It puzzles me why sensitivity is ever specified in volts when every other specification is in dB (usually dBV, often A-weighted). Using decibels converts multiplication and division operations into addition and subtraction, so it's easier for the consumer to understand.
dBV = 20 * log (V)
So, for 1.5mV/Pa and 15mV/Pa:
20 * log ( 0.0015 ) = -56.5dBV/Pa
20 * log ( 0.015 ) = -36.5dBV/Pa
Every decibel measurement is actually a ratio: dBV is referenced simply to 1V, so it's easy to understand (so the calculations above were actually 20 * log ( 0.015 / 1 ), for example). The reference level is stated in the suffix: "V", "u", etc.
dBu should be archaic, but it's still popular with preamp manufacturers. No one knows why (well they do know, but refuse to admit it's silly). So while all microphone specifications are in dBV, you have to convert one way or the other to reference to amplifiers. That is confusing and unhelpful, so I make all of my specifications in dBV.
Anyway, dBu is referenced to the voltage required to drive a 600 ohm load to 1mW. No one actually cares about power in a preamp circuit, and most preamps are no longer 600 ohm loads. The "u" means "unterminated", so it's just a reference to a voltage rather than an actual amount of power. The voltage works out to:
(.001 * 600) ^ 0.5 = 0.775V
So to calculate sensitivity in dBu, you'd have to do this:
20 * log ( 0.0015 / 0.775 ) = -54.3dBu/Pa
But since that is annoying, we can merely observe:
20 * log ( 1 / 0.775) = 2.2dB
So you can convert from dBV to dBu by adding 2.2dB, or just 2dB if you are lazy.
Next up is dBm, which is referenced to the voltage required to drive a specified load to 1mW. This can vary, but for audio it *usually* means 600 ohm, so it's the same as dBu. But be careful, because for RF signals it will be referenced to something else, usually 50 ohm or 75 ohm. RF signal transfer must pay attention to proper termination, so dBm actually matters there. It does not matter for audio signals, at least before the power amp stage (which are strangely not specified in dBm or maybe dBW, but simply in watts), so dBm is even more archaic than dBu.
Whew, that was a lot of math. Remember, preamp manufacturers are to blame!
Here's another converter site:http://www.sengpielaudio.com/calculator-db.htm
I can understand that recording dialog with a ribbon mic would require a low-noise preamp, but understand that there is no such thing as recording "without noise". That's physically impossible. The best you can hope for is electrical noise that is quieter than ambient acoustic noise. For starters, a ribbon mic has self-noise, it's just not specified. The ribbon mic's transformer has thermal noise from its primary winding, which is stepped up (often 1:40 or so) by the transformer. The goal is to make the resistance of that winding as low as possible, but typically you end up with a self-noise figure of 17dBA or so. The noise of the mic preamp, which really can't be much better than that, gets added to the mic's self-noise, so figure for an M160 that the best possible self-noise would be about 21dBA, or about the same as a good lav mic, or many shotgun mics (except the condenser shotgun mics do not put the same noise and gain requirement on the preamp). Of course when you're recording to tape you have far less dynamic range available and you have to max out levels to get that, so yes you might need 70dB gain there. But for a condenser mic recording a loud source into a digital recorder with >90dB dynamic range, you would never need 70dB--and a minimum gain of 18dB could be problematic.