# Calculating SPL from voltage output of a microphone with MAX4466 amplifier

I'm using Electret Microphone Amplifier - MAX4466 with Adjustable Gain and I'm getting the output voltage ranging from 1.5V to 3V (which, to my knowledge, is already amplified by MAX4466). I know that the sensitivity of the microphone is -44dB @ 1kHz. It is also stated on the mentioned product page that the gain can be adjusted from 25x to 125x by adjusting the trimmer pot on the back of the breakout as needed.

I'd like to know a way to figure out the op-amp gain and ultimately, calculate SPL dB (Sound Pressure Level) from the measured voltage. Any help is appreciated. Thanks.

Microphone sensitivity is quoted at

-44 ±2 dB, f = 1KHz, 1Pa 0dB = 1V/Pa

This means that for 1 Pa RMS sound pressure on the microphone you get -44 dBV RMS at 1kHz. Still confused?

Well, -44 dBV translates to $10^{-44/20}$ volts = 6.31 mV RMS and 1 Pa translates to a decibel sound pressure level of 94 dB SPL.

So your microphone produces a 1kHz sinewave of amplitude 6.31 mV RMS when subjected to a SPL sinewave of 94 dB.

As for the gain of the Maxim circuit, without a circuit diagram I can't help you any further.

• Thanks. I've added a circuit diagram that I hope what you asked for. Jan 18 '16 at 3:56
• Mid-band gain (1 kHz) is about 11. The upper -3dB point is about 16 kHz (high frequency) and the lower one about 16 Hz. Jan 18 '16 at 8:47
• Firstly, thank you so much for your time. Those are the ideal/calculated values, aren't they? Is there a way to measure the actual gain? And also, I'd like to know where you got those values from. Jan 19 '16 at 14:37
• @J.Jay the mid-band gain will be as accurate as the resistor tolerances. If you want to measure it, use a signal generator and an oscilloscope but if the resistors are 1% or better then there is no point because the visual accuracy of the scope screen is probably worse than 2%. Jan 19 '16 at 14:42
• Gain of a non-inverting op-amp amplifier = 1 + R2/R1. R2 is 100k and R1 is 10k hence mid-band gain = 1 + 10 = 11. At the higher frequencies the upper 3 dB point is when the 100pF has 100k impedance = $\dfrac{1}{2\pi F C}$ or F = $\dfrac{1}{2\pi RC}$ Jan 20 '16 at 15:33