Why does the gain change from day to day with REW??

Socket

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Todays newbie question
I am measuring the frequency response and EQing the speakers in my car
Have been doing this for weeks but recently the measurement levels (SPL) from one day to the next seem to rise and fall at random
When taking 20 or 30 measurements on any one day the levels are stable, but if I log in the next day and do the same measurement, the level has dropped or risen by as much as 5 dB
The frequency response is pretty close (within 1 db across the full range) and neatly lines up over the previous days measurements after dialing in the appropriate offset, but its almost as if there is some gain adjustment going on in the background
The HU volume is always set on -20db, using the same periodic PN track each day, using the same moving mic method each day. Helix V8 Mk2 DSP, most up to date version of REW, ACO, ATF PC tool and director firmware
I have the control input volume in REW unticked and the Standard Gain Setup selected (not potentiometer on device) selected in the AFT PC tool

Why would the levels (gain) be different from one day to the next when nothing has changed? what could be causing this? Is there another auto gain setting in REW or the DSP that I am not aware of? Problem with the umik1 or the mic cable?
It is literally driving me nuts !!!

Thanking you in advance
 

jschwender

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Philipd dss940
What OS, what hardware, what drivers?
 

Socket

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What OS, what hardware, what drivers?
OS= Win 10 Hardware = Laptop, umik1 (USB connection to laptop), HiFi walker digital player (blue tooth streaming period PN track to Helix V8 DSP) Volume on digital player set to 90, Volume on Helix commander set to -20db. Have been using the same hardware for months now. Drivers? Do you mean software drivers? ATF PC tool version 4-78a, REW version V5.20.3 Director updater V 1-77-2_10 ACO v13 from memory. If you mean speaker drivers Focal Utopia No 7s, three way active front,
 
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jschwender

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Philipd dss940
Are you playing stimulus signal as a file from the player, and not directly from REW via soundcard?
 

Socket

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Are you playing stimulus signal as a file from the player, and not directly from REW via soundcard?
No, not played directly from REW. Used REW pink noise generator to create a WAV file and copied that to the digital player and I play the track from there. Only using periodic full range pn. Not sweeps or tones
 

jschwender

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Philipd dss940
There is not so much that can go wrong with the Umik1, as it uses a simple usb class driver. Very unusual.
 

skid00

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"using the same moving mic method each day "

Are you using a robotic arm, with solid mounting points on the car chassis? :)
 

Socket

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There is not so much that can go wrong with the Umik1, as it uses a simple usb class driver. Very unusual.
Yes, very strange. Might try a new cable see if that helps
"using the same moving mic method each day "

Are you using a robotic arm, with solid mounting points on the car chassis? :)
Hahaha Yes same MMM.... No not bolted to the subframe
 

skid00

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My AV System  
Main Amp
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Carver Amazing Platinum Mark IV
I think you missed my (smart-axxed point)... Your mic is *never* in the same exact place, so you get different boost/modes.
 
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