Headphones impedance

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. 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 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 MD recorders etc., you cannot use more than one pair of headphones at the same output simultaneously.