REW Beta Release SPL measurements and calibration

MAST

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Hi, I tried looking for an answer to this question on the forum but couldn't find one, so here it goes. I'm really new to the audio scene, and I'm starting to use REW. I have a USB interface and a SoundID measurement microphone. I did the soundcard calibration, and I have the microphone calibration files. With both of this information, shouldn't it be possible to obtain the SPL value directly? To use the SPL meter, do I have to calibrate it against an additional SPL meter. Is this true? Am I doing something wrong, or am I missing some software tool?
 

John Mulcahy

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With both of this information, shouldn't it be possible to obtain the SPL value directly?
No. The missing pieces of information are the mic sensitivity, interface mic preamp gain and any other gain settings along the audio input path. You could use a mic calibrator rather than an SPL meter to provide the reference info REW needs to work out the overall effect of the sensitivity and gains.
 

MAST

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I know the mic sensitivity, and I can set the gain of the mic in the interface to zero. Is that enough?
 

dcibel

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No. We can't assume that the minimum position on the input gain knob is a value of 1 (0 would make for no audio). Absolute SPL is rarely needed however, but if you want any accuracy in this value, an SPL calibrator is a must. Main downside is that a decent SPL calibrator can cost USD$100+, so you have to really want that SPL value. Even if you calibrated once with gain at minimum then never touched the knob again, it would not be an ideal situation, with minimum gain setting SNR would suffer for most measurements at reasonable volume.
 
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