# From pressure to SPL : microphone calibration using sound level meter calibrator and sound level meter

I have a microphone + signal conditioner with adjustable gain and filter. I capture amplified signal using oscilloscope. I'm using this signal to compute equivalent continuous sound level

With signal conditioner, I set needed gain and A-weighted filter. From oscilloscope signal V I can get pressure p = V/ (gain x mic sensitivity)

We can suppose A-weighted filter filters out frequency out of audio range. So I can easily compute leq = 20 log10(rms(pa)/pref), pa is time domain pressure A filtered, pref is reference pressure = 20e-6 pa

I used a sound level calibrator to check if it works. I get a leq of ~94dB and 114dB when applying 94dB and 114dB level with calibrator.

I also have a sound level meter, I also checked it with sound level calibrator. Now, in my room (some noise because of oscilloscope...) with A filter it measures ~46 dB(A). Now I put my microphone close to it, with microphone I get a leq of 55 dB. When changing sound level meter filter from A to C results are better : sound level meter measures ~56dB. I changed microphone acquisition duration but leq results are same.

• How can we explain such leq difference when sound level meter and microphone measure dB(A)?
• How can I calibrate my microphone with sound level meter?
• is 55dB the noise floor of your system perhaps? – kippertoffee Jul 2 '19 at 10:45
• Can you tell us what kind of sound level calibrator you use? Or please explain in detail how the geometrical set-up changes when using the mic and the sound level meter. – user6522399 Aug 1 '19 at 12:40