[u]Here[/u] are your basic op-amp circuits. They can be used as a starting-point for making a microphone preamplifier.
If you build a non-inverting amplifier you should add a resistance between the input and ground to keep it from floating when no microphone is connected, or in case the mic is AC coupled. Most microphones are low-impedance and you can use a 1K resistor.
If you build an inverting amplifier, the impedance of the mic gets added to Rin to determine the gain and since the microphone impedance is not generally constant over the frequency range, you can get frequency response variations. To minimize that, Rin should be much greater than the mic impedance.
If you have an electret mic element it has to be powered. R1 on the [u]SparkFun Microphone Breakout Board[/u] powers the electret condenser and C1 isolates the microphone power from the amplifier.
If you copy that schematic, leave-out R2, R3, and C3. These components bias the output at 2.5V, which means silence is 2.5V and AC audio swings above and below that bias.
With a single-ended power supply (no negative supply) the op-amp won’t swing negative (OK in your application) but it also won’t go quite down to zero. And with a +5V supply you wont quite get 5V out. There are rail-to-rail op-amps but I don’t know if they go exactly to the power supply limits or just very-close.
A [u]Peak Detector Circuit[/u] will give you a varying DC voltage that “follows” the audio peaks. The decay time is determined by the C1 RL time constant.
Normally with SPL readings you wouldn’t necessarily want to use the peaks but most of your engines should have similar sound characteristics so you should be able to get reasonable calibration (as long as you have a real SPL meter for calibration).
I have a cheapy microphone that looks like it’s meant for an Arduino project that kinda works, but it isn’t consistent in how it “measures” different frequencies.
Again, if the sound characteristics are similar you should be able to get reasonable calibration/accuracy.
But, it’s tricky to make “real” SPL meter. SPL measurements are usually [u]weighted[/u] and averaged, and SPL meters are calibrated with known sound levels.