Feb 21, 2005 10:29 am Not to fret! (no pun intended)
Impediance is no simple concept. It is a science all on it's own. And yes 'ohm' is the most used unit to express impediance.
Now, relitive to volume; Impediance is not a good value to use to judge volume or sound pressure levels. Two headphones or speakers with the same impediance will have big differences in efficiency. That is to say one may be much louder than the other. Unfortunately, most headphone advertisements do not indicate efficiency ratings. They would be expressed in some term of dB produced per millivolt of signal if they did, most likely. But again marketers will use the oddest ways of presenting specs; to their advantage obviously. Some headphones like the Sennheiser HD-280 will say something like 'These babies are Loud!'.