I have severall pairs of headphones and when playing music on my walkman if the headphones are changed the lowdness of the music differs with out the volume being adjusted. Why is this??? i know you probably think deerrr stupid question but anyway, Why? the reason i ask is, i'm going to purchase a pair with a big frequency response range and want to understrand more about what i am buying.
Cheers Dude

[TameRacingDriver]

There a couple of things that can affect the loudness of a headphone.

The main one however, is a rating called sensitivity. In speakers, this is usually rated on a scale of loudness (db) @ 1 watt @ 1 metre distance. In headphones, this is usually rated as loudness (db) @ 1 mW power. Distance is not a factor since headphones are used directly on the ears anyway.

So, if you get a headphone that is rated at say (for arguments sake) 86 db @ 1 mW, then it takes 1 mW of power to give 86 db. If you then go to a headphone that is 96 db @ 1 mW, then it will sound to your ears roughly twice as loud (since a doubling of perceived volume is not a doubling of decibels) even at the same volume.

Just to make matters even more complicated, impedance (or resistance) is also a factor. A 64 ohm headphone will require twice as much voltage to reach a given volume than a 32 ohm one!