There is a closed question
on Physics Stack Exchange and I thought this might fit better in engineering (also as one of the comments there has been suggesting). And my question here is related to that but a bit different.
If we want a remote listening device which eavesdrops (collects) the sound signal and then transmits it to some receivers far away (so this process may require a type of electroacoustic transducer to convert the sound signal to electromagnetic waves, etc.）, typically how much power/energy would this entire device require, with respect to the distance of the transmission (receiver)? What are the limitations in its engineering and powering perspective?