I have found this which I do understand (huurah!). It apparently was originally posted by cool_torpedo and is taken from boomanas thread on full sized headphones and a guide to newbies (hate that word!). Hopefully I have been able to higlight the main points, so the highlighting is mine;
OK, here we go, since I can't find the thread nor the reply, I'll try to explain a bit -again- how the impedance/sensitivity thing goes.
Headphones' drivers are transducers that convert AC electricity into sound. Dynamic drivers, the most common ones, use a coil -wounded wire around a hollow cylinder- into a magnetical field provided by a magnet, to convert the AC voltage into movement which is transferred to a membrane. The membrane's movement is transferred to the air particles in front of your ear. For the frequency characteristics of that vibration, and its pressure level, your brain interprets it as sound.
The voice coil of the driver has an impedance, which is the opposition it presents to the AC source (the amp or any headphone out) to the free flow of electrons thru it. The lower that impedance, the more freely the electrons travel
and the closer is the scenario to a short-circuit. This means that your source of electricity needs to pump more current intensity to correctly drive the transducer. So you can take two conclussions from this:
- What makes the AC to drive any coil is its voltage
. The minute variations of voltage follow the signal originally recorded.
- The current intensity is important to keep the coil excited, and you need more current intensity the lower is the impedance.
There's a relation between the current voltage and the intensity which is the power measured in watts. Power is the product of the voltage and the intensity: P=V*I. This is why amps are rated for their power output and not only for their voltage capabilites.Up to this moment there's no relation between the impedance and how loud the transducer will sound. However there's a parameter named sensitivity which tells you how loud will a transducer "sound" for a given amount of power you're feeding it.
The sensitivity is rated in dB/mW for headphones, so a pair of phones delivering a SPL of 100dB/mW are more sensitive (can sound louder) than a pair rated at 90dB/mW.
So the easy or hard to drive a pair of phones is, depends on both parameters, the sensitivity and the impedance. The worst case would be a pair of phones of very low sensitivity and also a very low impedance. Why? because they'll be asking to the source more watts to sound equally loud as a more sensitive pair,
and an important part of that power will be asked in the form of current intensity, which is something that most portable players, headphone outs in receivers and players, etc. aren't designed to deliver. This is the case of cans like AKG 701 or Denon D5000.
If your cans are low impedance but are very sensitive (the case of Grados and most IEMs) then despite their asking more current from the source, they still manage to sound very loud because they need very little power to do so.Most people tend to think that low impedance equals to louder sound, but this is plainly wrong. It all depends on the sensitivity and how much power the cans need to give a high SPL.
Also take into account that not all manufacturers offer their sensitivity values and not all them do in dB/mW but do in dB/mV. It's not much of a problem, you just need to convert the mV in mW knowing the phones impedance.
If anyone has any questions, please don't ask me! Thanks to torpedo and hopefully you do not mind this bastardised reproduction of the original.