# Pls help me understand the relationship between input level, dBFS and SPL levels



## AustinJerry (Apr 2, 2010)

For some reason, I am having difficulty understanding the relationship between the Control Input Volume and the dB FS Output level shown on the REW SPL Meter. This misunderstanding arose after I entered the Sensitivity parameter for my UMM-6 USB mic into the calibration file, and then tried to verify that the REW SPL reading was indeed matching up to the SPL level measured by my external RS SPL meter (Slow, C-weighting).

Here is a test I conducted:

First, set the Preferences screen so that I can control input volume (shown as 1.0 in the attached screen shot).









Next, I generate a 1Khz sine wave test tone at -20dBFS, and adjust the computer output so that I am getting a reading of 70dB on the external RS SPL (with the SPL placed adjacent to the tip of the UMM-6).









Now, having heard in a separate thread that the control input volume should be set to 33%, I adjusted the input to read .33 and observed the reading on the RS SPL. Sure enough, it was registering 70dB! This seems to confirm my initial objective, i.e. make sure the sensitivity parameter in the calibration file was indeed resulting in a calibrated UMM-6. Note that the dBFS is registering -57.1.









Next, in order to test my understanding, I adjusted the input up to 1.0, which resulted in a dBFS reading of -45.2, and an SPL of 72.3dB.









Finally, I adjusted the input down to its lowest value of .001, which resulted in a dBFS reading of -74.9, and an SPL reading of 82.6dB.









Can someone explain the relationship between input level, dBFS, and the SPL reading? Or point me to a good explanation?


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## JohnM (Apr 11, 2006)

REW attempts to compensate for changes in the input volume setting and adjust the SPL figure accordingly, if it gets that right the SPL figure should stay the same over input volume settings from full scale down to pretty low levels - at the bottom end the mic's noise contribution can give some deviation. The REW adjustments are based on some data a user measured from his UMM-6, recording the change in the input dB FS figure at different input volume settings. That was done on an XP system, however, and the behaviour can be different for Win 7 and different again for Win 8. I don't have any measurements for Win 7 or Win 8 though, so REW uses the XP data and hopes for the best. 

If you'd like to measure how the dB FS figure changes as input volume changes for Win7 and/or Win8 and send me the result I can use that to update the volume handling for the UMM-6. Note that the changes may go in steps, as the mic's internal gain adjustment tends to be stepped so a series of input volume settings (e.g. .99, .98, .97 etc) can all result in the same dB FS figure, then at some point the figure will change by a dB or more. Probably best to start with a level that gives a higher dB FS reading than -45 at full volume though, otherwise the readings at low settings will be dominated by noise - place the mic close to the speaker driver to get higher levels.


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## AustinJerry (Apr 2, 2010)

OK, John, Here are the UMM-6 readings. I used an input signal at -20dB FS and varied the microphone input volume using the volume control on the REW Soundcard Preferences screen. I am running Windows 7.

View attachment UMM-6 readings.txt


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## JohnM (Apr 11, 2006)

Excellent, thanks Jerry.


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