Well, if you'd like to relate datasheet-listed sensitivity to needed gain, it's pretty straightforward math.
Standard professional line level audio is considered to be +4dBu, so we can reasonably say we want a preamp with sufficient gain to bring up the signal to this level. We just need to compute the output level of the microphone in dBu (almost surely a negative number) and subtract it from +4 to get the gain required in dB.
In case you're bothered by the unit change (from dBu to dB), just know that it's due to the nature of logarithms (and thus, dB). That is, we're comparing two signal levels to a known reference level (the u of dBu), and then comparing them to each other (the u drops out of the equation and we could have used any reference in place of u to get the same result)
We have a sound pressure wave that can be expressed in Pascals ((Pa), standard unit of pressure). The sensitivity can be expressed in (mV)/(Pa). The product of these two quantities is the number of millivolts of electrical potential in the signal sent from the microphone to the preamp in response to that many Pascals of pressure.
The Wikipedia page for sound pressure has a table that you can check to get a feel for rough Pascal values of various sounds. For the purposes of this explanation, I'm going to pick a value at the top of the approximate range for "normal conversation at 1 m" : [2*10^-3 (Pa:RMS)]. (According to the same table, this is about [60 (dB:SPL)])
Now let's say the microphone has a (somewhat "typical") sensitivity of [10 (mV)/(Pa)]. When transducing the sound, it should send the preamp:
- [10 (mV)/(Pa)] * [2*10^-2 (Pa:RMS)]
- = [2*10^-1 mV:RMS]
- = [2*10^-4 V:RMS]
To convert this into dBu, you have to take the logarithm of the ratio of the voltage you have to the reference voltage for dBu: [.7746 (V:RMS)]
- 20 Log([2*10^-4 V:RMS]/[.7746 (V:RMS)])
- = 20 Log(.0002/.7746)
- = 20 * -3.588
- = [-71.76 (dBu)]
You can also use this handy dBu calculator and reference page:
(Try typing .0002 V into the rightmost calcultaor, you'll get the same -71.76 dBu result)
The same site also has another reference page about sensitivity:
So, anyway, we have a signal of [-71.76 (dBu)], and we want it to be [+4 (dBu)], so:
- [+4 (dBu)] - [-71.76 (dBu)]
- = (4 + 71.76) (dB)
- = [75.76 (dB)]
We need the preamp to give about 76 dB of gain to get the signal all the way up to standard pro audio level, although with standard use, you'll probably want some headroom, and may not want to give it the full 76.
If you run through the math for a few other sensitivity ratings, you can get a feel for what the numbers really mean. Doubling the sensitivity (to [20 (mV)/(Pa)]) doubles the voltage and gives a 6dB boost to the output (that's 6dB your preamp doesn't need to provide). Halving the sensitivity is a 6dB cut. This is perhaps the most useful rule of thumb to know. Consider a mic you are familiar with and find out its sensitivity, then compare other sensitivities to this one with the above rule in mind.