Absolute dB SPL calibration of digital MICs

[akl]

A digital MIC with calibration data has the enormous advantage of NOT needing an additional calibrator.
(Of course the provided CAL-data has to be correct).

A digital MIC like all USB-MICs may be used as a calibrated Sound Level Meter right out of the BOX
(after applying the cal-data and an appropriate software).

I use REW and an UMIK-1 from miniDSP .

But absolute dB SPL calibration of REW is sometimes out of order.
This had been about one year ago but than corrected and seemed to follow the cal file again.

v.5.20 b 39
earlier Versions of REW asked for a CAL-file, when UMIK-1 was connected.
This one did not but showed a SPL value (white) that used a former special test cal file. (But did NOT show an engaged CAL-file).
When starting REW the cal-files seems not to be cleared. The numbers are white (!). An 'old' (cleared) cal-file is NOT forgotten but engaged.
Hitting the calibrate button asks for a umik-1 cal file, this will be selected, but still the 'old' one is engaged.

A new start of REW will show proper values displaying the name of the last selected cal-file.

beta 39 gives no correct hint, which cal-data it is using.

The day before, the SPL value did not change, even if different cal-files selected and the SENS factor was altered by +-10 dB !
The mixer volume had no influence 30 ...100.

win 8.1
Java 8 241 64 bit
mixer 58 (30...100 with no influence)
--

The second problem is:

The factory (minidsp) stated SENS factor is wrong
by more than 4 dB !
org miniDSP:
Sens Factor =-0.015585dB, SERNO: 7037
correct ( +- 0.3 dB ):
Sens Factor =4.25dB, SERNO: 7037
--

instead of 'SENS factor' I would recommend the max. dB SPL value ( at 0 dBFS ). This is a very easy to handle calibration figure and also shows a helpfull specification of the used mic.
correct ( +- 0.3 dB ):
max SPL =116.7 dB, SERNO: 7037

This also shows an important flaw fo umik-1:
The umik-1 has a to high default amplification. This should be changed, possibly using an improved AD. The usability and accuracy of the mic will improve if the mic is able to handle 125 dB SPL without complicate dismanteling.

Also it would be very nice, if minidsp would provide the correct data of max SPL at the DEFAULT state .
miniDSP stated 133 dB SPL, but right out of the box it is just 18 dB less == 115 dB SPL. I measured 116.7dB SPL. There are 2 bit missing for >125 dB.

The instruction is missing to change the amplification [shift of binary bit] (may be hidden somewhere). You have to open the MIC. I had a lot of problems asking for for the instruction.

REW is a great software and together with an improved umik ...
Have fun !

 

[John Mulcahy]

You can see the cal file REW is using with the Cal Files button on the Measure dialog or by looking at the entries in the Cal Files preferences.

 

[akl]

You can see the cal file REW is using with the Cal Files button on the Measure dialog or by looking at the entries in the Cal Files preferences.

NO, that is the problem,
all fields are empty, all is cleared,
but the old file is still engaged !

 

[John Mulcahy]

Mixer volume should not have an influence, REW reads the volume control and allows for the corresponding UMIK gain change.

I have fixed the meter still appearing calibrated after clearing the cal file.