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Discussion Starter #1
Hi

I'm having difficulty interpreting the UMIK-1 calibration data and correctly measuring the signal from an acoustic calibrator using the REW SPL tool.

Hardware:
- UMIK-1
- Bruel & Kjaer 4230 Pistonphone (94 dB SPL, 1000Hz). I'm using the B&K 1/2" adapter.

Software:
REW version 5.01 Beta 12, Build 2849 on Windows 7 (32 bit).

The pistonphone doesn't give a sealed connection to the UMIK-1 so I'm using modelling clay to ensure seal. REW detects the UMIK-1 on startup and request the calibration file which it appears to load successfully. See screenshot of REW Preferences attached.

When I measure the SPL with the UMIK-1 in place in the pistonphone, in a quiet room, I get a reading of about 60 dB SPL (C) before switching on the tone. When I activate the pistonphone I get a reading of 108.7 dB SPL (C) - much higher than expected (even allowing for free field vs pressure microphone issues).

The sensitivity factor given in my UMIK-1 calibration file is -14.7 dB.

Now, 108.7-14.7 dB SPL = 94.0 dB SPL, which is the nominal pistonphone output.

So, am I doing something silly here or is there something wrong in the way REW is using the UMIK-1 sensitivity data?

thanks for any ideas

Gerry
 

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It may be that the input level setting is not being taken into account correctly, or that the effect of the level setting is different on Windows 7 than it is on Windows 8. To get some extra info on this, please tick the box to "Control input mixer/volume" on the REW soundcard preferences and record how different volume settings alter the reading with the calibrator in place. I suspect it will read correctly with the volume set to 1.0. To get to the bottom of this it would be great if you could record the REW SPL meter dB FS readings (the figure just below the weighting curve selector buttons) with the calibrator in place at as many volume control level settings as you have the patience to record, ideally at least in 0.1 steps from 0.0 to 1.0.
 

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Discussion Starter #3
John

Apologies - I haven't been able to get back to this till now. I just downloaded and installed REW Beta 13.

Test conditions similar to my previous message, except PC is now:
LENOVO ThinkPad T61 running Windows 7 Professional (32 bit)

Apart from when REW Input Volume is set to 0.000 (readings below), I got a reading of 92.6 +/- 0.1 dB SPL as I changed Input Volume from 0.00 to 1.00. This compares reasonably well with the calibrator value of 94 dB SPL (1000 Hz) - allowing for the fact that the UMIK-1 is being used here in pressure rather than free field conditions.

Thanks very much for your help.

Gerry

REW Input Volume, dB SPL (C), REW dB FS
0.00, 77.6, -76.1
0.10, 92.6, -41.1
0.20, 92.6, -35.1
0.30, 92.6, -32.1
0.40, 92.6, -29.1
0.50, 92.6, -28.1
0.60, 92.6, -26.1
0.70, 92.5, -25.2
0.80, 92.6, -23.1
0.90, 92.6, -22.1
1.00, 92.6, -21.1
 

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Thanks for that. Looks like there is a dB still to find in that cal chain, but I think I know where it is hiding. :)
 
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