# Converting arbitrary mic input number to dB SPL

I'm trying to convert an arbitrary input number from a laptop microphone into SPL. The microphone specs are available on https://docs-emea.rs-online.com/webdocs/105f/0900766b8105f0e3.pdf.

From what I've read on another article, the equation is 20 x log (V1 / V0), V1 being the voltage measured and V0 being the reference level. The reference level (V RMS) I've found online for -42db is 0.001776 but I'm unsure what the V1 number should be. According to the other article, it is the gain that the preamp has applied but I'm not sure if this relates to what I'm doing, or how to find this number.

Any help would be much appreciated.

The article I'm referencing is How to convert Volts in dB SPL.

• Wait... "The reference level (Vrms) I've found online for -42dB is [this] but I'm unsure what V1 [should be]." This is confusing. Are you saying that your target specification for your gain is -42dB? Also, you say that according to "this source", you've made particular conclusions, however you haven't provided those sources so how do we know what you're talking about? Lastly, you say that you're confused on how to solve for V1 or how dB gain might be related to what you're looking for, yet you've provided an equation for gain in dB. You're contradicting yourself. This is confusing to read. – user103380 Mar 12 '18 at 18:07

To calculate the real sound pressure level (SPL) from the sample values from the sound card, you would need several things:

1. The sensitivity of the microphone (voltage produced for a specific, real sound pressure level.) This is given in the specifications of the microphone. The rating in the datasheet you linked to is kind of odd. It says 0dB is 1volt per pascal, then says that the sensitivity is -42dB. So, that's 0.008V per pascal. Note that it is +-3dB. So, there is considerable variation in the sensitivity of individual microphones.

2. You will need the gain of all amplifiers between the microphone and the ADC. It is unlikely that you will have information on all the amplifiers in the sound card.

3. You will need to know what voltage the full scale reading of the ADC equates to. It is unlikely that you will have this information.

Since you can't get much of the information you need, and what you can get isn't precise enough, you will have to go another route.

The usual way would be to compare your microphone/sound card setup to a known sound pressure level. In other words, you need to calibrate your measurements.

1. Turn off any automatic gain control in the line in of your sound card (or microphone input.)

2. Set the gain of the line in (or microphone in) to an easily reproduced setting (full up is easiest.)

3. Setup a sound source, your microphone, and a calibrated SPL meter so that the SPL pickup and your microphone are at the same distance from the sound source.

4. Make several measurements (at different volume levels) using both your microphone and the SPL meter. Note them down.

Using the table of measured values, you can calculate the true SPL from any measurement you make with your microphone - but, you must always use the same gain settings and setup as you used in the calibration setup.

For audio, my calculations are as follows;

1kHz @ 1m
mV ( rms ): 10, 8 , 1 , 0.1 , 0.01
dB (V/Pa): -40,-42,-60,-80,-100
dB( SPL ) : 94 , 92 , 74 , 54 , 34

0 dB(SPL) was the old way to define pressure where 0dB (SPL) was the human hearing threshold of a mosquito at ~ 1 m distance.

But for IEC standards 0 dB = 1V/Pa

• Sensitivity = -42 dB +/-3 @ 1kHz into a 2.2K load with 2V supply. Load affects gain.

• -40dBV=10mV from 1V = 0dBV and -6dB is 50% of voltage

• so -42dB (V/Pa) = 8 mV is half the pressure of a Jack Hammer @1m = 100dB(SPL).(Wiki)
• Since 2 Pascal = 100 dB SPL then 1 Pascal or 100-6=94dB SPL

Just remember this is for 1m distance and not 1mm or 10m.

• Cheers Tony! This helped a lot – Alfie Mitchell Mar 15 '18 at 22:45