[faisal2003456]

I know that this has been asked/explained many times, but I still can't draw the picture in my mind.
 
For example:
The Cowon J3 outputs 29 mW of pair to each speaker to a pair of 16 ohm headphones.
 
The RE0 have an impedance of 64 ohms and a sensitivity of 100dB/1mW.
 
In theory they would receive 16/64 or 1/4 of 29mW, so they would receive 7.25 mW.
 
Do I multiply the sensitivity and power received. That would give me a crazy number of 725dB. I know that can't be right. What am I missing? 
 
Another side-question. Sony lists the Walkman X series as having an output of 5mW per speaker. They don't list on what impedance that number was achieved. How do I work out the power it would supply to a pair of oh say 32 ohm headphones?

 

[paulypaul]

100db is heaps loud enough.
so to drive the RE0s you only need to give them 1mW (in theory).
So the Sony X will drive them easily.
The only phones you would need to give more power to would be Hi Impedance.
A higher impedance headphone generally would need more power to drive it and opposite for a lower impedance.
Some headphones are say 300 - 600 ohm so you would not be able to drive them to a good volume with say a sony X (enter the headphone amp).