# How to calculate sound pressure levels using microphone sensitivity rating

This is sort of a cross between a computer science and Physics question, but I would be grateful if anyone here could help me.

My objective is to calculate the sound pressure level (in dB) of a sound generated 1 meter away. I don't have a SPL meter, nor do I have the money to buy one, so I would like to use my condenser microphone to calculate this. The microphone I have is the Blue Yeti, which has a sensitivity of 4.5mV/Pa (reference value 20 microPascal).

So my question is is there a way that the digital output of the microphone can be directly monitored and recorded in units of Volts. Hence, I would use these values to calculate Pascals and then dB. Can anyone point me to a (preferably free) program that can do this?

Also please feel free to comment on my experimental method. Is my reasoning in calculating the SPA sound when it comes to this?

• If you can establish the sensitivity of your computer input, not a problem, but I wouldn't expect particularly precise results. Absolute acoustic measurements are notoriously hard. NIST has some rather nice articles about the subject here: nist.gov/calibrations/acoustic_measurements.cfm – CuriousOne Jun 28 '16 at 5:40
• How would one go about finding the sensitivity of their computer input? Also, the measurements do not have to be precise, I'm fine with a rough estimate of the voltage output. Thanks for the help! – LPC16 Jun 28 '16 at 5:43
• I have oscilloscopes etc. with the proper calibration, which makes this easy. Without that I would start with a voltage reference (e.g. an LT1019 linear.com/product/LT1019), which I would chop to a 1kHz square wave using a CMOS switch/multiplexer, add a buffer opamp and a variable voltage divider made from 0.1% resistors (possibly selected down to 0.01% of relative tolerance), then another opamp buffer to counteract the load impedance of the input stage. This will get you to absolute AC signals with better than 1% voltage error to calibrate the response of the computer input. – CuriousOne Jun 28 '16 at 5:50

## 1 Answer

This has been answered in the comments, but just for completeness I'll write a formal answer here.

Your problem is that computer sound cards were never intended for precise measurements and it's very unlikely the gain will have been documented. In any case the audio device driver sits between you and the sound card and it's hard to know exactly what that is doing to the readings. You would need to calibrate your computer sound card, but that requires an SPL meter and of course that brings us back to where we started.

The obvious way to do this is with an oscilloscope, but a good oscilloscope will be even more expensive than an SPL meter. There is oscilloscope software available for PCs, for example a quick Google found this one, but I don't know how they deal with the calibration issue.

I wonder if it's worth looking for similar software for your phone. It's possible that if you have one of the popular models of phone there may be software available that is pre-calibrated for the model of phone. If so, you could either use your phone to do the measurements or use your phone to calibrate your PC sound card.