UMM-6 wrong SPL level

[theblond]

Hello,

I'm using REW with a Dayton UMM-6 measuring microphone for a while now. I've done some SPL measurements today and was confused by the readings.

If I measure the background noise right now, i got the following readings for different input volume:
Input volume 100%: 35 db(A)
Input volume 0%: 45 dB(A)

Windwos reports me an input volume range of 0 dB to +20 dB.

Shouldn't the input volume be compensated? Why is the higher input volume quieter and which value is right?

The calibration files starts with:

"Sens Factor =-19.59dB, SERNO: 1880171"
4.6758 -1.3435 15.33
...

Curious that there is no line that says: Sensitivity -12.34 dBFS

Kind regards

 

[John Mulcahy]

Curious that there is no line that says: Sensitivity -12.34 dBFS

That is used for analog mics, not USB mics.

Windwos reports me an input volume range of 0 dB to +20 dB.

Sounds odd, AustinJerry previously provided data for the percent to dB mapping of the UMM-6 on Win10 which varied between -17.3 dB at 0% and +12 dB at 100%.

-17.3, // 0
-15.9, // 10
-14.6, // 20
-13.2, // 30
-10.6, // 40
-9.3, // 50
-5.4, // 60
-2.6, // 70
1.3, // 80
6.6, // 90
12.0 // 100

 

[theblond]

Got it, this strange behavior appeares only at low SPL levels near the noise level of the mic. If I measure a tone of 60 dB or more, the input volume has nearly no effect on SPL readings.

 

[John Mulcahy]

this strange behavior appeares only at low SPL levels near the noise level of the mic

That's normal for any mic. Measurement chain noise gets boosted by the mic gain setting along with the signal so as you approach the measurement chain noise floor it becomes more difficult to separate the signal from the noise.