All this jibjab in the science forum and no one has mentioned impedance matching yet? In simple terms, you get optimal power transfer from your amplifier when your headphone impedance matches your amp impedance. I think the recording industry design standard (don't ask me to name the agency, I don't remember) is actually 120 ohm, but no one actually goes by that.
It's an ISO standard. Some manufacturers go by it, some don't. The Beyerdynamic A1 amp is close (heh) with Zs=100 ohm.
I don't think impedance matching is something you want to do with headphones, instead you want impedance bridging.
("since it is more important to accurately control the loudspeaker than to drive it with the maximum amount of power available")
Here's my take on things:
- you want optimal power transfer
Maximum efficiency (Zs~=0) instead.
[...]
If I had the knowhow to build a voicecoil myself, I would... but I don't. I did however make myself a couple impedance adapters (like the Etymotic 4P/4S adapter) in 90 and 260 ohm flavours and plugging into my Bottlehead Crack amp which is designed for high impedance cans.
Here's the problem. It's not designed for such a load.
If I plug in a straight 32 ohm can, it's hard to control and I can barely turn the volume knob without it going too loud, but no only that it doesn't sound very good. The high range is ok, but the low end sounds all flabby and undefined. Plug in an adapter and the most noticeable effect is volume attenuation. My listenable levels are now within a comfortable 10-2 o'clock sweep and the definition has increased noticeably (I hesitate to say "significantly" since we all hear things differently), especially in the low end.
Putting resistors in series with speakers .. is not a good "solution". In fact, the result of doing so is flabby and uncontrolled bass.
Now this is just my experience in adding resistance to the output line. The only effect here should be noise rejection and impedance load matching to optimize power transfer.
Noise rejection isn't needed with the right amp/device that's been designed for such a load.
Power transfer: see above
In the end, you want to headphone to see the "same" voltage the amp is seeing on the input (bridging), and not a "distorted" version of it (power transfer), right?