# Please help me understand some basics...



## -Axle- (Sep 28, 2014)

I've gone ahead an read a lot of the FAQs and threads on REW and the miniDSP UMIK-1. I've also googled some sources but am having difficulty trying to get answers on some aspects that are glossed over that I am not certain I understand.

Its likely because of being an outsider to audio calibrations. If anyone can help me understand, I'd highly appreciate it.

1.) Many of the microphones that can be purchased will come with a calibration file (or one can be created). Why is this necessary out-of-the-box? Shouldn't the microphone be accurate from the manufacturer (after they've ran their initial calibration)? In other words, why bother leaving the calibration file a manual process instead of baking-in to the unit? My mind around this is thinking about video calibration, where a colorimeter will drift with time and require correction, but out-of-the-box would be accurate. Will microphones also drift with time?

2.) The miniDSP UMIK-1 will come from the manufacturer with its own calibration file, but you can also opt to purchase one that is "uber" calibrated by Cross Spectrum labs to:
-make it more accurate below 20Hz and above 20kHz?
-why doesn't miniDSP do this by default? Cost?
-how far off would the measurements without cross-spectrum's additional calibration (+/-1dB? +/-3dB? +/-5dB? etc.)
-Does cross-spectrum's calibration affect the accuracy from 20Hz to 20kHz as well? And if so, how far off was it originally? (+/-1dB? +/-3dB? +/-5dB? etc.)

3.) I hear mention of using an SPL in conjunction with the microphone together, but I don't understand what is being accomplished. I understand an SPL isn't good (by default) at measuring frequency response. Does that mean that microphones aren't good at measuring SPL levels? Shouldn't it be better than an SPL?

4.) Anything you've read above that gives the impression I may have some concept backwards or distorted?

Thanks in advance. This seems like a great community.


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## JohnM (Apr 11, 2006)

1) Mics do not have any built-in means of flattening their response, adding that would be very costly
2) Getting reliable data at very low frequencies is difficult and time consuming, which would add time and cost to the end of line calibration. The UMIK as standard also comes with on-axis calibration only, whereas the CSL mic includes a 90 degree file (for use with the mic pointing straight up rather than at the speaker). It's difficult to say how much difference there is between the factory and CSL-calibrated UMIK's, I haven't seen many comparisons - main differences are likely to be at the lowest frequencies, CSL goes much lower than the factory cal. Either should be within a dB or so from 20 Hz to 20 k.
3) A calibrated mic that includes a sensitivity calibration figure can provide accurate SPL readings. The UMIK has a factory-generated sensitivity figure. CSL do not have a setup that is suitable for measuring sensitivity to an accuracy level they would be happy with, the factory data can be copied into the CSL cal files.


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## jtalden (Mar 12, 2009)

-Axle- said:


> ...
> 
> 3.) I hear mention of using an SPL in conjunction with the microphone together, but I don't understand what is being accomplished. I understand an SPL isn't good (by default) at measuring frequency response. Does that mean that microphones aren't good at measuring SPL levels? Shouldn't it be better than an SPL?


Expanding a little on this part of your question:

You may be getting confused between SPL (sound pressure level) and SLM (sound level meter). The SLM is a handheld meter including readout that is designed to measure noise level (SPL) in wider freq bands. An SLM is commonly used for general noise level measurements, for example, determining if hearing protection is required. 

For EQ of our audio system we want to plot the SPL vs freq for our room/speaker setup. We normally want to chart this accurately at each narrow band freq. We can do that best with a calibrated measurement mic. The calibration file that comes with the mic corrects for the actual variation in the mic in those narrow freq steps.

We could also calibrate a handheld SLM for use with REW, but those are not normally calibrated for this type of use. The design is also not optimal for use at high frequency or very low freqs. While these hand held meters can be calibrated by CSL and others, it is still not recommended to use them outside the range of maybe 20-3000 Hz. 

An SLM type readout is built into REW as a feature. It requires an attached a measurement mic or a SLM to detect the sound. REW can then report the readings in several ways. Chart the SPL vs freq or provide the same types of wider band measurements that the SLM provides on the handheld unit itself. All the REW SLM needs to be accurate is to match a reading on the handheld SLM so it can scale the reading properly. As JohnM stated, with a calibrated USB measurement mic the cal file includes the need info to set the correct SPL level in 
REW. With that type of mic there is no need for an SLM. Any measurement mic can be used without the need for a SLM. We normally really don't care if the SPL level reported is highly accurate. For EQ we are interested in the relative levels at different freqs. These will be the same even if the overall level is not set accurately.


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## -Axle- (Sep 28, 2014)

Thanks for the responses.


JohnM said:


> 1) Mics do not have any built-in means of flattening their response, adding that would be very costly


Thanks, I think I just clicked into the fact that I don't understand how a microphone physically works (I should find out).

I think I'm thinking of it too much like a speaker, where (for example) a tweeter is designed to yield a certain response curve by fluctuating the way handles its input / power (and that a speaker doesn't need a "calibration file" to do this, although we overlay EQ to deal with how our room interacts with the sound produced by the speaker / tweeter.



JohnM said:


> 2) Getting reliable data at very low frequencies is difficult and time consuming, which would add time and cost to the end of line calibration. The UMIK as standard also comes with on-axis calibration only, whereas the CSL mic includes a 90 degree file (for use with the mic pointing straight up rather than at the speaker). It's difficult to say how much difference there is between the factory and CSL-calibrated UMIK's, I haven't seen many comparisons - main differences are likely to be at the lowest frequencies, CSL goes much lower than the factory cal. Either should be within a dB or so from 20 Hz to 20 k.


Gotcha, thanks.



JohnM said:


> 3) A calibrated mic that includes a sensitivity calibration figure can provide accurate SPL readings. The UMIK has a factory-generated sensitivity figure. CSL do not have a setup that is suitable for measuring sensitivity to an accuracy level they would be happy with, the factory data can be copied into the CSL cal files.


So, the microphone can detect the frequency accurately, just not at what SPL its hearing that frequency? Is that correct?

Therefore, a legend (sensitivity information) is required to map out the varying SPL the mic "thinks" there is between hearing different frequencies?


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## -Axle- (Sep 28, 2014)

jtalden said:


> Expanding a little on this part of your question:


Much appreciated.



jtalden said:


> You may be getting confused between SPL (sound pressure level) and SLM (sound level meter).


I think so too.



jtalden said:


> For EQ of our audio system we want to plot the SPL vs freq for our room/speaker setup.


100% with you.



jtalden said:


> We normally want to chart this accurately at each narrow band freq. We can do that best with a calibrated measurement mic. The calibration file that comes with the mic corrects for the actual variation in the mic in those narrow freq steps.


I think I understand, but what is the difference between narrow frequency steps vs wide frequency steps? I'm thinking you do a sweep from 15Hz to 20kHz and correct peaks and valleys based on what the microphone records. Is wide frequency bands implying instead of a sweep, just playing pink noise and trying to address peaks and valleys on that kind of reading instead? Presumably that's not for HT use, no? Am I close?



jtalden said:


> We could also calibrate a handheld SLM for use with REW, but those are not normally calibrated for this type of use. The design is also not optimal for use at high frequency or very low freqs. While these hand held meters can be calibrated by CSL and others, it is still not recommended to use them outside the range of maybe 20-3000 Hz.


But the miniDSP UMIK-1 is not a SLM, correct? My goal is to use it for HT use, which I assume its suitable for.



jtalden said:


> An SLM type readout is built into REW as a feature. It requires an attached a measurement mic or a SLM to detect the sound. REW can then report the readings in several ways. Chart the SPL vs freq or provide the same types of wider band measurements that the SLM provides on the handheld unit itself. All the REW SLM needs to be accurate is to match a reading on the handheld SLM so it can scale the reading properly. As JohnM stated, with a calibrated USB measurement mic the cal file includes the need info to set the correct SPL level in REW.
> 
> With that type of mic there is no need for an SLM. Any measurement mic can be used without the need for a SLM. We normally really don't care if the SPL level reported is highly accurate. For EQ we are interested in the relative levels at different freqs. These will be the same even if the overall level is not set accurately.


Gotcha, so for an SLM, you need to level set the SPL measurement as its not an SPL meter, but with a calibrated microphone (like the UMIK-1) you don't need to level set the SPL measurement as its already been done. Correct?

Thanks again for the responses.


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## jtalden (Mar 12, 2009)

-Axle- said:


> ...
> I think I understand, but what is the difference between narrow frequency steps vs wide frequency steps? I'm thinking you do a sweep from 15Hz to 20kHz and correct peaks and valleys based on what the microphone records. Is wide frequency bands implying instead of a sweep, just playing pink noise and trying to address peaks and valleys on that kind of reading instead? Presumably that's not for HT use, no? Am I close?


Yes, you are close enough for a general understanding. A sweep has freq steps that are so close that it looks continuous. 



> But the miniDSP UMIK-1 is not a SLM, correct? My goal is to use it for HT use, which I assume its suitable for.


Yes, It is a very good choice for use with REW and very popular now. Since you have the UMIK-1 there is no reason to own or to worry about an SLM. It is not as suited to the task. I was just trying to shed light on the confusion between SPL and SLM. 



> Gotcha, so for an SLM, you need to level set the SPL measurement as its not an SPL meter, but with a calibrated microphone (like the UMIK-1) you don't need to level set the SPL measurement as its already been done. Correct?


With the UMIK-1 your must download the cal file needed. REW will ask you to load it and then you are done with all calibration steps. It is just slightly more complicated using CLS calibration.

The SLM is a SPL meter. It is just designed for other purposes than measuring the wide range of acoustical properties that REW is designed for. To use it with REW there are several calibration steps needed to get the same levels (charts) as the UMIK-1. Even then its accuracy fades outside of 20-3000Hz for the inexpensive units. There is no need to understand the SLM further if you plan to use the UMIK-1. If you just want to learn more about SLMs then just search for "sound level meter" and also for "A weighting" and "C weighting". You can start *HERE*.


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## -Axle- (Sep 28, 2014)

jtalden said:


> With the UMIK-1 your must download the cal file needed. REW will ask you to load it and then you are done with all calibration steps. *It is just slightly more complicated using CLS calibration.*


Thanks again for the replies. One last one.

I'm under the impression I just have to load the correct file from CSL (Chosen orientation + 5Hz-25kHz). Or is there another aspect?


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## jtalden (Mar 12, 2009)

Yes, it is necessary to take the first line on the UMIK-1 file (sensitivity value) and paste it as the first line in the CSL file. There are several threads that detail this. It is very simple if you have ever edited text files.


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## -Axle- (Sep 28, 2014)

Got it.

Thank you both again.


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