When you wondering; "What is Headphones Impedance" you need to know a couple of things for context.

In summary headphone impedance refers to the electrical resistance. This is shown in units called ohms.

Headphones with certain impedance requirements need a certain output capacity from the amplifier or sound source that they are connected to. The amplifier or sound source processes the output capacity to make sure your headphones provide an optimal sound.

  • Low impedance headphones plus high output amp = potentially risky.
  • High impedance headphones plus low power amp = poor sound quality.

The two important electrical specifications for a pair of headphones are the impedance of the phone transducer itself and its on-the-ear sensitivity. So-called low impedance headphones may vary from 75 ohms up to about 150 ohms. Phones in this impedance range may be directly plugged into the headphone jack routinely found on recording and playback equipment like an MP3 player.

Higher impedances, such as 600 ohms, are more useful in studio installations where many units may be wired in parallel for studio monitoring applications.

Headphone sensitivity is usually stated as the in-the-ear sound pressure level produced by one milliwatt (mW) of audio input.

Typical sensitivity ratings of AKG headphones for example run from 88 dB per mW to 105 dB per mW. You can see that very little power is needed to drive a stereo headphone pair to very high listening levels.

Doing a little bit of math will show that a signal of .775 Vrms will produce one mW in a 600 ohm load. That same signal will produce 8 mW in a 75 ohm load, a difference of about 9 dB. So it is clear that 600 ohm phones and the lower impedance models should not be intermixed in the same application.

The 600-ohm models are more rugged than low impedance models in that the higher resistance coils are less susceptible to burn out than low impedance models. To produce 200 mW in a 600-ohm load a signal of 11 V is needed.

This shows that the advantage of high impedance headphones is that they can be used with almost any amplifier output without any risk of being damaged by overload, and you can connect up to 10 or more pairs to the same output. However, they may be not loud enough with some portable recording devices.

Low impedance headphones will sound louder with devices with low output voltages such as portable recorders etc., you cannot use more than one pair of headphones at the same output simultaneously.

In short:

  • If your headphones are higher impedance you are not risking overload when playing sound through an amplifier.
  • Lower impedance headphones risk damage if you're using them with a high output amplifier.

View a complete range of headphones over at PMT Online.