Impedance

Impedance is a measure of electrical resistance specified in ohms. Speakers are commonly listed as 4 or 8 ohms but speakers are reactive devices and a nominal 8-ohm speaker might measure from below 4 ohms to 60 or more ohms over its frequency range. This varying impedance curve is different for each speaker model and makes it impossible to design an effective "generic" speaker level high-pass filter. Active devices like amplifiers typically have an input impedance between about 10,000-100,000 ohms and the impedance is the same regardless of frequency.

One issue that is especially important when choosing a pair of headphones is impedance. So-called low impedance and high impedance pairs should not be intermixed. Low impedance headsets 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 sets normally have an impedance of around 600 ohms; these are more useful in studio installations where many units may be wired in parallel for studio monitoring applications. This means that low impedance headphones will sound louder when plugged in devices with low output voltages such as portable CD players, etc. In reality, because of the limited power available from portable players, headsets for portable use should have a maximum impedance of 64 ohms.