SPL meter is way off with USB calibrated mic

[ivanAV]

Hey !

So - I've spent a few hours reading every related forum post and testing various settings in REW, to no avail ; hopefully someone will be able to help

Problem: REW's SPL meter is way off (and digits are red, indicating a lack of calibration) despite having an individually calibrated USB mic with a "Sens factor" line in the cal file, which if I understand correctly means I don't need to do any SPL calibration.
Note: I don't have a SPL meter to compare with but my ears tell me REW's SPL meter can't be right, which is confirmed by measuring the voltage at my loudspeakers' terminals when playing a 1kHz tone and comparing with the loudspeakers' sensitivity specs (in that case, 85dB @1m for Kef LS50s passive). The difference isn't small - eg. 20-30dB.

My setup:
- OS: fedora 36 x86_64, gnome.
- REW: 5.20.9, also tested with 5.20.11 beta, both with the bundled JRE.
- Omnitronic MM-2USB USB microphone with individual calibration, bought from Hifi Selbstbau

lsusb shows the mic as 'ID 0d8c:0134 C-Media Electronics, Inc. Usb Microphone'

Starting with a clean install of REW and wiped prefs.xml:

- Selecting 'Microphone [plughw:2,0]', with the 'Mic (Mic)' input (the other settings are default - I haven't changed anything else):

- Define the calibration file:

the calibration file's first lines are:

"Sens Factor =-17.987dB, SERNO: 7034567"
10.00 -7.05
10.59 -7.32
[...]

clicking on the wheels icons shows:

roomeq_wizard0.log.txt (full file attached) shows the following, which seems to indicate that the sensitivity value is properly parsed:

INFO: Mic sensitivity -17.987 dB for serial number 7034567 from MM2USB_BZ_90d.cal

- As far as I understand the few settings above should be enough to be able to do calibrated SPL measurements, but the SPL meter's digits stay red + the measurements are off by a huge margin (in the picture below it should be around 67dB according to the speakers' sensitivity and measured voltage):

FWIW: REW (re)sets the input volume in the soundcard pref tab, as well as OS-wide (pipewire) volume when starting ; I've tested with setting the mic's volume both to 0 and max in gnome before starting REW, and after starting REW the volume is set to a kind-of middle value:

While doing SPL measurements, changing the os-wide volume changes the SPL values. With the new beta (5.20.11) mentioning directly accessing alsa devices I thought REW would access the mic directly (ie. bypassing pipewire) but that doesn't seem to be the case.

Any idea ? It could be linux-specific but I don't have a Windows machine to test on; however even if pipewire ends up being the culprit, why are the digits in REW's SPL meter red when using a calibration file, instead of white ?

Thanks !

(and obviously thanks to @John Mulcahy for this great piece of software )

 

[John Mulcahy]

REW only accepts sens factor entries for those few mics for which it has data on the relationship between the input volume setting and the corresponding gain in dB, currently UMIK-1, UMIK-2, UMIK-X, EARS, CARIO-A2B, UMM-6, Omnimic and Beta Three. Other mics can use the "Sensitivity" entry format per the Cal files preferences, the figure will be valid for whatever input volume it was measured at. I'll try and make it more clear when a Sens Factor has been found but the mic is not recognised so the figure cannot be used.

 

[ivanAV]

Thanks a lot for your quick reply !

I had read that REW used the "Sens Factor =..." for recognized/known mics but after seeing that the cal file was parsed properly in the log, + having received an (automatic) email with calibration files and instructions from the guys at Hifi Selbstbau where it was mentioned to use a "Sens Factor =..." line for use with REW, it didn't occur to me to test with a "Sensitivity..." line, l'll try asap; thanks again!