Impedance, damping and sensitivity

Impedance

Impedance is expressed in Ohm.
Often it is mistaken for resistance as resistance is also expressed in Ohm.
Resistance (Ohm's law) is DC resistance.
Impedance is the resistance when an alternating current is used. Basically, it is frequency dependent.
A simple example is an analog interconnect and a digital interconnect like a 75 Ohm SPDIF cable.
If you measure resistance using a multi meter, both will have the same value pretty close to zero. If you send a signal with a high frequency over the cable a true SPDIF cable will have a 75 Ohm impedance and any other coax cable will have some value but most of the time not 75.

Damping

Most of the time the headphone out of an amp is a matter of connecting the headphone out to the same power amp as used by the speakers. To avoid frying the headphone, a resistor network is used to reduce the power substantially.

Nothing wrong with this approach but the result is an amp with a high output impedance.

A rule of the thumb is that the damping factor, the impedance of the headphone divided by the impedance of the amp, should be 8. Often this value is criticized as being to low.

This is the reason why traditional headphones do have a high impedance. You can drive them with a high impedance output and still have a decent damping ratio.

Driving high impedance headphones is no problem for a stationary amplifier. Power consumption is not an issue.
Cell phones, however, don't have big amps as this will drain the battery.
Tiny amps being frugal on power is what they need.

This gave birth to a whole new type of low impedance headphones, often 32 Ohm or less.
They can be driven by a couple of milliwatts hence by any mobile.

 

It is not a good idea to use a cell phone to drive a high impedance headphone (300 ohms or more). It will not have enough power.

It is not a good idea to drive a low impedance headphone (32 Ohm) with conventional equipment. You will have insufficient damping.

 

More about damping.

Sensitivity

It looks simple, high impedance headphones for powerful stationary gear and amps with a high output impedance (valves!) and low impedance for portable use.
Practice is more complex.
There are headphones with high impedance that are easy to drive and low impedance headphone that are hard to drive. This is called sensitivity.
It is the amount of power needed to reach a certain level of loudness.

 

1. Electrical Impedance: What is it? - Electrical4U
2. Headphone Impedance Demystified: Do I Need a Headphone Amp? - Headphonesty
3. The Complete Guide To Headphones Sensitivity Ratings - Arthur Fox