Feature Request - Separate Cal file for external mic preamp

[cimera]

When using REW with a measurement box to switch between spl and impedance setup it would be helpful to have the option to add a calibration for the necessary external mic preamp in addition to the mic cal file only in case of spl measurements. From my understanding at the moment you whoul have to add both cal files and apply the sum as the mic cal file.
Would it be possible to add such an option? Maybe an additional dialog to measure the frequency response of the mic preamp would also be helpful.
If someone has another good solution for this I would like to hear it =)

Thank you in advance!

 

[Wayne A. Pflughaupt]

All you need is a soundcard calibration file. See the Help Files for info on how to create one. It can be loaded separately from the mic calibration.

Soundcard Preferences

www.roomeqwizard.com

Regards,
Wayne A. Pflughaupt

 

[cimera]

I think this is not what I mean. With the measurement box you are using the same input of the sound card for spl measurements and impedance measurements. In case of spl measurements there is an external mic preamp (not part of the audio interface) in the signal path. In case of impedance measurements the same input measures the voltage drop over the speaker (without the pre amp in the signal path).
Take a look at the "schematics" of the measurement setup:
Sorry can't post links here as I am new to the forum. Just Google for "arta messbox".

 

[sm52]

cimera
the microphone preamplifier is not connected during sound card calibration. The soundcard calibration file does not contain any correction for microphone preamplifier effects. Microphone correction file also does not contain microphone preamp effect correction.

From my understanding at the moment you whoul have to add both cal files and apply the sum as the mic cal file.

There is no need to summ these files.

 

[cimera]

Hi sm52,
yes that is how I understand it too.
But from my understanding the frequency response (and phase response) of the mic pre amp should be calibrated out of the meassurment to have a accurate meassurement or am I wrong?
If the mic pre amp is not calibrated out the resulting measurement should be the sum of the "actual" meassured frequency response AND the pre amps frequency response. In my case (Img Stage Line MPA-102) this would result in a -1.6 drop in frequency response @20 Hz.
Here the frequency response of the MPA-102 (with soundcard cal @ 48 kHz sampling rate):

So the total frequency response of the meassurement system whould be different for SPL and impedance meassurements:
frequency_response_spl = frequency_response_mic + frequency_response_preamp + frequency_response_soundcard
frequency_response_impedance = frequency_response_measurementbox + frequency_response_soundcard

In case of Impedance meassurements both components are calibrated out with soundcard cal and the impedance cal (inside the meassurement dialog).
In case of spl meassurements only soundcard (via soundcard cal) and mic (via mic cal file) are calibrated out.

Summing up the frequency response of the mic and the pre amp and applying the result as the mic cal file whould be a workaround to calibrate out the impact of the pre amp. At least I would think so.

 

[John Mulcahy]

Mic and soundcard cal files are not used for impedance measurements, the impedance cal process deals with everything itself. For SPL measurements a soundcard cal file can be created that includes the mic preamp in the loop, that is typical usage with analog mics. Getting levels right can be a little tricky (an attenuator at the mic preamp input can make things easier).

 

[sm52]

Maybe you are reasoning correctly. Suggest a way to do this. I have not heard anyone worried about this. This may be because usually the microphone preamplifier, which is intended for measurement, has a very flat response. Or, when a microphone is calibrating at the factory (a cal file is created), the effect of the preamp is taken into account in the microphone correction file.

 

[cimera]

@John Mulcahy That would be a solution. I didn't know that the sound card cal isn't used in case of impedance meassurement. Thank you!

@sm52 As it's only -1.6 dB@20 Hz I would say this is mostly flat but in the same ballpark as the frequency responce of the sound card (which is calibrated out). As you are saying many people are calibrating their mic WITH the pre amp which sould have the same result as adding up both frequency responses. But as @John Mulcahy stated that the soundcard cal is ignored in impedance measurements the pre amp could also be calibrated together with the sound card which should be the easier solution.