The receiver spec's how big of a signal goes in to give a desired result.
The microphone spec's how big of a signal goes out given a specific input sound level.
So the first gets a smaller number if more sensitive, the second increases if it's more sensitive.
Totally different industries, totally different notions of how to go about it.

A receiver's sensitivity figure in dBm tells us how much power must be coming in the antenna port
to achieve a specific result, in this case the "minimum discernible signal".
A power level zero dBm is arbitrarily defined as 1 milliwatt, and since typical signals at the receiver
are much lower in power, the receiver sensitivity is a fairly large negative number.

The microphone's sensitivity is defined as how big of a voltage signal we get coming out
when the microphone is presented with a sound pressure of one Pascal, and zero dBV is
arbitrarily defined as one Volt.  

Most of the speakers I buy state something like: "Response from 20 Hz to 20 kHz".
Not very precise.
They don't actually say what the response is, could be argued that catching fire is a response of some sort.

Here's a webpage on speaker sensitivity:
    http://www.psbspeakers.com/articles/Guide-to-Speaker-Specifications

"Sensitivity
Sensitivity is most easily defined as the speakers’ ability to effectively convert power into sound. The traditional way of measuring a speakers’ sensitivity is using the standard of 1 watt/1 meter. Meaning a microphone is placed 1 meter away from the speaker to measure the sound output (in decibels) with 1 watt of sound played through it. "

So like the microphone, they spec how big of a signal goes out (sound in dB) when given a specific input (1 watt of electrical power).
Makes sense, as would have been the same engineers working with microphones and speakers back when these things were defined.


Next question: 
What is 0 dB of sound, and shouldn't that dB have a letter following it to tell us what the baseline is when finding that ratio?

Answer: 
Zero dB of sound is defined as the minimum discernible signal for the typical human ear.
No final letter because, well, it's a different group of engineers who defined it and they didn't care about the resulting confusion.

Jerry 


On Mon, Jul 16, 2018 at 11:50 PM, iz oos wrote:

Thanks Jerry, I had always thought the opposite, the lower the more sensitive just like sensitivity in a receiver. So, I was wrong. What about sensitivity in headphones and speakers?