Some people may neglect impedance when they buy headphones for music. However it is one of the most important parameters. You may not get the best sound from the headphone if the impedance is too high or too low, but an impedance mismatch would not cause big disaster anyway.

What is it?
Headphones drivers are equal to transducers, by which AC electricity is converted into sound. As the most commons ones, dynamic drivers convert AC voltage into movement by using a coil, a wire winded around a hollow cylinder, into a magnetic field which is provided by a magnet. The movement is transferred to a membrane, which produces vibration. The vibration is delivered to the air particles to ear. Afterwards brain would interpret the vibration frequency and pressure level into sound.

The driver's coil has impedance which impedes the transfer from AC electricity to sound. The lower impedance a driver has, the more freely the electricity can flow and travel. The voice coil of a headphone's driver largely determines its impedance.

Headphone Impedance varies from 16 ohms to 600 ohms. Normally impedance under 50 ohms is regarded as low-impedance headphones. These headphones do a great job at home with receiver as well as for on-the-go listening with Bluetooth-enabled phone. Thus a great majority of wireless in-ear, on-ear and over-ear headphones are low-impedance ones. Mixcder's Bluetooth in ear headphones (about 16 ohms) and over ear headphones (around 32 ohms) are low-impedance designs, which are much better suited to use at home or take out. As for high-impedance cans, they are more suitable for home than on-the-go use.

Relationship between impedance and sensitivity
Impedance has no relation with the sound volume a headphone has. But the other parameter--sensitivity decides how loud a headphone sounds for a given amount of power. It depends on two parameters--impedance and sensitivity to examine if the headphone is easy or hard to drive. The higher sensitivity and lower impedance a headphone has, the less power it needs to drive it and the louder sound it can be heard.

Some people wrongly think that low impedance produces louder sound. In fact, the sound volume is decided by the sensitivity and how much power the headphones should have to give a high sound pressure level (SPL).