# Accuracy of measurements...



## Exocer (Apr 19, 2006)

I own the newer RS analog SPL meter and am getting excellent measurements with my Sealed Avalanche 15 sub. I wonder if they're too good to be true? Im dying to get a calibrated mic asap though... Here are some measurements.

Nearfield.








Listening position










What are some ways I can test for accuracy? The spl meter seems to have found the Tuning frequency of my Natalie P's to be 28hz (in-room) instead of my goal to tune them to 32hz...pretty close. My Velodyne DPS 10 looked like this before EQ









and after









Seems like the thing is working...but I cant believe the Avalanche is that flat in-room with no sort of EQ behind it.


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## brucek (Apr 11, 2006)

> What are some ways I can test for accuracy?


Well, when Sonnie and I were creating the calibration files for the three RS meters, measured against the ECM8000 that Sonnie had professionally calibrated, we overlayed our final responses using the cal files we had developed.

We felt that if we measured the same signal with the three different RS meters (using our newly generated cal files) against the ECM8000 and the responses were fairly close, then we'd be OK. 

Here is a set of response graphs of the three types of RS meters using the cal files posted here at the Shack against the ECM 8000. We actually had two digitals at the time.


Here's the 10Hz to 150Hz graph....









Here we look a little closer from 10hz to 60Hz to check accuracy.










Here we separate the different graphs to get a closer look.... You see the ECM8000, an old analog, a new analog and two new digitsl using the new cal files......









They're close enough for me. Is that close enough for you?

brucek


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## Sonnie (Apr 11, 2006)

Robert... that is remarkable response on your Avalanche 15.... awesome indeed with no eq at all.

I wouldn't think the meter would be more than a db or two different, but then again you could have gotten one with a bad capsule inside... only way to know for absolutely sure is to measure it against another SPL meter or a calibrated mic.


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## Exocer (Apr 19, 2006)

Bruce, they're definitely close enough for me. I wonder about my particular SPL meter, not your calibration files...You guys have done a remarkable job 

Sonnie, I guess that'd be the only way to know for sure huh?
I guess i'll be confident with the measurements i've got so far. At least they're not totally unbelievable like 120db at 10hz from a single sealed 15" sub.


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## Sonnie (Apr 11, 2006)

You seem to do a lot of speaker projects that need measurement... I'd recommend the Behringer ECM8000, especially for full range. Go ahead and get it calibrated. Leonard has just sent his off to another fellow to compare calibration with our recommended guy... so you might wanna wait til he gets his back and lets see how close mine is first. If his is drastically different, I may end up getting my re-calibrated. :yikes: Here we go again??? :dumbcrazy: Maybe not! raying:


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## Exocer (Apr 19, 2006)

I'm hoping the measurements are close enough for satisfaction. Is there a thread about this? I am very interested in keeping track of this experiment :T


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## Ilkka (Jun 8, 2006)

Without some heavy EQ/LLT, the near field response you show can't be accurate. It should start sloping much higher with around 12 dB/oct. slope. I recommend getting a calibrated mic (or at least non-calibrated ECM8000) and measure again.


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## Guest (Oct 9, 2006)

Hi all, l'm wondering if would be able to steer me in the right direction here.......l have a newer analog rs meter and am wondering what allowances l will have to make with it just using it manaully c weighted for my sub.......any information would be appreciated........cheers ken


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## brucek (Apr 11, 2006)

> l have a newer analog rs meter and am wondering what allowances l will have to make with it just using it manaully c weighted for my sub


See here and download the Excel workbook for your specific meter and the compensation values are built into the file.

Or, you can download the XXXX.cal file from that page (which is just a text file) and use the values of meter compensation.

brucek


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## Guest (Oct 9, 2006)

brucek said:


> See here and download the Excel workbook for your specific meter and the compensation values are built into the file.
> 
> Or, you can download the XXXX.cal file from that page (which is just a text file) and use the values of meter compensation.
> 
> brucek




Thaks for that brucek
this means that my ib is down around 25db @10hz ( nearfield )...that sound about right??...cheers ken


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## brucek (Apr 11, 2006)

> this means that my ib is down around 25db @10hz ( nearfield )...that sound about right??...


Well, if you use the newrsanalog.cal file, you would add +7.29db @10hz to the reading you get with the meter to compensate for the inaccuracy at that frequency. 

brucek


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## Guest (Oct 9, 2006)

brucek said:


> Well, if you use the newrsanalog.cal file, you would add +7.29db @10hz to the reading you get with the meter to compensate for the inaccuracy at that frequency.
> 
> brucek





thanks bruce........yeah that is corrected level......my drivers were brand new when l did the measure a few weeks ago so maybe they're broken in more now and have changed slightly...either that or there is some attenuation going on in my avr's lfe.....cheers ken


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## Guest (Jun 18, 2008)

i AGREE WITH YOU:clap:


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## Ricci (May 23, 2007)

Bruce,
I've got a question about the CM140 cal file. How was this file determined? I'm interested in the correction below 20hz and most importantly the 10hz and lower correction values. I recently measured my output at 5hz with one of my subs with slow C weighting and 80-130db range, 5hz sine wave. The meter showed 105.1db. Now if I add the correction value of 24db to this my true SPL would be 129.1db at 5hz? That's why I'm wondering about the correction values down low.


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## brucek (Apr 11, 2006)

> I've got a question about the CM140 cal file. How was this file determined?


It was created by comparison measurements taken near field, using Sonnies subwoofer and his calibrated ECM8000 (which is calibrated down to 5Hz). Quite a few Galaxy meters were tested and they were actually quite consistent. When the cal file was complete we loaded it and did a comparison against the ECM (with its own cal file loaded) and the two results tracked very closely.

But it matters not, because we found that the galaxy tracked quite close to a real C-Weight curve and so even if you cleared your cal file and simply checked the C-Weight box instead, it would still be fairly accurate. Our file just trims it up a bit. A real C-Weight curve compensate about 25dB at 5Hz (so you can see the cal file is close).

Either way, I'm not a fan of measuring anything below 10Hz...... why bother.

brucek


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## Ricci (May 23, 2007)

Thanks.

Why? Because there is content in movies down that low, and I'd like to try to reproduce it if possible. Also because why not?:devil:


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## NEO Dan (May 27, 2007)

Bruce, 
Like Ricci I am very concerned about one thing in particular:

The LF consistency of the ECM8000 and CM-140 has been called into question.

Ilkka has basically declared that unless the Mic is calibrated it is not to be trusted for LF measurement.:gah:

Has anyone found this to be the case?


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## lcaillo (May 2, 2006)

This is the case with any measurement tool, particularly at the extremes of the operating range.


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## Ilkka (Jun 8, 2006)

NEO Dan said:


> Bruce,
> Like Ricci I am very concerned about one thing in particular:
> 
> The LF consistency of the ECM8000 and CM-140 has been called into question.
> ...


As harsh it may sound, I wouldn't trust in any microphone that hasn't been individually calibrated, especially when it comes to the extreme ends of the operating range. Down to 20 Hz is usually okay, but measuring 5 Hz or similar with an uncalibrated microphone (and the rest of the measuring loop) is like taking a stab in the dark.


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## Ilkka (Jun 8, 2006)

I have posted this before but here's a comparison of 5 uncalibrated ECM8000 microphones. You can see that there is already ~7-8 dB maximum different at 10 Hz. How about at 5 Hz then? One simply can't use a universal calibration file if you want to measure frequencies that small.


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## brucek (Apr 11, 2006)

> One simply can't use a universal calibration file.


For sure, and we try and make a point on our download page that if you want accuracy, then you better get a calibration. The lower you measure, the greater the possibility of inaccuracy.

But again, we also let people know that for home use, a movement of the mic element a few feet can easily result in several dB of level change, so it is our opinion that the universal cal files will be fine to use in a home theater.

I do remember when Sonnie was tesing the Galaxies from the group buy, he couldn't find one that wasn't the same as all the rest. They were amazingly consistent.

During the testing, after the calibration files were developed, we did a bunch of comparison testing.

Here's a plot down to 5Hz of three different ECM8000's versus three different Galaxies. It's close enough for me (and you can actually read the vertical scale ).

But again, I would never measure to 5Hz. Makes no sense. There's next to no information that low (please don't show me those crazy spectrographs), and the inaccuracy of the mic/meter is called into question.









brucek


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## Ilkka (Jun 8, 2006)

brucek said:


> But again, we also let people know that for home use, a movement of the mic element a few feet can easily result in several dB of level change, so it is our opinion that the universal cal files will be fine to use in a home theater.


Yes, definitely fine to use in a home theater. My comment above was meant to those trying to measure frequencies down to 5 Hz or similar.


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## Ilkka (Jun 8, 2006)

brucek said:


> It's close enough for me (and you can actually read the vertical scale ).


It's 5 dB/div.


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## NEO Dan (May 27, 2007)

Ilkka,
thanks for the explanation


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## NEO Dan (May 27, 2007)

brucek said:


> For sure, and we try and make a point on our download page that if you want accuracy, then you better get a calibration. The lower you measure, the greater the possibility of inaccuracy.
> 
> But again, we also let people know that for home use, a movement of the Mic element a few feet can easily result in several dB of level change, so it is our opinion that the universal cal files will be fine to use in a home theater.
> 
> ...



Bruce, 
thanks for posting this up. I think it looks reasonable, and I wouldn't expect much more. Is it cheaper to buy a new 5Hz calibrated Mic, or have on cal'd down to 5Hz? I've got an ECM8000 if it matters at all.

Thanks, 
Dan


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## Ricci (May 23, 2007)

Thanks for the explanations guys. I was just getting some crazy measurements that weren't making sense to me. I guess I will look into getting my meter or ECM8000's calibrated, or stick to 15hz and above for now.


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## brucek (Apr 11, 2006)

> Is it cheaper to buy a new 5Hz calibrated Mic, or have on cal'd down to 5Hz? I've got an ECM8000 if it matters at all.


I don't know if they even sell microphones pre-calibrated to 5Hz. It's too low a frequency for most to even consider. Sonnie had to special request to have his microphone calibrated down that low and I believe he paid extra for the service.

Here is the lab he had his done at. I think normal microphone calibrations would likely extend to 20Hz. 

I remember when Sonnie said he was going to calibrate his microphone to 5Hz and that he wanted to create cal files for the Galaxy and Radio Shack meters down to 5Hz so we could provide them on the download page, I was quite resistent to the idea (and still am...  ). I think 15Hz would be perfect, and perhaps in a weak moment, I might go along with 10Hz, but 5Hz is beyond reason IMO. The RS meters wouldn't measure that low, so we stopped at 7Hz. I hope no one is seriously using an RS meter down to 7Hz hehehe..........

brucek


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## Ricci (May 23, 2007)

Bruce,
There is a contingent of DIY subwoofer guys with loads of experience and very capable systems that have single digit extension and there is quite the discussion of exactly what is occuring below 10hz in room over at AVS. It involves varying theories on room gain, distortion levels and the audibility of those distortions, measurement accuracy, etc., etc. We are looking for some means of accurate measurement that is preferably affordable. It's kind of like exploring the final frontier of bass. That's the reasoning behind it, it's kind of uncharted waters.


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