I got quite intrigued by the design of audio devices ever since getting my ER4 earphone, which claims to reproduce the pressure field in ear canal as what it would be in reality.

The only thing sounding fishy to me is how could a soundcard with an output impedance of 40 Ohm drive a 45 Ohm earphone without voltage division taking place. A search on Google told me nothing besides audiophiles selling their impedance matching snake oil. It just didn't seem like many people had done a serious measurement on this matter, so I decided to do one, and also for what will happen if an op-amp is used to boost the current delivery.

The report can be downloaded from here.