# Extereme LF weirdness



## zheka (Jun 11, 2010)

Hello,

I am playing with a newly bought Galaxy meter that was calibrated down to 5Hz.

However the results I am getting at the extreme LF do not look realistic at all.

As you can see in the attached REW measurement snapshot I am getting a 30dB jump between 15Hz and 5Hz. Is it even theoretically possible?

I brought it up to Herb of the Cross Spectrum and he confirmed that the calibration files look correct. Besides I tried measuring with RS meter using generic cal file and registered the same weirdness so it's unlikely that the meter is to blame. 

What am I doing wrong? Could it have something to do with the signal dropping below the measurement range (50dB - 100dB) ?

Thank you


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## EarlK (Jan 1, 2010)

zheka said:


> What am I doing wrong?


- In my ( minority-based ) opinion , the only thing you have done wrong here is believe that the compensation files ( from either Herb or the generics used for the RS meter ) will help give you meaning data , from below 15hz . 
- That's not a belief I share .

:sn:


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## zheka (Jun 11, 2010)

EarlK said:


> - In my ( minority-based ) opinion , the only thing you have done wrong here is believe that the compensation files ( from either Herb or the generics used for the RS meter ) will help give you meaning data , from below 15hz .
> - That's not a belief I share .
> 
> :sn:



I am not sure that 30dB jump at 5Hz can be explained by wrong calibration files.

Even if we assume that the actual FR at 5Hz is the same as at 15Hz, which is highly unlikely, in order to get the kind of curve the meter would have to be much more sensitive at 5Hz than at 15Hz, which is even less likely IMHO.

What gives?

here is the compensation values for the Galaxy 1/3 octave 5Hz to 80 Hz

5 -30.49 0
6.3 -23.99 0
8 -19.35 0
10 -15.72 0
12.5 -12.62 0
16 -10.03 0
20 -7.9 0
25 -6.16 0
31.5 -4.74 0
40 -3.58 0
50 -2.66 0
63 -2.01 0
80 -1.65 0


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## EarlK (Jan 1, 2010)

Okay, I'm a 2-chnl guy ( here at home ) with no HT or PCHT . I also don't run subs here ( only at work ) . Now, my bias is disclosed.

Here's a pic I made ( for other purposes which is why it stopped at 20hz ) to check out the accuracy of my old RS analog meter ( levels were carefully matched, positions duplicated, etc. etc. ) .










For my purposes the two track each other well enough from 30hz to 1K .

If you dig around the archives you'll find a poster ( from a couple years back ) who did comparisons of the RS meter to 3 other calibrated mics / I share his conclusion , "That the RS compensation file ( as it sits now ) when combined with the "C-Weighted" EQ , ends up over compensating the RS meter ". I see this over & over .

I'll shutup now ( I'm done ) .:dontknow: :whistling:

:sn:


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## zheka (Jun 11, 2010)

I am not overly concerned about sub-bass. 
But I do want to make sure that this is not a symptom of other things being wrong in my set up.
Being new at all this, I suspect that a rookie mistake somewhere is the most likely explanation


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## JohnM (Apr 11, 2006)

There are a couple of potential causes of that characteristic low frequency rise, boosting of low frequency acoustic noise by the cal file corrections (can eliminate that contribution by clearing the cal file and leaving the C weighting box unchecked) and the high output impedance of the RS meter interacting with leakage currents and 1/f noise in the soundcard input stage.


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## zheka (Jun 11, 2010)

JohnM said:


> There are a couple of potential causes of that characteristic low frequency rise, boosting of low frequency acoustic noise by the cal file corrections (can eliminate that contribution by clearing the cal file and leaving the C weighting box unchecked) and the high output impedance of the RS meter interacting with leakage currents and 1/f noise in the soundcard input stage.


John, thank you very much for your reply.

Please take a look at the attached measurement graphs.

Do you think can we rule out the first possible cause you cited?


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## zheka (Jun 11, 2010)

JohnM said:


> There are a couple of potential causes of that characteristic low frequency rise, boosting of low frequency acoustic noise by the cal file corrections (can eliminate that contribution by clearing the cal file and leaving the C weighting box unchecked) ...


just to make sure, do I leave the meter in C weighting mode and only uncheck C weighting box in REW?


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## JohnM (Apr 11, 2006)

zheka said:


> just to make sure, do I leave the meter in C weighting mode and only uncheck C weighting box in REW?


Yes.

There is still some rise without any calibration correction, which I suspect is due to the output impedance/input stage interactions. Hard to know whether what remains is the noise floor of the measurement without looking at the noise level of the impulse response.

Since V5.01 beta 7 the amount of boost provided by the combined cal file corrections is limited to 20dB by default, which would reduce the effect of the cal corrections.


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## ricardo_lee (May 30, 2012)

Zheka, what are you measuring? Are these ported speakers? What brand, model & size?

On the 2 curves you show, everything below 14Hz is just noise.


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## zheka (Jun 11, 2010)

JohnM said:


> Yes.
> 
> There is still some rise without any calibration correction, which I suspect is due to the output impedance/input stage interactions. Hard to know whether what remains is the noise floor of the measurement without looking at the noise level of the impulse response.
> 
> Since V5.01 beta 7 the amount of boost provided by the combined cal file corrections is limited to 20dB by default, which would reduce the effect of the cal corrections.


Should I worry about it? Does it make my measurements above, say, 15Hz suspect?

Thank you


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## zheka (Jun 11, 2010)

ricardo_lee said:


> Zheka, what are you measuring? Are these ported speakers? What brand, model & size?
> 
> On the 2 curves you show, everything below 14Hz is just noise.


The two curves are for comparison of measurements with and without cal file and weighting.

It is a trio of subs - CS18.2 and a pair Epiks legends. All sealed design.
I do not expect the subs to extend this low. I measured out of curiosity only because my new meter is calibrated down to 5Hz.

The problem is that it measures 95dB at 5Hz. Some mighty noise it is, don't you think?


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## ricardo_lee (May 30, 2012)

zheka said:


> The problem is that it measures 95dB at 5Hz. Some mighty noise it is, don't you think?


Do the same measurement 8 times and overlay. Then do 8 measurements with the speakers *disconnected*. This will tell you what is noise and also how reliable the measurements are.

What length sweep (how many seconds) ?

You should do this with any acoustic measurements.

95dB at 5Hz is not uncommon with hot air central heating and air-conditioning. It is a serious problem when recording in certain large venues. There are also windowing effects inherent in FFT methods which interact with noise in subtle and not so subtle ways.

But enough pontificating on my part. I'm _really_ interested to see the above repeated measurements.

BTW, as a microphone designer, I don't believe any 'calibration' of omni measurement mikes below 100Hz. You can easily make an omni flat to 1Hz or below. Most such 'calibrations' are just measurement error. That's not to say, there are no bad 'measurement' omni mikes.

What mike are you using?


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## JohnM (Apr 11, 2006)

zheka said:


> Should I worry about it? Does it make my measurements above, say, 15Hz suspect?


No


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## zheka (Jun 11, 2010)

ricardo_lee said:


> Do the same measurement 8 times and overlay. Then do 8 measurements with the speakers *disconnected*. This will tell you what is noise and also how reliable the measurements are.


seems like such an obvious thing to do yet it never occurred to me before. Thank you!



ricardo_lee said:


> What length sweep (how many seconds) ?


 I am using 512k samples length which translates to 12 seconds or so. I will check tonight to make sure.




ricardo_lee said:


> What mike are you using?


Galaxy Audio CM-140 SPL Meter calibrated by Cross·Spectrum Labs


thanks again


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## zheka (Jun 11, 2010)

JohnM said:


> No


great, thank you.


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## zheka (Jun 11, 2010)

ricardo_lee said:


> Do the same measurement 8 times and overlay. Then do 8 measurements with the speakers *disconnected*. This will tell you what is noise and also how reliable the measurements are.


here it is.
I also attached an RTA snapshot overlaid with the "speakers off" measurements. As you can see the boost is not reflected in the RTA readings.
I have no idea what to make of it other than the LF boost must be an artifact of the generated signal going through the sound card.


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## ricardo_lee (May 30, 2012)

Zheka, the good news is that you are getting good & reliable measurements down to 15Hz. Your curves are very consistent.

The bad news is everything is rubbish below that.

Difficult to tell where it's coming from but breakthrough in the sound card is a distinct possibility. The rubbish is very consistent which points to that. RTA isn't a correlated measurement so less likely to pick up it's own signal.

Does the rubbish appear when you do a loopback test on the soundcard?


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## EarlK (Jan 1, 2010)

As mentioned a few times now, nothing below 15hz is to be believed .

One could always alter the Galaxy's calibration file ( & replace all the db numbers below 16hz to match the 16hz calibration coefficient of " 10.03 0 " ) .



zheka said:


> 5 -30.49 0
> 6.3 -23.99 0
> 8 -19.35 0
> 10 -15.72 0
> ...


This will help remove the visual distraction of seeing the huge LF boost .

Here's a pic of a sweep I made ( late last year ) in response to someone with a similar inquiry ( though I'm unsure of it's relevancy here ) .










This is the venerable Behringer UCA202/222 USB soundcard with a frequency limited sweep run through it ( I was trying to duplicate the other threads OP's pic which is the reason for the truncated range ) . He was using the UCA202 with a Radio Shack SPL meter .

NOTE : In this instance, inputs & outputs ( of the card ) were unterminated . I can only conclude that the extreme LF rise was circuit leakage of some description ( or FFT correlation noise of some sort ? ) .

:sn:


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## zheka (Jun 11, 2010)

ricardo_lee said:


> Zheka, the good news is that you are getting good & reliable measurements down to 15Hz. Your curves are very consistent.
> 
> The bad news is everything is rubbish below that.
> 
> ...


No boost with loopback sweeps. Iit must be due to the mic/sound card interaction just like JohnM suggested.

Being able to trust the measurements up to 15Hz is more than enough for me.

Thanks you.


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## zheka (Jun 11, 2010)

mine is Behringer UCA202 too. I was getting the same boost with both Galaxy and RS meters.

anyway, as long as I can rely on readings within sonic spectrum I am content. 

Thanks! 



EarlK said:


> As mentioned a few times now, nothing below 15hz is to be believed .
> 
> One could always alter the Galaxy's calibration file ( & replace all the db numbers below 16hz to match the 16hz calibration coefficient of " 10.03 0 " ) .
> 
> ...


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