakers ... So what is it ?

And last. What should the ideal level ? For example, I have a huge boost at 20 Hz but where should it be in an ideal situation ? 

Thanks.


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## JohnM (Apr 11, 2006)

Welcome! You are getting there, but need to fix a couple of things. To get levels showing correctly you need to calibrate your SPL reading. You then need to check what you have loaded as the calibration files for your mic and/or soundcard, from the look of those measurements you have got a measurement loaded as a calibration file by mistake.


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## WhereIsMyMac (Jan 14, 2015)

First of all, thanks for your really fast answer ! 

You must be right about my calibration file cause I did another set of measurements a few days ago and those measurements looked a lot different !
Plus the ones I just posted don't reflect that huge + 16 dB I measured at 136 Hz the other day. And I'm sure this + 16 exists cause when i knock it down with an EQ I can't clearly hear a difference.

I think l'll do another calibration file as I don't really know how to check it as you told me.

Thanks again !

EDIT: You were absolutely right, my calibration file wasn't good. I did another one and I got my 136 Hz issue back


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

Also, your graphs would be more meaningful if properly scaled. Fair warning, it will make them look much worse!

Getting Graphs Ready to Post

Regards, 
Wayne


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## WhereIsMyMac (Jan 14, 2015)

Thanks for the advices. I did another 4 measurements. Here they are. Properly scaled with a 1/6 Octave smoothing.
































So what do you guys think ? Am I in a lot of trouble ? Is 75 dB my reference level ?


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## acoustat6 (Mar 7, 2008)

Yes 75db should be your reference level. Is that what you independent SPL meter reads?

Bob


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## fusseli (May 1, 2007)

+/- 5 dB from reference is probably common, great speakers are 3 or less. The 100 Hz region could use some EQ and the 1-2kHz dip is probably inherent in your speakers. Are the measurements taken while driving a single speaker or both? Sweeps into high frequency range with both speakers playing isn't a "realistic" test and will cause anomalies in the results.


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## WhereIsMyMac (Jan 14, 2015)

acoustat6 said:


> Yes 75db should be your reference level. Is that what you independent SPL meter reads?
> 
> Bob


Almost. It reads a little bit more. Around 77 dB or so.




fusseli said:


> +/- 5 dB from reference is probably common, great speakers are 3 or less. The 100 Hz region could use some EQ and the 1-2kHz dip is probably inherent in your speakers. Are the measurements taken while driving a single speaker or both? Sweeps into high frequency range with both speakers playing isn't a "realistic" test and will cause anomalies in the results.


Yes, I did the measurements with both speakers playing. It's a mixing room actually and what I did was to put the mic at the mixing position towards the wall in front of me. Do you suggest that I should leave the mic where it is and just turn one of my speakers off. My speakers are DynAudio BM5-As.

I watched all the REW video presentation and they didn't say we should measure with only one speaker playing ! They should !

When you say anomalies, you mean phase cancellations and things like that ?


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## fusseli (May 1, 2007)

WhereIsMyMac said:


> Almost. It reads a little bit more. Around 77 dB or so.
> 
> 
> 
> ...


Ya, since either speaker is playing a sinusoid perfectly in-phase the spatial separation would cause them to not sum perfectly once the signal becomes acoustic. Anyone correct me if I'm wrong. I would look at R and L only, overlaid on each other (All SPL view). There's also the different distances to room boundaries impacting reflections and bass. Definitely leave your mic where it is.


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## WhereIsMyMac (Jan 14, 2015)

It makes sense. I'll give it a shot tomorrow. Ok, i'll leave the mic where it is but would you point it towards the speakers or not ? My first guess would be to let it as it is.


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## Talley (Dec 8, 2010)

How about some no smoothing and some 1/48 smoothing?

I don't like any more smothing beyond that.


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## fusseli (May 1, 2007)

I would point it at the speakers unless you have a special 90-degree calibration to point it upwards.


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## WhereIsMyMac (Jan 14, 2015)

Talley said:


> How about some no smoothing and some 1/48 smoothing?
> 
> I don't like any more smothing beyond that.


I was just following the forum's recommandations about posting measurements. But why not. I'll post them in a few minutes.



fusseli said:


> I would point it at the speakers unless you have a special 90-degree calibration to point it upwards.


I'll see if it makes any difference but it shouldn't. I just remember that my mic is omni.


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## WhereIsMyMac (Jan 14, 2015)

Here they are:
- 1/48 Octave Smoothing
































- No Smoothing


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## JohnM (Apr 11, 2006)

That 136 Hz peak is huge! Are you in a concrete bunker? 

Worth attaching an mdat file, that way people can play with smoothing themselves and look at other views, such as the waterfall.


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## WhereIsMyMac (Jan 14, 2015)

Lol, i know ! I don't know how it's even possible to get a peak that huge. And I have a Primacoustic London 12 Kit in my room !!
You're right avec the mdat file. Give me 5 minutes.


EDIT: Here
View attachment My Room.zip


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## Talley (Dec 8, 2010)

WhereIsMyMac said:


> Lol, i know ! I don't know how it's even possible to get a peak that huge. And I have a Primacoustic London 12 Kit in my room !!
> You're right avec the mdat file. Give me 5 minutes.
> 
> 
> ...


Where was your pink noise levels when you ran this test?... at 85db?

(mind you I am not a pro at this...)

Major resonance at 43hz, 85hz and 135hz. Are these room modes?


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## WhereIsMyMac (Jan 14, 2015)

You mean the pink noise to check the SPL before doing the measurements ? No, it was 77 dB.

Yes these are room modes.


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## JohnM (Apr 11, 2006)

Thanks for posting the file.

Good points:
- very clean measurement chain, soundcard looks great. What are you using as the mic?

Things to fix:
- you have loaded the soundcard calibration file as the mic calibration also, the file is very flat so it isn't having any real effect on your measurements but you should remove it. Go to the Mic/Meter tab of the Preferences and click Clear Cal, and uncheck the box for C weighted SPL meter.
- you are using V5.00 of REW, better using the latest V5.10

As for the room, those are big resonances, will be difficult to hear anything else at the low end. The speakers also cut off very sharply at 40 Hz. Here's the waterfall:









What does the room layout look like? If you have scope to sit elsewhere you could try measuring a few other positions. A quick way to do that is to play REW's Pink PN test signal and look at the response on the RTA window using these RTA settings, the response will update as you move the mic around:


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## WhereIsMyMac (Jan 14, 2015)

Thanks a lot for your answer. I have an Apex 220 as my measurement mic. Soundcard is an Apollo from Universal Audio. Very nice interface !!

About the sound card calibration file and the C weighted SPL meter setting, i must have misread the manual. I thought I read to do that.
In my room, there is only one place I can sit because it's a mixing room. 

The dimensions are:
Lenght: 13 ft
Width: 10 ft
Height: 7,5 ft

In the room itself, there is just my desk, my rack and a few thing like a guitar amp, mic stands and 2 guitars.

About the version, I don't see any 5.10 but 5.01 which is a beta. I'll check myself at the download page.

I also did some measurements 1 speaker at a time as suggested earlier.

*Left:*
No Smoothing:





























1/6 Octave Smoothing:





























*Right:*
No smoothing:





























1/6 Octave Smoothing:


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## acoustat6 (Mar 7, 2008)

Hello, Which make and model SPL meter are you using? A Radio Shack meter (analog or digital) is fine for this purpose.
Move your reference line up to 77db. 

Bob


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## WhereIsMyMac (Jan 14, 2015)

I don't have a real SPL meter for now. I use an app on my iPhone, I know it's not the best thing in the world .... But it gives me an overall idea !



acoustat6 said:


> Move your reference line up to 77db


I have no clue how I do that ? And why did it go to 75 dB ? During SPL calibration, I enter a value of 77 dB. ???


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## acoustat6 (Mar 7, 2008)

Hello, ok the iPhone app is good enough for now. the next time you go to measure use the iPhone spl meter with the pink noise and set it at 75db. then perform a measurement. post that measurement. do not move anything or change any settings on you system. Mark where the master volume control is with a piece of tape and a pen, as you need to get back to this setting exactly.
Bob


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## WhereIsMyMac (Jan 14, 2015)

The pink noise from REW when you check levels ?


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## WhereIsMyMac (Jan 14, 2015)

I did another measurement as you suggested acoustat6.








But I'm a bit confused now ...
I don't have any SPL calibration line as you can see on my screenshot. SPL calibration has been done of course ! I checked Mic or Z Weighted SPL Meter in the Mic/Meter preferences tab.

As you can see here, the SPL calibration box is grayed out.








And this dB scale on the left is confusing me a lot. Before I re scaled my measurement to post it, the Soundcard calibration was at 0 and now, it's at 80 dB ... What did I miss ?

I use these settings








I know it's a lot of questions but I need to know what I'm doing to use REW correctly !!

Thanks everybody.


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## acoustat6 (Mar 7, 2008)

Hello, Yes that is correct, pink noise at your seating position. Both speakers, set the pink noise to 75db on your iPhone app and hit OK in REW to set the level. That way your iPhone app and REW SPL meter read the identical levels.
Bob


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## WhereIsMyMac (Jan 14, 2015)

That's what I did. Do you have any idea why I don't have the Mic/Meter curve on my measurements ?


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## JohnM (Apr 11, 2006)

There isn't a mic/meter curve because you don't have a cal file loaded for your microphone.

The cal file curves are drawn at the middle of the plot, regardless of the plot level (otherwise you would rarely be able to see them).

SPL calibration is not a cal file and doesn't result in a cal curve, it tells REW how to convert the input level from yoru soundcard into an SPL figure.


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## WhereIsMyMac (Jan 14, 2015)

Ok cool ! Can we say my setup is correct for measurements ?!


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## JohnM (Apr 11, 2006)

Sure, looks fine.


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## WhereIsMyMac (Jan 14, 2015)

And what do you think of my room response ? Seems a little bit all over the place no ?


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## JohnM (Apr 11, 2006)

Pretty much, yes.


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## WhereIsMyMac (Jan 14, 2015)

Does this look like Flutter echoes, standing waves, or ?? What do you think ?


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## JohnM (Apr 11, 2006)

At the low end looks like sharp modal resonances (aka standing waves). Moving elsewhere in the room, even if only by a few feet, might have a significant effect, can try that by just moving the mic somewhere else.


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## WhereIsMyMac (Jan 14, 2015)

Ok thanks i'll give it a shot ! And what about the high mids and treble ? They are like up and down and up and down. Really don't know how to interpret that ...


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## JohnM (Apr 11, 2006)

That's normal, sound travels to the mic along many paths as it reflects from the surfaces of the room and that produces comb filtering at high frequencies. Apply some smoothing to see what the underlying trend looks like.


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## acoustat6 (Mar 7, 2008)

Hi again, Looking better, so that last measurement was set at 75db according to your iPhone SPL meter? If so drop the target line (the black line currently on 80db) to 75db. Then in the set axis and limits box set it to read on the left 45 and 105db (not 45-115db). Then the frequency 20 - 20,000hz (not 20-25,000hz).

Bob


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## WhereIsMyMac (Jan 14, 2015)

JohnM said:


> At the low end looks like sharp modal resonances (aka standing waves). Moving elsewhere in the room, even if only by a few feet, might have a significant effect, can try that by just moving the mic somewhere else.


Do you have any idea how can I attenuate them ? I don't have any basstraps in the back corners of the room, would that help to have basstraps there ? For now, I have a Primacoustic London kit 12:










JohnM said:


> That's normal, sound travels to the mic along many paths as it reflects from the surfaces of the room and that produces comb filtering at high frequencies. Apply some smoothing to see what the underlying trend looks like.





acoustat6 said:


> Hi again, Looking better, so that last measurement was set at 75db according to your iPhone SPL meter? If so drop the target line (the black line currently on 80db) to 75db. Then in the set axis and limits box set it to read on the left 45 and 105db (not 45-115db). Then the frequency 20 - 20,000hz (not 20-25,000hz).
> 
> Bob


Here are 3 screenshots of my last measurement with the good scale and everything:


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## JohnM (Apr 11, 2006)

Bass traps should help, but see what you can achieve by moving things around first. Some big rolls of loft insulation can work well as bass traps, although maybe not the most aesthetically pleasing


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## WhereIsMyMac (Jan 14, 2015)

You were right, the low response is totally different when I move the mic. These are made from 3 different positions.
























For the last one I put the mic 3 feet behind my mixing position and high frequencies tend to be a lot more "stable" if I may say except above 10 KHz but it seems to be my mic that causes that.


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## JohnM (Apr 11, 2006)

The first position looks pretty good - a bit of EQ could pull down that peak just above 40 Hz and some bass trapping would help with the 100 - 200 Hz region.


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