"Negative decibels" in Friture's sound intensity scale?

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Moya Benegas Ángel

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Aug 14, 2019, 12:55:25 PM8/14/19
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In Friture, the sound intensity measurement scales like the one always on the left of the display or the ones in the 2D Spectrogram, FFT, and Octave Spectrums,go from a relative 0 on the -200dB, to around 0dB or more (I've never gotten past +5dB with the program). I'm trying to figure out how this scale would translate into the actual decibel scale used for acoustics, which starts at 0dB (the threshold of human hearing) and goes all the way up to around 100~200dB. 
It would be very beneficial for researchers like myself to have this information more available so Friture can be used to study the physics of sound

Does anyone have any idea to how this goes?
Thanks.

David Macklom

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Aug 14, 2019, 1:41:55 PM8/14/19
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Moya, 

Unfortunately, Friture does not provide the ability to calibrate a microphone to a specific sensitivity of the microphone in use, therefore the measurements that are presented in Friture are not referenced to any absolute value.  Typically in an acoustic laboratory the microphones will all be calibrated using a calibrator that is presenting a 1000Hz tone at exactly 94dB to the microphone.  That calibration provides the measurement software with a sensitivity reference (with units like mV/Pa or similar) so that what the microphone is 'hearing' can be referenced absolutely to the threshold of human hearing which can be expressed in pascals, which is a unit of pressure and is equal to 0dB.  Friture does allow you to adjust the scales of the graphs, but those adjustments only move the view and not the reference.  Somewhere in the code there is some kind of reference value, but I could not tell you where that is, or what that is, in any case, that setting is not exposed for the user to change in any way.  

Best Regards, 
David
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